Fur-Bearing Mammals of California

Transcription

FUR~BEARING
MAMMALS
OF CALIFORNIA
Their Natural History,
Systematic Status, and Relations to Man
BY
JOSEPH GRINNELL
JOSEPH S. DIXON, AND JEAN M. LINSDALE
<>CONTRIBUTION FROM THE
MUSEUM OF VERTEBRATE ZOOLOGY
UNIVERSITY OF CALIFORNIA
IN
Two
VOLUMES' VOLUME
II
UNIVERSITY OF CALIFORNIA PRESS
BERKRLEY, CALIFORNIA·
1937-
,
[GRINNELL, DIXON, LlNSDALE: FUR-nEARING MAMMALS OF CALIFORNIA: PLATE VIII]
GOLDEN BEAVER
BEAVERS
GOLDEN BEAVER
.Castor canadensis subauratus Taylor
NAMEs.-Castor fiber~ part; Castor canadensis, part; Castor americanus; Castor
pacificus; Castor subaU1'atus; Castor subauratus subauratus; Castor fiber var. canadensh; Castor fiber americanus; Castor canadensis pacificus, part; American Beaver, part;
OTHER
California Beaver; Pacific Beaver, part; Beaver, part.
General characters.-Size, largest of all the rodents (in adults, length of body about
30 in., of tail alone, 16 in., weight upwards of 40 lb.); strong incisor teeth, orange
brown in color, protruding from mouth; pelage dense, consisting of long, glossy overhair and thick underfur; eyes and ears small; tail flattened horizontally, broadly paddleshaped, scaly; hindfeet large, toes completely webbed. (See PI. VIII, frontispiece.)
Descrt'ption.-Adults: General tone of color, golden brown. More exactly: Underfur, very nearly unifor~ all over, varying around bister, warm sepia, and cinnamonbrown, paling slightly on hinder part of belly and around muzzle; overhair on back,
dose to ochraceous-tawny, on lower surface where much shorter and scantier, pinkish
buff; on base of tail brightening to orange-cinnamon and even vinaceous-rufous. Because of notable gloss of overhair, the color tone varies greatly with the angle at which
it is viewed. As a pelt is turned this way and that in good light, an impression of metallic gold is given. Length of longest overhair on middoqum, 2 in. (50 mm.), on belly,
IYs in. (28 mm.); depth of underfur on back, %-in. (I9 nun.), on middle of belly,
)':i-in. (I3 mm.).Whiskers relatively short (longest up to 2% in. [70 mm.]), pale
yellowish brown in color.
Soles of both hind and fore feet and nose pad, naked, with skin rough, blackish
brown in color; tops of feet devoid of underfur, covered with short overhair, russet
in tone on hind feet and dull cinnamon-buff on forefeet. Front foot relatively small,
but all five toes well developed and each armed with a heavy claw, the longest (middle
one), Va-in. (22 mm.) long and %6-in. (5 mm.) wide. Toes';f hind foot each have
a claw which projects beyond margin of webbing; claws of outermost three toes, stout,
wider than high, the longest, I in. (25 mm.) long and Ya-in. (IO mm.) wide; claws
of inner'two toes much smaller, second with horny blade underneath its claw which
leaves a narrow slit; this' second-toe structure constitutes the so-called louse comb.
Claws dark brown at bases to horn color at tips. Tail hairy at base continuously with
body, terminal flat part (about % total length) scaly, meagerly beset with short hairs;
"scales" wide and short, tending to form transverse and diagoI!-al rows; but these are
not perfectly regular in arrangement a~ on a carp's body.
Young, taken from house, small (length, including tail, 15 in. [380 mm.], weight
2IYz oz. [610 gm.]): LIke adult, but pelage "woolly" in texture and general color
darker. Underfur on back deep clove brown; on lower surface, fuscous; overhair lusterless, snuff brown on ·back, pale buff below. Feet like adult's; tail similar to adult's but
flat part relatively narrower, and scaly surface shows fine hairs, of same dark brown
color as rest of tail.
The general features of the beaver's skull are shown "in figure 258. Detailed descriptions and illustrations have' already been published, by Taylor (1916); and need not be
repeated here. Our measurements from skull~ of the Californian races, as available to
us, are given on pages 630 and 722.
Fur-bearing Mammals of California
Color vart'ation.-As· far as can be seen from external examination, the sexes are
alike with regard to size, color, and texture of pelage. Only one molt takes place each
year, occurring
slowly through summer and fall. The fur does not become prime
before the first of December. Winter-taken pelts show but slight variation in color.
The tone of the underfur is wonderfully uniform. Such variation as is shown in the
overhair generally seems to be caused by the amount of "tipping"; whereas most· of
this is individual, it may also be modified by abrasion and, perhaps, fading. The "golden
brown" cast of color in Castor c. subauratus is so constant as to be diagnostic when comparison is made with C. c. frondatol'.
.
very
MEASUREMENTS (IN MILLIMETERS) OF SKULLS OF ADULT OR SUBADULT
Castor canadensis subauratus
FROM CENTRAL CALIFORNIA
:1i
No.
Sex
M.V.Z.
16383
31081
31082
31083
31084
31108
31255
31276 .
31285
31302
i
ill'..
I
!
8987
12654
12669
31079
31273
31278
31290
31293
31294
31296
Locality
"Ii,
.!!
j
0
.p
•8
;'l
~
••
;'l
~
•
'S
;'l
M
.~
"
jj
~
~
:>:•
r:5~
1
~
~
'S
1 ""~~a
"8
to
'il§
:Be ~~
·:di
lie
8"
0
'S
=-6
,,0
eli,
~~
P
'E.~
~~
,,8
l"
~
Grayson •••...•••.•••...••..•
Mendota ....•. , ...•.• , ... , ..
Mendota .•..•.. , .... , •...••.
Mendota .........•...• , ...•.
Mendota ...••...••..•.•....•
Fair Oaks ..•.•....•.•...•..•
Hopeton .. , ............... ,.
Snelling .... , ...•............
Hopeton •.••..... , ..... , •...
Snelling •. , ... , ..............
120.6
127.0
121.0
121.5
125.0
126.8
123.6
126.8
124.5
122.9
96.0
105.6
100.3
102.8
99.3
99.3
95.9
98.3
94.7
93.7
25.6
28.3
26,9
26.7
27.1
29.6
25.6
26.7
26.5
25.5
65.2
68.8
6S .8
66.5
65.9
67.8
68.0
70.1
66.8
63,6
49.3
56.8
S3.7
53.4
53.1
52.1
52.9
57.7
52.8
5:3 .8
26.1
27.9
24.7
24.8
25.8
26,S
26.6
28.2
25.2
26,4
11.1 19.0
10.1 19.1
11.9 19.7
12.7 ....
10.4- 18.4
11.9 19.6
12.:3 19.2
13.3 19.2
11-.9 20.3
12.8 19.:3
31.7
33.9
31.8
32.0
32.5
33.9
34.6
33.2
33.733.0
~' Grayson ..•..••. , •.•.. " ... -.'
~' Grayson., .. , .....• , ....• , ...
113,1
126.4
115.2
115.0
127.3
130.5
123.3
116.0
123.2
126.1
88.8
103.1
94.0
92.1
97.3
97.5
92.8
90.8
93.9
99.1
23.3
28.1
25.4
24.0
27.1
27.3
24.5
26.0
26.5
28.6
63.5
70.2
66.4
60.0
69.7
68.4
66.1
64.8
67.7
69.3
50.2
54.2
51.0
48.0
57.3
56.6
52.7
51.6
51.1
54.2
24.0
28.0
25.7
23.5
27.9
27.5
24.8
24.3
26.S
26.5
10.8
10.0
12.4
12.8
13.6
11.9
11.2
12.0
11.4
13,4
18.4
18.3
18.8
18.6
21.2
19.5
18.5
19.2
19.7
19.5
29.8
33.2
30.9
30.8
34.2
34.2
31.3
29.7
32.3
33.6
General average, both sexes ... 122.7
96,7
26.5
67.7
53.1
26.0
11.9
19.2
32.5
,,'
''""
,,''"
""
""
"
~,
Grayson ...... , ......... , ....
~' Mendota." .. , .....•..••.• ,.
~
~
~
~
~
~
Snelling ........ " •. ,." .... ,
Snelling .... " .. , ............
Snelling, ......... " ..•.. , ...
Snelling ...... ,., .. , .. , ........
Snelling .. , ...• , .... , .••... ,.
Snelling ........•.•.... , .... ,
",'
~-
~]
~ ->'"- -~o
~~
---
- - - - - - - - - - - -- -
* Tne sex as here recorded is as determined and written on the labels by the trappers, But, because of the difficulty
known to have been experienced in sexing beavers, the accuracy of the records in those specimens starred in the table is,
in our minds, open to some doubt; for this reason, separate averages for the sexes are not given.
Measurements.-The following are average and extreme external measurements, in
millimeters: Of 9 adult males: Total length, 1091 (1025-II75); tau vertebrae, 371
(35(}--405); scaly part of tail, 12ox28r; ratio,' per cent, width of tail to length of tail, 42.8
(37.9-48.3); hind foot, 188 (180-195); ear from crown, 26 (23-28). Of 15 adult females: Totallength, II07 (103o-r200); tail vertebrae,383 (345-420); scaly part of tail,
r20 X 295; ratio, width of tail to length of tail, 40.5 (31.5-46.8); hind foot, 188 (175205); ear from crown, 28 (27-29). Of 24 adult examples (9 males and 15 females):
Total length, I rOI (1025-1200); tail vertebrae, 379 (345-420); scaly part of tail,
120X289; ratio, width of tail to length of tail, 41.4 (31.5-48.3); hind foo1; 188 ('75205); ear from crown, 27 (23-29)'
,
-'~
--,--_.
Fig. 258. Skull of golden beaver, side view, male (no. 3I085, Mus. Vert. Zo01.). trapped on February I7, I920, near Men~
dota, Fresno County. X 1. Noteworthy features include, as correlated with the beaver's special mode of existence: the huge
incisor teeth for gouging and grasping, their .housing in the massiv~ rostrum and mandible, the great development of ridges
for attachment of powerful muscles, and the corresponding relatively small size of the brain case.
MEASUllliMENTS (IN MILLIMETER.S) AND WEIGHTS (IN POUNDS) OF 61
Castor canadensis subattratus
FROM VICINITY OF SNELLING
II>
No,
M.V.z.
Date
Ago
Sox
Weight
Total
length
.g•
1 "•
~
31269
31271
1920
Feb. 25
Feb. 26
13SX290
12SX305
46.5
40.9
115 X275
41,8
HOX290
llOX275
llOX285
115X250
40.0
38.6
125 X275
llOX290
45.4
115X295
119X295
39,0
40,7
40.3
1130
350
347
355
415
135 X325
adult
adult
adult
adult
d'
d'
d'
d'
8
adult
,d'
47
1080
370
adult
adult
adult
adult
adult
adu,lt
0
0
0
0
0
0
36
33598
33602
33605
33608
1922
Dec. 5
Dec. 7
Dec. 8
Dec. 14
Dec. 17
Dec. 20
35
34
41
48
38»
1045
1015
1012
1055
1153
103u
33612
33614
1923
Jan.
Jan. 7
adult
adult
0
0
4'
46
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
adult
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
50
Feb. 28
Feb, 29
Mar.
Mar.
31302
31291
,,"e, 6
Mar. 8
31299
Mar. 14
,,
~~~
.9~B
350
405
350
370
370
370
395
365
365
39
37
1080
34
42M
43
38
1025
1150
1175
1170
40
1035
42Ylj
lOSS
45
1100
37)1
35
36»
50
1045
1027
d
8~t!
~
d'
d'
d'
d'
d'
d'
d'
d'
d'
31276
31255
31285
~i1 ~
'il.~
~
adult
adult
adult
adult
adult
l).dult
adult
adult
adult
31274
'"
~"O;;::
]
e
~
'"
0 '"
]
~
~
§
a
J!
--- -'""
185
195
185
195
190
190
185
180
190
23
27
27
28
28
27
26
"
I
177
41.5
180
180
195
26
27
27
29
I
150X295
50.8
190
28
380
342
360
375
370
375
120X290
118X282
118X287
118X290
123 X300
115X285
41.4
41.8
41.1
40.7
41.0
40.4
180
178
180
185
187
180
25
25
27
28
29
28
1095
1120
370
400
117X300
130X320
39.0
40.6
187
184
28
27
1140
400
345
365
420
380
400
385
142X310
125 )(305
117X290
115 X310
118X300
125 X305
125 X285
110X295
130X295
131X279
110X284
125X290
90X285
120X295
112X295
45.8
40.9
40.3
37.0
39.3
40.9
43.8
37.3
195
180
185
205
205
195
190
185
195
175
179
185
180
187
187
1030
48.3
46,0
37.9
.,
1922
33597
33601
....
33607
Dec. 6
. Dec. 12
Dec. 17
Dec. 20
1050
llBX290
1923
33616
33596
,\1
... "
.....
31270
31301
31273
31277
31278
31283
31254
31290
31293
31294
31296
31297
31298
31300
Jan.
,
1920
Feb.
Feb.
Feb.
Feb.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
Mar.
24'
24
27
28
1
2
5
5
8
10
10
11
11
11
15
....
35
35
41
45
45
42%
44
38»
37Y.i
50
36
45
45
1100
1030
1090
1135
1180
1105
1100
1200
1050
1097
1105
1050
1107
1115
(Continued on next paul
385
395
365
375
390
367
395
380
44.0
46.8
38.0
43.1
31.5
40.6
37.9
"
,.
27
28
29
27
28
28
"
"
"
"
"
"
"
Golden Beaver
MEASUREMENTS (IN MILLIMETERS) AND WEIGHTS (IN POUNDS) OF 6I
Castor canadensis subauratus
FROM VICINITY OF SNELLING-(Concludcd)
"e
~
No.
M.V.Z.
Date
Ago
Sox
Wcight
Total
.g
length
t
jj
~
~'O~
~
'B
");
"
..!;
~
ro
1922
33595
5
2-year-old
0'
Jan. 6
2_year-old
2-year-old
0'
0'
Dec.
31
.
~
~~~
d
8...!::t:
uS
11
c:>,.-;::
"
~
§
j
o,s ..
:.::
.g
.~'S
~
"
iii
"'
d
,
~
950
320
lO7X260
41.2
177
25
980
"0
110X270
40.7
170
...
26
}923
33613
33615
Jan. 7
J1
35 (very
....
...
........
...
fat)
..
1920
31275
31288
Feb. 28
Mar. 7
33604
Dec. 15
2-year-old
2-year-old
0'
0'
32
28
1030
350
340
105X280
1000
95X265
37.5
35.4
175
180
26
25
2-year-old
9
25
930
JJ7
}OOX265
37.7
165
28
2-year-old
9
22
940
345
96X270
35,6
160
28
175
160
170
24
1922
1923
33611
Jan. 5
(poor)
1920
31284
31292
31272
Mar. 5
Mar. 10
2-year-<Jld
2-year-old
Feb. 27
2-year-old
33600
Dec.
,
yearling
33603
Dec. 14
33610
1923
Jan. 3
31286
31289
31279
31282
31281
31287
31280
1920
Mar. 6
Mar. 8
Mar. 2
Mar. 4
Ma,. 4
Mar. 6
Mar. 2
33599
33606
33609
1922
Dec. 8
Dec. 17
Dec. 23
32187
1921
April 25
9
9
9
"
1000
375
lO$X270
38.8
30
28
1020
380
350
lO5X275
lUOX270
38.1
965
0'
0'
18
20
882
840
315
295
lOOX254
39.4
yearling
87X23S
37.0
168
160
24
26
yearling
0'
20
865
300
93X2SS'
36.5
160
25
yearling
yearling
yearling
yearling
yearling
yearling
0'
0'
0'
9
9
850
960
280
9
925
820
865
860
340
320
345
270
280
300
70X220
90X240
87X245
10X270
75X210
72X220
80X230
31.8
37.5
35.5
38.8
35.7
32.7
34.7
165
175
175
170
150
162
170
25
y~ar1ing
19
18"
18
22
20
19
18
y~arling
yearling
yearling
9
9.
9
19
13
20
880
730
900
320
255
300
92X2S2
62X200
95X260
36.5
31.0
36.5
168
140
160
2J
2J
about 17 days
0'
380
115
30X 90
33.3
74
12
37,0
..
2J
1922
~
1%
"880
..
24
24
23
24
24
25
Note.-The table shown above gives the age of specimens as determined from body measurements and weights. In
this grouping) age of specimens also accords with size and general development of the skulls; but if age be determined
alone by the degree of emergence of the permanent upper premolar teeth, then nos. 31289 and 31280 would be classed as
two-year-olds, and no. 31272 as a yearling. "Adult"=thirty-two months old or older, 341bs. or more in weighti "twoyear-old"=about twenty to twenty-two months old, 28 to 33 lb.; "yearling"=about eight to ten months old, hs to 22
lb.
!
r
Average and extreme measurements-in millimeters of 20 adult skulls, both sexes
supposedly in equal numbers, are as follows: Basilar" length, 122.7 (II3.1-I30.5); zygo-
matic breadth, 96.7(88.8-105.6); interorbital width, 26,5 (23.3-29.6); mastoid b,,;adth,
67.7 (60.0-70.2); greatest length of nasals, 53.1 (48.0-57.7); width of nasals, 26.0
(23.5-28.2); vertical diameter of foramen magnum, 11'9 (10.0-13.6); transverse diameter of foramen magnum, 18.7 (18.3-21.2); ratio vertical to -transverse diameter of
foramen magnum, 62.0 (52.9-69.2) j alveolar length of upper molar series,,' 32.5
(29.7-34. 6).
TABLE SHOWING: NUMBER IN LITTER, SIZE, WElGliT AND DEVELOPMENT OF EMBRYOS
Castor canadensis subattratu!, FROM FEMALES CAPTURED
JANUARY 8 AND MAllCH 16, 1920 AND 1921
Date
Locality
:1
II
i
:.·.·.I
'.
;,1
'
I
Jan.
Jan.
Jan.
Mar.
Feb.
9, 1921
16, 1921
16,1921
2,1920
3,1921
5
2
5
2
Snelling, Merced Co..... .....
Netherlands Cut, Yolo Co.....
Snelling, Merced Co.... , . ...
Snelling, Merced Co."., ......
Mendota, Fresno Co... " . , ..
Netherlands Cut, Yolo Co... , ,
Snelling, Merced Co, ..... " . '
Mar.
Feb.
Mar,
Mar.
Feb,
Feb.
Mar.
5,1920
3,1921
15, 1920
11, 1920
20,1920
27,1921'
2,1920
4
2
1
Snelling, Merced Co.
.. " .. ...
Snelling, Merced Co., ..•. , " . ,
!i.e
~
::9
Netherlands Cut, Yolo Co....
Snelling, Merced Co. ...... ....
Netherlands Cut, Yolo Co..•..
.
~~
tOe
z"
I
5"
..1:•
~
.S
.S
~
~~
---
1.0
1.8
7.5'
13.0
22.2
30.0
35.8
80.0
88,0
1
1
110
130
140
150
200
227
240
. 302.2
315.0
Mar, 8,1920
3
285
354,0
~r,
4
325
485,0
11, 1920
3
2
.....
i.'
Avetage number in litter ••.•..••..... ,' ....
..
2.7
Development
~E
25
32
53
65
67
3
OF~
BETWEEN
,
Body naked, feet and tail not fully formed
Body naked, tail round
: Body naked. tail slightly flattened
'Body naked, "tail fl.at"tened
Body naked, tail flattened, hind feet pa.r~
,tially webbed
Body naked, "louse comb" distinct
Body naked. feet fully webbed
Body naked, tail scaly but hairless
Body naked, tail scaly but hairless
,
,
.......
Body well haired, vibrissae well developed
Ove'rhair on body 3 mm. long, hairs ap~
pearing on tail
; Overhair on body 7 mm. long; under£ur
scant
Overhair 10-13 mm. long, underfur thick,
8~lQ mm. long; body fully deVeloped;
ready for .birth.
I
..
Note.-Besldes the above mentIOned, two old females WIth enlarged utenwere taken February 27, 1920, and another
female, believed to be a two-year-old, in similar condition, March 5, 1920.
•
~or details, see tables (pp .. 632, 633). For discussion of variation in proportiqns of
tail, see text, p. 642.
Weigh~.-Average of 9 adult males, all taken in February and March, weighed
before skinning, 40 lb.; extremes, 34 lb. and 45 lb. Average of -14 adult females, of
same season, 42 lb.; extremes, 35 lb. and, 50 lb. for det·ails.see t~.ble (p. 632) and for
discussion see text (p. 637).
Type iocality.~Grayson (San Joaquin, River near its confluence with the Tuolumm;:
River), Stanislaus County, California (Taylor, r9<2, p. 167).
Distribution area.-The lower courses of the San Joaquin and Sacramento rivers and
the larger tributaries Df these, from Tulare Lake (formerly), Kings County, and Kings
River (formerly), near Sanger, Fresno County, north to Sacramento River and Butte
Creek, in vicinity of Marysville Buttes, and (formerly) to the McCloud and upper
Sacramento rivers; in Shasta County, and eVen (in 1883) to Sisson (now Shasta City),
-------------
Golden Beatier
in Siskiyou County.
(Se~ fig. 259.) Altitudinal range, from sea level up to
1000
ft.
Life zone, Lower Sonoran and, locally, Upper Sonoran.
Specimens examined.-Skulls, skeletons, skins or embryos, contained in the Museum
of Vertebrate Zoology: Am~rican River near Fair Oaks, Sacramento County, I; Cache
Slough, 10 mi. riorth of Rio Vista, Solano County,_~; San_ Joaquin River near confluence
with Tuolumne River (vicinity of Grayson and Westley), Stanislaus County, 10; Merced River, near Snelling and Hopeton; Merced County, 48; San Joaquin River near
Mendota, Fresno County, 15. Total, 75.
Besides the material mentioned above, there is contained in the Museum a skull
bearing the following data: Sespe River, Ventura County, California; :May 19, 1906;
r! adult. As suggested by Taylor (1916, pp. 446,
449), corroboration of this record is
needed before it can be credited as an indication of a point of occurrence of beavers in
a natural state.
The fact that beavers exist in California at the present writing (1931),
though in smaller numbers than formerly, is unknown to most people living in this State. Doubtless this is because the beaver, despite its relatively
large size, is aquatic in habits, mainly nocturnal, and, in California, l~aves
comparatively little sign of its activities. The more northern and eastern
races are famous for the construction of houses and dams, and for the ultimate formation of meadows resulting from the dainming of streams. Our
beaver, a bank dweller, in adapting itself to life in and along sluggish lowland streams that are subject to great seasonal fluctuation in volume, seems
to have lost in some measure its constructive proclivities.
The beaver, the largest rodent in California (see fig. 260), equals in size
a setter dog and averages 40 pounds or more in weight. Its body is stout,
squat, and short-legged; the head joins the body directly, without any constriction at the neck; the eyes and ears are small; the hind feet are broad
and webbed; and, most curious specialization of all, the tail is horizontally
flattened, paddle-shaped, and scaly, not hairy. The coat, of dense underfur
and long overhair, is rich both in color and in texture; hence the pelt has
been in great demand from the beginning of Western history.
As already intimated, everywhere it occurs, or did occur, in North America; the beaver is variable geographically. Some twelve races or subspecies
have been distinguished under separate \lames, three of which are credited
to this State, Of these three, the golden beaver occupies the great ~entral
valley of California, which comprises the lower drainages of the San Joaquin and Sacramento rivers. Curiously, in our State, at the present time,
no beavers (save by introduction) live in the high mountains, although
here mountain streams and bordering growths of deciduous trees abound,
like those which constitute ideal abodes of beaver in Colorado and elsewhere. Indeed, the golden beaver, so far as we know, never existed at an
altitude higher than about 1000 feet, and all the existing colonies live below
the 300-foot level. The most persistent are near sea level.
Of the other two subspecies of beaver inhabiting California, the Shasta
beaver is limited to the drainage basins of the Klamath and Pit rivers, in
1.. Castor' c. shastensis
2.. Ca.stor c. 5uba.uratu5
.3.... Castor c. frondator
DISTRIBUTION MAP
MUSEUM OF VERTEI3RATE ZOOLOGY
UN'VE"S'TY OF eAUFO .... ' ...
Fig. -259. Assumed former general range of beavers in California outlined; Spots indicate stated
places of occurrence, chiefly prior to 1924. Ranges of the three races shown: I, Shasta beaver;
2, golden beaver; 3, Sonora beaver. Three record stations outside these ranges are shown with a
question mark, since in each of them uncertainty exists with respect to the occurrence or to the
raCe represented.
the extreme northern part of the State, the Sonora bea:ver to the lower Colorado River and its distributaries. With regard to California, of the three
races named, the golden beaver is, and probably always was within history,
the most important to human interests. (See map, fig. 259.)
-
,
Golden Beaver
There is really slight difference between the golden beaver and· other
races in the gross structural characters. A person familiar with eastern or
northern beavers must not expect to find any conspicuous peculiarities about
the Californian animals. The color of the coat, more brighdy golden brown
than in more northern races, is distinctive only for skins subjected to close
comparison; the two chief differentiating features of the skull, broadness
of nasals and flattened oudine of the foramen magnum, are found in the
great majority of examples but require careful study of series of specimens
for their appreciation. Because of the relative slightness and variability of
these diagnostic characters, we prefer to consider the form subauratus as
a subspecies of the North American species Castor canadensis, rather than
as a full species by itself. The closeness of the relationships of all the North
American beavers indicates common ancestry at no very remote period of
time, phylogenetically and geologically speaking, or else an exceedingly
slow and incomplete process of differentiation.
We have heard it said that certain sections of the country produce extra- .
large beavers. The golden beaver has been referred to.as of maximum size
am~ng the North American races. The fact is, according to the evidence
now accumulated, that all beavers continue to grow at a greater or lesser
rate throughout their lifetimes; So that old and hence large individuals are
likely to be found anywhere.
The largest California beaver of which we have authentic record was
trapped by Edgar Standiford in the fall of 1895 on the Merced River about
three miles above Snelling. Mr. Standiford testifies that this animal, a fat
male, alive in the trap when found (not drowned and hence not full of
water), weighed 82 pounds on standard platform scales which had been
tested and found accurate. The weighi1;lg of this Goliath beaver was watched
by several citizens of Snelling, and in March, 1920, two of these men verbally corroborated Standiford's record.
Of 104 beavers caught in the vicinity of Snelling between January 15 and
March 15, 1920, 40, including the largest, were weighed and the heaviest
was found to be a female which tipped the scales at 62 pounds. Early in
January, 1920, 2 large beavers, collected for the Oakland Public Museum,
were trapped near Mendota, Fresno County. These 2 animals weighed 53
and 58 pounds, respectively, and were said to be the largest of the 35 beavers
trapped in that locality in January and February, 1920. Of a mated pair of
middle-aged adult beavers, taken March 5 and 6, 1920, in an isolated slough
near Hopeton, Merced County, the female, containing two small embryos,
weighed 42 liz pounds, and the male 43 pounds.
All the data available to us tend to show that the two sexes are of practically the same dimensions and weight. See tables on pages 632 and 633.
Judging from a series of seventeen embryos now at hand, a beaver at
Fig. 260. Photograph showing relative sizes of a beaver and an average
man. In this golden beaver, just trapped, as held up, alive, the distance from
the end of the nose to the tip of the tail was 44 Yz inches. The animal weighed
37 pounds. Mus. Vert. Zoo1., no. 3238.
birth weighs close to I pound. We have record of a week-old youngster
which weighed 25 ounces. The average weight of seven "yearling" beavers
was 20 pounds; five "two-year-olds" averaged 30 pounds. From weights
obtained at the close of the rutting season, at which time the adult males
,
Golden Beaver
were not so fat as the pregnant females, the average weight of nine adult
males was 40 pounds, whereas fourteen adult females averaged 42 pounds.
(See table, p. 632.) However, even among the yearlings and two-year-olds,
the females were found to be heavier than the males of corresponding age,
so that with regard to weight the females equal or surpass the males.
Because of its great size and special structure, we may infer that the beaver's tail serves important uses. And, indeed, the functions of this member
proy.~ to be important and varied. In the water the beaver uses its tail as a
Fig. 261. Photograph of a golden beaver. "galloping," taken on February 26,1920, near Snelling, Merced County. 'The beaver is not a speedy animal on land; a man can easily outrun one.
However, it is not easy to keep pace with a beaver and operate a camera at the same time. The
humped posture of the beaver here shown is' characteristic.. Mus. Vert. Zool., no. 3239.
rudder by means of which to steer the body upward, downward, or sideways; it is also used to prevent the animal from swimming in circles when
towing branches of trees. In directing the body in a laterally changing
course, the tail is tilted sidewise so that when viewed in cross section, instead
of appearing horizontal as it does when the animal is swimming straight
ahead, it would appear more nearly vertical in position. Beavers customarily communicate a signal of alarm by violently splashing or "popping"
the water with their broad tails, applied flatways. Furthermore, on land the
tail serves regularly as a prop or "third leg" when the beaver sits up on its
hind feet to reconnoiter or when at work cutting trees. And finally, when
the beaver runs, it gallops and thuds the ground forcefully with its tail.
This then serves to support the body momentarily at a certain point in the
sequence of locomotory movements and to help propel it forward. (See
fig. 261.)
.
The tail is also said to be used, in times of stress, as a scull to assist in
propelling the body rapidly through the water; but no observation of ours
confirms this idea.
The tail of an adUlt beaver is shaped like the blade of a canoe paddle and
is about 16 inches in length (see fig. 262). Some writers believe that certain
North American Indians, in designing canoe paddles, took the tail of the
beaver as a pattern. The large muscular basal part is covered with coarse
Fig. 262. A beaver's tail is flattened horizontally, has a scaly surface, and is shapeq something
like the blade of a canoe paddle. In an average adult the scaly part of the tail is 12 inches long,
5 inches wide, and
I
inch thick in the center. The tail is used as a rudder; also to "whack" the
water, thereby producing a warning sound. From freshly trapped male, no. 16383. Mus. Vert.
Zool., taken in San Joaquin River, near Grayson, Stanislaus County, December 2, 1911.
overhair and soft dense underfur like that on the rest of the body, but the
terminal two-thirds is Battened and scaly. In a full-grown golden beaver
this scaly part is roughly 12 inches long, 5 inches wide, and I inch thick in
the center, tapering to thin margins. The outside covering of this part consists of a layer of tough blackish brown skin which appears to be covered
with scales like those on a fish. Actually this is only a superficial resemblance, for the epidermis composing the beaver's scales is continuous and
merely folded, not imbricate like the shingles on a roof (see fig. 263)' The
largest "scales" are in the center of the tail above and below the vertebrae;
their width is greater than their length. Their usual form is ·shown in figure 264, but there is much variation in form and size. A selected average
Golden Beaver
-
"
scale for ,an adult is 5 millimeters wide, 3 millimeters long, and supports 7
hairs. The scales are arranged more or less regularly in rows that extend
diagonally across the tail. The number of these scale rows across the middle
of the tail averages twenty for both young and adults; that is to say, a young
beaver is born with its full quota of scale rows.
Rooted beneath each scale and protruding backward and slightly upward
through its posterior margin, seemingly from between the scales, may be
seen a number of stiff brownish hairs, usually as long as the scale itself (see
fig. 263). These are best seen in embryos or in young individuals, since in
later life they are kept worn down through
abrasion by external objects. In adults they
are most easily observed along the edges of
the tail, where they are subjected to a miniFig, 263. Longitudinal section of the
mum of wear. In a beaver embryo no millidermis of a beaver's tail, x 2; to right
is anteriorward. A beaver's tail appears meters long, the scaly part of the tail is 26
to be covered with scales, but this is be~
millimeters long and IO millimeters wide;
cause the tough black epidermis or
outer skin, a, is folded. The so-called
there are no hairs on the body or tail, alscales are not separate like the scales on
though the scaly pattern of the tail is plainly
a fish. The short coarse "scale hairs," b,
are rooted in the underlying fatty tisvisible. In a larger embryo, 285 millimeters
sue of the tail, c, and protrude backwaI:d through the rear margins of the
in length, the scaly part of the tail measures
"scales." Drawing diagrammatic.
60 by 25 millimeters. There is a thin gelatinous coating over this part through which appear numerous scale hairs
2 millimeters long. A third embryo, V.5 millimeters long, fully developed
and ready for birth, has a well-scaled tail measuring 70 by 27 millimeters.
On a typical scale of the tail of this embryo are as many as three stiff brown
hairs from 5 to 6 millimeters long. A juvenile golden beaver (no. 32I87)
about seventeen days old, has a scaly tail 30 by 90 millimeters, over which
are numerous, long, and unbroken scale hairs.
The proportional dimensions of the scaly part of the tail for many years
have been used by mammalogists as characters in differentiating between
the various geographical races of the American beaver (Castor canadensis).
Perlpps this is because the unusual structure and function of this highly
specialized member has attracted the interest of naturalist and trapper alike.
From a critic's point of view, however, the diagnostic value of any character must be inversely proportional to its individual variability. Let us, therefore, inquire into the individual variation in size, proportions, and shape of
beavers' tails.
Between February 24 and March I5, I920, one of us (D.) was able to
obtain measurements, in-the-flesh, of 36 specimens of Castor canadensis
subauratus, all from one locality. Hitherto, lacking field measurements, the
dimensions of the scaly parts of beavers' tails have been obtained only from
dried museum specimens, and this, admittedly, has not been satisfactory.
The tails of the 36 specimens, with but two exceptions, were all measured
by one person. All the tails were "in-the-flesh," not dried, when measured.
By eliminating all the yearling and two-year-old beavers, a series of 24 adults
is left for consideration. In these adults, the scaly part of the tail is more slender, on the average, in females than in males, a difference not great enough,
however, to use as a sole criterion for determining sex. In I5 adult females
the breadth of the scaly part of the tail averages 40.4 per cent of its length
Fig. 264_ Part of golden beaver's tail (same as in fig. 262) near middle, X2, to show
form and size of the "scales." These are largest near, the axis of the tail, at right, and·
decrease in size toward the margin, at left. From Mus. Vert. ZooL, no. 656.
(extremes, 31.5 per cent and 46.8 per cent); in 9 adult males the average
is 42.8 percent (range, 37.9 per cent to 48.3 per cent). The largest tail of
the' series, measuring I42 by 3IO millimeters, belongs to a female. The range
shown is greater among females (I5.3 per cent) than among males (IO.4
per cent).
Our examination of the 24 adult beavers shows that shape and size of the
scaly part of the tail are exceedingly variable (see table, p. 632). The outline of the tail otherwise than as shown by ratio of width to length is also
notably variable. For the two sexes together, the total range of variation in
the proportions of the tail is I6.8 per cent. Among all the seven described
races of beaver inhabiting western North America from Alaska to Mexico,
as listed by Taylor (I9I6), the extreme total variation of this character is
. only I6.5 per cent. Extremely broad-tailed individuals (usually old males)
occur in distinct and widely separated forms. The type of Castor c. phaeus,
from Admiralty Island, Alaska, has a tail the width of which is 54-I per
cent of its length (Taylor, op. cit., p. 438). Mearns (I907, p. 353) indicates
the maximum for Castor c. frondator from Arizona and Sonora as 54.3
per cent.
From the figures given above, it is believed that the importance. of the
size, shape, and proportions of beavers' tails has been greatly overestimated.
,
,
Golden Beaver
This member is subject to such great variation (16.8 per cent in a series of
24 adults of one subspecies taken from one locality at one time) and the
overlap between subspecies is so great that its value as a diagnostic character, even when averaged, seems doubtful.
The following experience (D.) is given as typical of what an earnest
observer may expect to see of the behavior of beavers under favorable conditions. A beaver pond near Hopeton, Merced County, was visited at7
o'clock in the evening of April 15, 1921. By this hour several beavers were
out, swimming back and forth across the pond. Only the small dark head
of each animal was to be seen above the water, and this might readily have
passed for a floating chunk of wood had it not been for the V-shaped silvery
ripple that broke thesmooth surface of the dark water. One of the animals
swam swiftly past within ten feet of the observer, who had hidden himself
behind a bush. Even at that short distance, he heard no sound of splashing
or rippling made by the swimmer.
The pond was visited again at daybreak the next morning. At 5 A.M.,
when daylight was growing fast, a beaver was sighted swimming boldly
upstream below the dam, toward a well-worn beaver trail which led over
the middle of the dam from the stream to the pond. The beaver, a huge
fellow and apparently the patriarch of the pond, crawled quietly out of the
stream and clambered up onto the dam, the water dripping from the long
glistening overhairs. 1ltanding erect on the crest of the dam, braced by his
tail and two large broadly webbed hind feet, helooked like a gigantic prairie
dog. After standing motionless, listening intently for several seconds, he resumed his customary position on all four feet, waddled down the upstream
declivity of the dam 'and disappeared under the water without making the
least splash or noise. In a few seconds he appeared about eight feet offshore
and then turned around and swam back along the face of the dam, stopping
here and there to inspect some leaks near the crest. T~ice he scooped up
mud between his forepaws and his chin aJid plastered it over the breaks.
Mter making sure, as it seemed to the observer, that the entire dam was in
good order, he turned and, passing close to the watcher, who remained hidden, swam away upstream.
When the beaver was sixty yards distant, the observer suddenly turned
his head to get a better view; apparently the beaver had been looking back
over his shoulder, for he lifted his broad tail high in the air, brought it
down on the surface of the water with a "pop," signifying danger, and disappeared beneath the water in the direction of his den. This was a burrow
in a high bank under a valley oak (see fig. 265).
Fig. 265. Entrance to burrow of golden beaver in a slough bank under
an oak. This entrance was 12 inches wide and 14 inches high. Ordinarily such
an entrance is entirely covered by water, but the one shown here is exposed as
the result of a break in the dam below. Photographed on August 7. 1920, one
mile west of Hopeton, Merced County. Mus. Vert. Zool., no. 3284.
-.
Golden Beaver
At 5:<>7 another beaver, of middle size, came up the channel, crossed the
dam, slid into the water noiselessly, dived 10 feet, and then swam, top of
head above water, straight for its home. At 5: I5, a third individual, also
medium sized, came up the slide, stood motionless upright on the crest of
the dam and then, entering the water as the others had done, swam thence
straight on home. At 5: 20, a fourth, large beaver forged up the channel to
the dam, and stood on it for a moment until it caught sight of the observer
partly hidden 75 feet away. Bolting into the water, "popping" its tail as it
.dived, it went off under water toward its home burrow for a distance of
~oo feet before reap?earing on the surface. At 5: 25 the fifth and last beaver went over the dam without taking alarm, dived quietly, soon came up
again, and swam on toward its home.
·During these performances, because the air was moving gently from the
direction of the dam toward him, the observer kept as silent and motionless as possible behind the partial screen of a tobacco bush, giving the beavers little chance to detect his presence except by sight. The fact that two
of the five animals did catch sight of him testifies to a certain sharpness of
vision despite the relatively small size of the eyes. It was further apparent
that beavers see much better by daylight than they do in the dark.
An adult male beaver, caught at Snelling, Merced County, ·was held by
one hind toe in a No.2 Victor trap. The toe was little injured and the beaver was released in an open pasture and later turned loose in a pond to give
the observer opportunity for close watching of its manner of walking, running, swimming, and diving. This the observer (D.) proceeded to do with
notebook in hand. On land the beaver usually maintained a humped-up
position in which the back was strongly arched (see fig. 26I). When standing, more or less erect, the hind feet were planted well apart and forward
under the middle of the body. The entire hind foot from tip of toes to heel
was placed firmly on the ground and the fully webbed toes were widely
spread (see fig. 266). Most of the time, the beaver's hind legs and feet supported approximately three-fourths of the load.
When the beaver attempted to run, it ~ssumed a clumsy lope or gallop,
first raising the head and neck and quickly thrusting both front feet forward in unison. The moment the weight of the forward part of the animal came to bear upon the forefeet, the back was arched and the hind feet
quickly slid forward under the middle of the body. At the same moment,
or an instant before, the tail, which had been carried slightly recurved along
the ground, was pressed forcibly against it (see fig. 26I). In this way the tail
was used as a lever to help support the heavy posterior part of the body while
l
the hind feet were being moved forward. The beaver's body is thus provided
with three places of support, the front legs together, the hind legs, and the
tail. A galloping beaver is like a large measuring worm In action, though
much more rapid in its movement.
When walking, the beaver ambles along somewhat like a slowly moving
rat, swaying from side to side as a result of the diagonal instead of parallel
Fig. 266. When a beaver sits up on its hind legs, the tail may extend forward underneath, as in this photograph, taken February 26, I920, near Snelling, Merce.d County,
or straight out behind. Either way, it se~ves as a balance or prop. Mus. Vert. -Zoo1.,
no. 3240. This animal had been injured slightly.
use of thefore and hind legs; that is, the legs of each pair operate alternately;
The broad scaly tail then simply drags behind the body, without giving any
apparent service.
Good, clear-cut beaver tracks are hard to find. These animals rarely leave
tracks, since they spend most of their time either in the water or in their
dens. Moreover, when they do go ashore their routes, which are often well
defined, frequently pass over sand or mud bars where the tracks are usually
brushed out when the body and tail drag over them. Those that are to be
found are naturally the ones left by the hind feet rather than by the forefeet.
Golden Beaver
If the bank of the river or pond is abrupt, claw marks and prints of both
fore and hind feet will sometimes be found at the "slides" where the beavers
"haul out." (See fig. 267.) Claw marks and tracks of the forefeet are usually
Fig. 267. A well~traveled beaver trail. This path was 9 inches wide, 4 inches.
deep and 36 feet long, It'led from the main slough over the top of a reclamation
levee to a tule pond that had been cut off by the dike. Photographed on January 4,
19:'U, 01) Cache Slough, near Dozier, Solano County. Mus. Vert. Zoo!., no. 3458.
in evidence where beavers rak~ up mud and decayed vegetation to form
their musk mounds. (See fig. 268.) On the night of March 2,1920, a beaver
crossing a mud bank near Snelling made ~ series of tracks in the soft mud.
All except one made by a right hind foot were smeared over or obliterated
by the action of the animal's body and tail in dragging over them. The one
track that escaped obliteration measured, in soft mud, 6Yz inches in length
and 4% inches in width. The beaver was traveling uphill (see fig. 269);
hence, the track, found on a sloping bank, was imperfect because not quite
all the heel pad touched the mud. Later the beaver was caught and proved
to be a large female that measured 43Yz inches from nose to tip of tail and
tipped the scales at 45 pounds.
Fig. 268. Tracks and claw marks made by a beaver's front feet may be seen in the foreground.
These were macie-when the animal scratched up sand to ~orm a scent mound. After depositing
scent the beaver had circled about, dragging the broad tail over the mound. Photographed on
March 7. 1920, near Snelling, Merced County. Mus. Vert. Z061., no. 3276.
The beaver is as graceful in the water as it is awkward on land. It is surprising that such a large animal can move through the water with so little
commotion. Indeed, it causes as little disturbance as a fish a foot long. When
the beaver is swimming under normal conditions, more than nine-tenths
of the body is submerged. Once, by actual measurement, 94 per cent of the
body was under water (see fig. 270). Ordinarily, the upper part of the animal's head is all that is visible, though sometimes the rump or the top of the
back will show also. When the animal's suspicions are aroused, only an inch
or two of the top of the head, including the eyes, ears, and nose, is exposed.
These most important organs of sight, smell, and hearing are thus kept
above the water where they will function up to the last moment possible.
The animal under observation did not use the front legs in swimming.
Goldetz Beaver
,
They were relaxed and "streamed" back against the breast, almost touching each other. The body was propelled forward by the broadly webbed
hind feet, operated alternately. In turning, the broad flat tail that ordinarily
trailed along just under the surface of the water, almost in line with the top
of the back, was tilted edgewise at an angle of about thirty degrees.
This beaver often tried to elude observation by submerging quietly, leav-
Fig. 269. Track, in soft mud, of hind foot of a beaver. This track, of an adult individual, was
6 Yz inches long and 4 inches wide. The imprint left by the tail is to be seen just in front of the
track. Photographed on March 7, I920, near Snelling, Merced County. Mus. Vert. Zool., no. 3244.
*
ing scarcely a ripple as it disappeared. In these movements the tail was used
as a horizontal rudder to steer the body downward. On the bottom of the
pool in 3Yz feet of water the beaver would lie stretched out motionless, and
when wrapped about with delicate wreaths of green algae, accumulated incidentally in diving, it resembled to a surprising degree an old waterlogged
chunk of wood. As far as could be seen, the beaver did not hold onto the
bottom. By actual count of two observers whci stood by, watch in hand, this
beaver remained under the water, on three successive dives, 2y.f, 2y.f, and
4Yz minutes, respectively. The extremes of many observations on this beaver
were 2 and 5 minutes. Another beaver, alive in a trap, remained under water
for I I minutes. The first-cited beaver, upon coming to the surface after diving, breathed rapidly and deeply at a rate of 6 respirations iIi IO seconds. At
such times the nostrils were distended, nearly circular in outline and almost
or quite as large as the eye (see fig. 27I). The beaver spent from I to 2 minutes above water between dives. In diving, upon alarm, the beaver's back
is arched and the tail raised clear and then brought down. horizontally onto
65 0
the surface with a resounding splash just before the animal disappears under the water. Thus it gives warning to its neighbors.
When injured or irritated the beaver under observation at Snelling expressed its displeasure by blowing through its nose, thereby making a hissing sound similar to that made by an angry goose. On. two occasions, at
night, calls undoubtedly given by beavers were heard from different directions. This call note of one member of the family to another, possibly mother
to young, is similar to the soft whimpering of a young puppy .
. On June I, 1918, in the middle of the forenoon, at Belldsland, near· Mend?ta, Fresno County, one of us (D.) investigated a large beaver house. As
i
I[I
Fig. 270. Diagrams of a beaver swimming. Left, side view; right, rear view.Whe.Q. swimming,
under normal circumstances, nine-tenths of the beaver's body is under water. The forelegs are not
used in swimming but are relaxed and "stream" back against the breast.
the observer crept noiselessly up onto the house, he heard a subdued gutturar or rumbling noise issuing from within the lodge every five to ten seconds. By lying down on the house and listening he was able to hear it more
distinctly. It might have been caused by beavers snoring intermittently; or,
more likely, as Mr. Ayres puts it, he could "hear their guts working." After
listening to this strange rhythmic sound on· a number of occasions, often
with ears pressed against the wall of the lodge within three or four feet of
the beaver, the observer concluded that Ayres was correct and that the sound
was the "rumbling" of the intestines. The animals at this time were feeding
almost entirely on the succulent underwater stalks of the cattail and on the
roots of a species of water lily, and it is possible that such material produces
much intestinal gas during the process of digestion.
The "plopping" of the tail has already been described, and may be considered a manifestation of "voice" because it is a means of audible communication. This device employed by a beaver may serve to transmit vibrations
through water to .individuals submerged at some distance away.
Golden Beatler
. The function of the ordinary beaver dam in California is to create a permanent pond (see fig. 272) which will: (I) afford a ready refuge from
enemies, (2) protect the beavers' house from invasion by keeping the entrance under water, (3) serve as a highway of travel,and (4) afford a route
for easy transportation of saplings and trees cut for food or for construction
purposes. In the northern part of North America two other functions of
the beaver pond operate importantly. In these cold regions, by means of
the special construction of the dam, the home pond is made deep enough
Fig. 271. Ordinarily a beaver when swimming keeps just his "conning tower" out of water.
His eyes, ear~, and nose' are thus able to function, while the rest of the body is under water, safe
and out of sight from animals in the air or on the surface. Photographed on February 26, I920,
near Snelling, Merced County. Mus. Vert. Zoo1., no. 3.243.
to keep it from freezing solid in winter; and sufficient depth of water is
provided for the storage of food materials beneath the ice for winter use.
Neither of these two uses of ponds applies to our golden beaver, which lives
in a warm region at low altitude.
After the main dam has been built, the beavers sometimes construct a
series of smaller dams across the stream below it. These smaller "supporting" dams, often extending nearly straight across the channel, back up the
water, and partly serve to reenforce the main dam. The principal function
of such a series of supporting dams is, however, to provide a chain of connected ponds, which together constitute a through highway for the beavers,
making accessible a greater area. As compared with the northern races, the
golden beaver builds relatively few dams. Those that are constructed in
California are low and of comparatively small size, since it is not necessary
-
Fig. 272. By damming streams at strategic points, beavers are able to form
ponds. These;': ponds afford protection for the beavers and their nurseries; they
make possible the growth of aquatic vegetation, thus increasing the beavers' food
supply; and they provide avenues of transportation to and from willow and other
trees which the beavers cut down for food. Photographed on November I, 1921,
near Hopeton, Merced County. Mus. Vert. Zool., no. 3790.
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,
Golden Beaver
to make the pond deep enough to prevent its freezing solid during the winter. The presence of permanent deep-water streams in our lowlands tends
also to make dams unnecessary. Beavers rarely attempt to dam the main
river channels, but devote their energies to building dams across the more
quiet old river channels, sloughs, and irrigation ditches. Their repeated
damming of irrigation canals in the dry season, when water is scarce and
essential to growing crops, has caused trouble and has given the beaver a
bad name in several localities in the State.
Measurements have proved that beaver dams are not so high as they appear to be. To our knowledge, the average beaver dam in California is 30
inches high and 50 feet long. A few dams are of small size, apparently because of a real scarcity of building materials; but this is not the usual cause.
The chief factors that determine the ultimate height of a dam seem to us
to be height of adjacent ground and kind as well. as amount of available
building material. The largest dam out of more than thirty, measured 4 feet
in height and ISO feet in length (fig. 273).
The following will give a good idea of the location and distribution of
beaver dams in California, as existing in I92I. On the north side of the
Merced River near Snelling an old river channel leads off from the present
main stream and runs through a .series of pastures and fields of corn and
alfalfa, uniting with it again six miles downstream. All new growth of
willow and cottonwood is nipped off and kept down by the cattle, sheep,
and hogs which crowd the pastures, so that the timber bordering the stream
now consists almost entirely of large trees, chiefly willows and valley oaks.
Although building material is rather scarce, seven beaver dams and three
beaver houses or dens were found in one mile along ithis stream. The beaver had long been protected in this territory and had become so accustomed
to the presence of pigs, horses, cattle, and even man, that they built their
largest dam beside the county road. Tracks made by pigs and beavers were
found intermingled in cow trails within a short distance of human habitations (see fig. 274).
At another place, just across the main river channel from the town of
Snelling, there is a tract of some 400 acres of river bottom where recent
dredging for gold has removed the soil and left only huge heaps of smooth
water-worn cobblestones. This rock-pile area, of very uneven contour, has
numerous depressions, many of which are deep enough to fill up with seepage water and form small ponds. When this bottom land was dredged,
three straight channels were left through the middle to car;y off the floodwaters which, when the river was up, threatened to engnlf the town. The
beavers soon built a series of dams across each of these channels and converted them into rows of long, narrow, beaver ponds. Each dam consisted
of one uninterrupted span across the channel, which varied in width from
30 to 100 feet. Some of them were nearly straight, bowing but slightly downstream. Others, where the current was swift, were decidedly crescent-shaped
with the curve downstream (see fig. 275). None of the dams was arched
against the stream. The dams stopped the current, and the silt held in suspension in the muddy waters was deposited over the bottom and sides of
the ponds. Many willow seeds, carried along and deposited with the silt
I
I
:1
,I
,I
I
I:,
i[
Fig. 273. Beaver dams in California are of relatively small 5iz~. The dam here 'Shown was
4 feet high and ISO feet long, the-largest that we have found within this State-, Photographed
on December 23. 1919. near Hopeton, Merced County. Mus. Vert. Zoo1., no. 3253.
around the margins of the ponds, had sprouted and flourished. The resulting thrifty growth of smooth-barked, slender willows furnished the beavers with an excellent and abundant food supply. The high banks about
the ponds afforded burrow sites.
Beavers respond quickly to conditions favorable to dam building. This
was strikingly illustrated by the behavior of a family of beavers near Hopeton. In the winter of '92(}-2' a large dead cottonwood tree, standing in the
middle of a grain field, was blown down in a storm. The farmer owning
the field chopped up the tree for firewood and dumped the dead brush in
several good-sized heaps at the edge of the field, which was sixty to seventy
feet distant from the stream. In a few nights the beavers discovered this
"windfall" of ready-cut and delivered building material and proceeded at
once to start 'building a dam across the stream. Instead of constructing it
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..- - , - - " - - - - - - - - - - - - -
Golden Beaver
close to the brush piles;" they carried the brush to the creek and then swam
upstream, towing the brush some hundred yards, where they ultimately
built a dam 50 feet in length and 2 feet in height across the stream just above
a shoal.
The naturalist, seeking an explanation for this unusual behavior, waded
up and down the stream, noting the width and depth of the channel as well
as the character of the bottom, and conCluded that for the beavers' welfare
the dam might just as well or better have been built opposite the brush pile
or slightly downstream and thus have saved the extra work of towing the
brush a huudred yards upstream against the current.
Fig. 274. Beaver "lodge" in a pasture. In the region ahout Hopeton, Merced County, beavers
were (when this photograph was taken, in 1921) valued highly by the fanners because dams
built by these rodents seryed to maintain the water level during the dry summer months. The
natural and inexpensive subirrigation thus furnished, through maintenance of a high water~
table, kept the pastures green and made possible good summer crops on the sandy soil of the
near-by cultivated fields: Mus. Vert. Zoo1 ... no. 36°5.
As far as could be discovered (D.), there was no pressing need for the
building of a dam by beavers at this particular time and place. Ordinarily,
beavers do most of their dam building in the late summer or fall when the
water in the streams is low. In this instance the work was done between
April I and April 20, when the water was high, and hence with extraordic nary difficulty. It is believed by us that these beavers responded to the stimulus 9f the brush pile for much the same reason that a boy with a sharp-bladed.
pocket knife, finding a piece of soft wood, picks it up and starts whittlingnot because he needs to carve out anything, but simply because the stimulus
-
and opportunity are afforded. Why the beavers chose this particular site,
so far above the source of materials, is not explainable.
Dams built bybeavers differ in form, composition, and constiuction from
those built by man. Ordinarily, a man-made dam on the Merced River is
Fig. 275. 1£ the current is swift when beavers build a dam, the structure will
usually bow noticeably downstrea~. Photographed on February 28, 1920, near
Snelling, Merced County. Mus. Vert. Z081., no. 3251.
composed of concrete and built on a foundation of solid rock across the main
channel at a point where the stream is narrow. Beaver-made dams near by
are built of saplings, sticks, mud, roots, and rocks (see fig. 276), usually
across broad lateral channels, and have for a foundation firm earth or clay.
Thus it will be seen that the two types of dams require different sorts of
location and that a site which might be best from man's point of view would
not be at all.adapted to the beaver's needs, or to its special building powers.
-
Golden Beaver
Much has been said ~nd written about the wonderful sagacity shown by
the beaver in building dams"arched against" the stream. An examination
of a number of beaver dams in process of construction throws a great deal
of light on this question and leads us to the conclusion that the facts about
beaver dam building have been subject to much misinterpretation. We do
!lot believe that, as at one time supposed, the curve of the dam is thought out
or planned by the beaver. Indeed, the form of the dam and its curve up or
down stream appearto be largely a matter of circumstance, as will presently
be explained. Beavers do seem to exercise some choice in deciding where
their dams shall be built; but our observations have shown that even in this
regard they sometimes make mistakes. Having fixed. upon a suitable site
where the stream bed is firm and will not readily wash out, and where a
suitable supply of building material is available, the beaver family begins to
build its dam.
If a beaver dam is to be successful, the Initial building unit must be of
small size, and elastic, so that strains and pressures are widely distributed
and easily absorbed. Construction of a dam often begins at the same time
on both banks of a channel and progresses toward the center of the stream,
where the two wings finally meet. The beavers start the structure by laying
a foundation of 'saplings and brush in the water at the margin of the stream.
Foundation saplings are usually laid with their trunks parallel with the current and are anchored and held in position by mud and rocks which the
beavers pile on them. When thus weighted down, the branches are forced
against the bottom of the stream and the dam is to some degree anchored.
As the building progresses, the channel becomes more and more constricted.
Be~ause the water has to pass through an opening that grows narrower each
night, the velocity of the current at this point increases rapidly; and the
material toward the center of the dam is gradually forced downstream before the increased current (see fig. 277). The amount or degree of the curve
of the dam downstream is determined in large part by the velocity of the
current at the time it is built. If the stream is swift, the curve will be great;
if the stream is small or the current sluggish, it will be slight.
Occasionally, a beaver dam will be found arched against the stream; but
we do not believe that this should be credited to any special "intelligence"
of the beaver. Beavers are quick to take advantage of any natural obstruction lying in the channel of a stream. Obstructions such as rocks, stumps,
trees, clumps of bushes, grassy islets, or gravel bars, anyone of which will
afford a natural support, are often used as starting points in building dams.
As the dam, which may then be built from near the center of the stream
toward both banks, increases in length, the channels on either side become
gradually narrower and the current swifter, with the result that the ends
of the dam curve more and more downstream. In new dams which are
arched against the stream, the starting point or obstruction can often be
found near the middle. As time goes on, the stump may rot or the tree die
and decay, and then someone comes along and finds a beaver dam which
he supposes was "knowingly" arched against the stream by the beavers.
Where the water is shallow or where there is little or no current, it is built
Fig. 276. Beaver dams are built of peeled sticks, saplings, and roots. Sometimes the mass is
weighted down with rocks, as here shown. The smaller openings in a dam afe often chinked
with mud and matted roots. Dead leaves and floating -debris drifting downstream from above
the dam automatically tend to keep it watertight. Photographed on February :28, 1920, neaf
Snelling, Merced County. Mus.Vert. Z061., no. 3254.
i
,
from one natural anchorage or support to the next. The resulting dam under
such circumstances may, when complete, form a letter S and be a hundred
or more yards in length. It has been our observation in California that the
majority bow downstream. Many bow both up and down stream in different parts of their courses, whereas orily a very few are arched symmetrically
against the current.
As the dam increases in height, the beavers often obtain mud and roots
by digging a trench across the bottom of the stream parallel to and just
above the dam. Material from this source is worked into and solidifies the
upper face of the dam. Silt, waterlogged sticks, and decaying leaves also
float downstream and lodge naturally against the upper face of the dam,
:~
,
;;1
-
- - - - - - - - - - - - - - - - . - - - - - - - - - - - - - - - - - - - - - - - - " " _ .. _ - - - - - - - _ .. _ - - - - -
Golden Beaver
where they tend to chink up the smaller openings and thus help to keep the
water level near the crest of the dam. Furthermore, fallen leaves are most
abundant in the streams in the late summer and fall months, at which time
the water in the stream is lowest and the beavers' need of conserving water
in the ponds greatest. Because many of the larger sticks which float downstream are deadwood, they are light and hence float on the surface. They
balance on the crest of the dam, tip over, and lodge in a slanting position
on the downstream face of the dam, where they break the force of the water
which at flood time pours over the top. The lower ends of the sticks often
become bedded in the mud and then become incorporated in the dam,
where they serve as supports or braces (see fig. 278). It is thus seen that some
of the features of the dams are purely fortuitous so tar as the beavers' own
efforts are concerned.
Beaver dams need constant repairing. They continually fall into decay
because the constituent material is for the most part soft, short-lived wood.
Beavers are ever on the watch for breaks in the dam and these are promptly
repaired, at any cost.
In some dams the crest is skillfully carried along on the same level so that
a little water trickles over all along. When the flow of a stream is small, all
the escaping water may seep through the dam. In others well-defined spillways are left for the water. Such spillways are often situated in clumps of
growing willows, with the result that the flowing water does not wash out
a section of the dam.
The wood and leaves which form a large part of the dam decay in a short
time and form a ready-made, well-watered bed for willow and cottonwood
seed. Bushes and even trees soon spring up along the crest of the dam and
their roots penetrate and strengthen the structure, which in time becomes
a solid earthen embankment bound together with interlacing fibrous rootlets (see fig. 279). One such dam, on the ranch of W. L. Means near Hopeton, is definitely known to have been built by beavers before I860. It has
been and still is (in I923) added to and kept in repair by them. This dam is
now used by man as a diverting dam for two irrigation canals which carry
water to several hundred acres of alfalfa near by (see fig. 280).
In general appearance the lodges of the golden beaver resemble large,
crudely made haycocks. They are rough in appearance because they consist
m~inly of coarse materials-slender poles, sticks, roots, and dead branches.
In Galifornia, beaver houses or lodges are few, relative to the numbers of
beavers. Conditions favorable to so-called bank burrows-permanent deep
water and high dirt banks-abound in the habitat of the golden beaver,
----
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--- - - - - - - - -
660
and burrows in banks rather than lodges usually suffice as homes. Houses
are built, for the most part, where the beavers have made dams in sloughs
and where the banks of the streams are not high enough to leave room for
dens in them above the water level. Four houses have been found that were
built in the middle of shallow tule ponds (see fig. 281). Seven large houses,
well hidden in dense willow thickets and subject to inundation, were found
on slight elevations of low ground bordering streams. Many of those examined have been in more or less open situations, on the margins of the beavermade ponds, from 3 to 20 feet from the water's edge.
1
\
/~
\b
A
e
/
\
oj
B
c
Fig. 277. Diagram demonstrating why beaver dams curve up or down stream. If the dam is
started from both banks and extended toward the center of the stream, the current at a is increased
and tends to force the material gradually downstream; the completed dam then bows downstream,
as at A. If the dam is started at some stump Of rock, d, in midstream and advanced toward the
shore, both ends, at band c, tend to curve downstream a:ld when finished the dam will bow
upstream, as at B. In water where. there is little or no current, the dam may curve both up and down
stream, as at C, according to the contour of the ground Of to the location of willow dumps, as at e,
which serve to anchor the dam and keep it in place.
Willow poles from I to 5 inches in diameter and from 6 inches to 14 feet
in length were found to form more than half of the material used in lodge
construction. Most of the willows are freshly cut by the beavers themselves,
although sometimes a great many loose dead limbs have been gathered up
and worked into the houses. Also, a great deal of alder and cottonwood is
cut and used. Dry redwood planks, fence posts, oars, barrel staves, and even
hoe handles have been appropriated by the beavers and worked into their
houses. One lodge, situated near the center of a cattail swamp, was composed
in large part of cattails and tules. Rocks, fibrous willow roots, bark, and dead
or decaying vegetation are often worked in among the interlaced sticks of
a house. Mud is used sparingly, and only a few well-plastered houses (see
fig. 282) have been found among all that have been examined. Inthe habitat
of the golden beaver, the mud coating of a lodge would never freeze solid
and would therefore be of litde value as protective armor in winter, as is
true in colder countries.
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,
Golden Beaver
661
No two beaver houses are exactly-alike with regard· to size, composition,
or site, In illustration of this variation, four houses representing the commoner types have been chosen for description.
House A, situated near Hopeton, Merced County, represents a transition
stage between the burrow in a bank and the aboveground house or lodge.
It is of particular interest because it affords a reasonable explanation of how
the lodge-building habit may have originated in the far distant ancestral
history of the beaver. The beavers apparently had begun by constructing an
ordinary burrow in a steep, 8-£oot, north-facing bank of the slough. The
Fig. 278. Diagram showing sectional view of a beaver dam. The foundation of the dam consists
of saplings, c, which are cut by the ,beaver, dragged to the stream, and arranged usually parallel with
the current. Brush is added; and then rocks, 'e, are piled on top, pressing the foundation saplings
~gainst the bottom and thus helping to keep them in place. As the structure ipcreases in height, mud
is dug from the bottom of the pond~ at tI, and used to chink the upstream surface of the dam, at b.
Sticks which fioat -downstream are often swept over the crest of the dam, as at f, and later become
embedded in the soil, as at d. Such sticks, with the foundation saplings, c, form a double ,set of
braces for the dam.
original den or nest cavity, situated about 6 feet from the water's edge, was
merely an enlargement at the end of the burrow. The den was reached by
two passageways under the water; one was short and direct and the other,
the back door as it were, p.aralleled the bank for several feet, then turned
and entered the slough three feet beneath the surface of the water. A welltraveled cow trail extended across and above this den, and appearances indicated that following a spell of wet weather the entire roof of the den had
caved in. The beavers removed the dirt that had fallen and clogged their
burrow, but the roof was gone; the only covering now consisted of a few
straggling roots from a near-by valley oak. The resourceful mammals then
set to work cutting willow saplings, which they placed crosswise over the
top of the open den. Soon driftwood, peeled sticks, and dead fallen limbs
were piled on top of this framework, until a substantial stick roof was completed (see fig. 283). Thus the den which had for its beginning a mere burrow in a bank, having caved in, was roofed over with sticks and transformed
into a house or lodge ..
The thatched roof of this house covered an area 12 feet long and 10 feet
wide. The inside cavity was 5 feet long, 4 feet wide, and 2 feet high. The
house was visited several times in March, 1920, but was found to be uninhabited. On August 7, 1920, it was again visited (D.), and the following
notes were written after a short period of observation.
Fig. :279. The soft wood used in heaver dams soon decays. Then willow and
cottonwood trees spring up and cover the darn. In time this becomes a solid
earthen embankment which is kept' from washing away by the roots of the
trees that grow upon it. Photographed on November I, 192I, near Hopeton,
Merced County. Mus.Vert. Zoo1., no. 3791.
"As I approached the house at'ten o'clock in the forenoon of an exceedingly hot day, I could detect the peculiar odor of the beavers when I was
still six feet away. I crawled cautiously out on the willow poles and sticks
Golden Beaver
7
that roofed the den and found that I could look down through the interstices. Just as my eyes were becoming accustomed to the shadows and gen_,
eral dimness of the place 'a splash and a gurgle announced the departure of
one of the old beavers. Peering about as best I could, I was eventually able
to make out another beaver curled up asleep on a bed of freshly cut cattail
stalks near the back entrance of the den. A slight move for a better peephole resulted in a twig snapping under my knee. This was immediately fol-
Fig. 280. The dam (in right background) was built by beavers more than sixty years previously, and had been kept in repair ever since by the animals. Man for years used it to divert
\vater into two canals which served to irrigate several hundred acres of alfalfa near by. One of
these canals is shown in the foreground. Photographed on February
22, 1920,
three miles
west of Snelling"Merced County. Mus. Vert. Zoo1., no. 3249.
lowed9Y two diminutive splashes and then a final big splash and gurgle, as
the baby beavers and another old one became alarmed and dived out of the
side entrance into the deep water of the slough.
"By removing seven of the larger sticks I opened the roof of the den sufficiently to stick my head and arms through and was able to examine and
measure the interior of the house. The floor was level and firmly packed,
so hard that I had difficulty ih making an impression in it with my thumb.
It stood about six inches above the general water level of the slough, which
was low at this season of the year. There were four separate beds, all made
of freshly cut and shredded green cattail stalks and blades. The central bed
was the largest and measured thirty inches in length, twenty-four inches in
width, and two inches in depth. Two small beds were located beside a larger
bed near the side entrance of the den [see diagram, fig. 284]. These three
closely grouped beds had been occupied by the other large beaver and the
two young ones. The latter were about the size of cotton-tail rabbits. There
were no traces of droppings or urine in this den, which was thus sanitary,
as it was also damp, cool, and well ventilated. Such houses as this, that are
thinly roofed, may well be compared to a screened summer-house, and investigation has shown that they are regularly resorted toby the beaver families during the heat of late summer when the thermometer in the general
range of this race frequently registers over 100 degrees Fahrenheit."
House B, the largest one noted, was near Mendota, Fresno County. The
structure, measuring 30 feet long, 16 feet wide, and 6 feet high (see fig. 285),
Fig. 281. Four beaver houses out of twenty~five, in the San Joaquin Valley, were built (at the
time this photograph was taken) on islets in·the centers of ponds,-as here illustrated. Ordinarily.
the houses in California are built on the banks of ponds. Photographed November I, 1921, at
Hopeton, Merced County. Mus. Vert. Z081., no. 3789.
was built around the base of a clump of large spreading willow trees that
grew on an island which is completely submerged at high water. In 1918
this house was regularly inhabited by eight beavers, which have since been
trapped in irrigation canals more than a mile distant from it.
House C, the smallest one that we have measured, was on an exposed
bank of an irrigation canal ten miles north of Firebaugh, Fresno Couuty
(see fig. 286). This little hut was believed to be inhabited by a single, smallish, presumably two-year-old beaver. It was 10 feet long, 9 feet wide, and
28 inches high, measured at the surface of the ground. The floor was 6
inches above the water level of the canal, which was then low. Appearances
indicated that this beaver had tried to dig a burrow in the bank of the canal
and, finding the bank too low for this, had piled willow boughs and mud
over th~ open end of the projected burrow. Then going inside the pile as far
as it could, the beaver, beginning under the center and turning round and
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Golden Beaver
round, had gnawed off e~ery projecting limb and twig until it had completed a suitable cavity with smooth, close-cropped walls, like the inside of
a large oval basket. The thickness of the roof above the cavity was 14 inches,
the upper 8 inches consisting almost entirely of mud which had been
scooped out of the irrigation canal six feet distant and shoved up gopher-,
fashion onto the house. The house was visited at the time (November II,
1920) when the mud coat was being applied. Tracks indicated that the soft
semiliquid mud had been gathered up and held between the front paws and
the chin, and that the beaver had then slid forward on its belly, propelling
itself up the bank by its strong hind legs and feet. From the points where the
Fig. 282. We have found only a few mud-coated beaver hOllse·~ in Cali£~rnia. Ra~ely or neve1would the mud coating freeze here in winter; it would not afford the protection from enemies
that it would in a cold country. Photographed on November II, 19:W; ten miles northwest of
Firebaugh, Fresno County. Mus. Vert. Z081., no. 3459.
mud had been obtained well-worn trails led up onto the roof of the lodge.
(See fig. 282.)
The cavity. within was 42 inches long, 36 inches wide, and 14 inches high.
A small but well-made bed 14 inches long, 12 inches wide, and 3 inches
deep, placed near the back of the cavity; consisted of weed stems, twigs,
.,\nd finely split willow wood. A typical willow sliver used in the bedding
measured 75 millimeters in length, 5 millimeters in width, and 3 millimeters in thickness,
. Although the mud-plastered roof had no openings in it for ventilation,
------
666
Fur-bearingMammals of California
there was a loophole between the back wall of the hut and the ground. This
opening, just behind the beaver's bed, was 3 inches wide and I inch. high. It
was concealed on the outside of the hut by dead weed stalks and growing
grass, enabling a beaver to he.ar and probably even to smell a stealthily
approaching enemy.
A dead and partly devoured mudhen, found in the center of the hardpacked floor of this house, had tooth marks on it which indicated that per-
Fig. 283. The transition from burrow to "lodge"
shown in this beaver hOllse (A <;If the
text) flC'l! Hopdon, Merced County. At first there was merely the usual burrow in a bank. Wet
weather came and the earthen roof of the den fell in. The beavers at once set to work and made
a roof of willow poles over the exposed den. One year after rCp'airs had thus been made, this
l?-ouse was used as a breeding den. ~is example gives a reasonable clue to the possible origination of beaver houses from mere burrows in high banks.
haps a mink had been at work, and not a beaver. The house had not been
visited by any beaver for several days before the time it was dug out.
.. In tlie neighborhood of Snelling in December, I922, all the larger beaver
houses examined had been heavily plastered on the outside with mud. The
preceding fall had been an unusually mild one. One house, an exceptionally
'old one, was in an exposed situation on a low bank at the edge of a slough.
On its sides were three runways up which the beavers had shoved mud to
plaster the house.
.
Apparently; rain, even it small amount of it,will keep beavers inactive
and within their houses. There was rain for part of the night on December
6, I922, in theSndlingregion. The following morning, there was very little
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Golden Beaver
.~~------..------
...
Entrance
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2feet
B
Fig. 284. Diagrams of interior of the beaver house shown in figure 283. A, sectional view; B, plan.
In August. 1920, the bed~ (at h, d, and a, c) were used by two large and two small beavers. Broken
line, I, shows reduced size of this house on April 15, I92I, when it was occupied by one adult:
female beaver and her week-old kitten. The baby beaver was found in a large freshly made bed
of cattails at e, after the mother beaver had become frightened and escaped by the back-door exit, g.
668
evidence that beavers had been active during the night. Breaks made in the
dams on the previous day had not been repaired. Only one of seven traps
had been sprung. The weather cleared on December 7, and then the beavers
were again active, repairing breaks in dams and moving about. There is
no apparent reason for this seeming trait of behavior.
House D, built in the fall of 1919 near Snelling, Merced County, was in
the middle of a willow clump that had grown up on the bank of a channel
Fig. 285. House B-(o£ the text), the largest beaver lodge found in the State, was near Mendota,
Fresno County. This house, photographed o~ June 2,1918, proved to measure at the water level
30,by 16 feet, with a height of 6 feet. In December, 1919, this house was inhabited by eight
beavers: This is the largest number of beavers that we have found in anyone house, and it prob~
ably included members of tWo seasons' litters besides the parents. Mus. Vert. Zool., no. 2757.
left by a dredger, and was made almost entirely of slender willow poles. It
had been inhabited by the builders, a single pair of beavers. In February,
1920, the dried foot of one beaver was found in a steel trap near this lodge,
and the decaying carcass of another was found in a burrow within a few feet
of the house. This evidence showed that the inhabitants of this home had
6eefi destroyed as a result of illegal trapping. The observer likewise noted
that the dam below the house had been cut by the trapper and the water had
drained from the pond, leaving the abandoned lodge high and dry so that
the main entrance was exposed above the water (see fig. 287). This house
measured 22 feet in length, 14 feet in breadth, and 6 feet in height. The
slender willow poles had been cut and trimmed of branches by the beavers,
then piled crisscross upon the lodge. These poles, which had not been peeled,
Golden Beaver
were from I to 2 inches in diameter and from 6 inches to 14 feet in length.
With them were also several pieces of an old 3-inch plank which the animals had found and dragged up onto the house. In taking this house to
pieces to examine and photograph.the interior, the observer (D.) counted
nearly 2000 sticks greater than liz-inch in diameter and more than I foot
in length. The largest was 4 inches in diameter and 4 feet in length; many
were 2 inches in diameter and from 8 to 12 feet in length; one was more
than 14 feet in length. A quantity of matted will0w roots .and small rocks
had been used to chink up the openings between the poles, but there was not
more than one cubic foot of mud in the entire structure.
Alfalfa
8 Feet '.
Fig. 286. Diagram showing a
small~sized
beaver house (C of the text). This lodge was on the
bank of an irrigation canal paralleling the San Joaquin River not far from Firebaugh. It was 10 feet
long, 9 feet wide, and 2 Yz feet high, and accommodated but one beaver. Burrow, b, opening into
canal; bed at n.
A clump of willow saplings. protected the egg-shaped nest cavity (see fig.
288). It was 42 inches l{)ng, 30 inches wide, and 18 inches high. The walls
were tightly chinked with short pieces of wood, matted roots, and rocks.
The bottom had been excavated in the bank and was thickly lined with
soft, shredded, reddish bark of the incense cedar. This bark, from cedar saw
logs which had been cut many miles up the Merced River near the boundary
of Yosemite National Park, had floated down the river from the sawmill at
Merced Falls. The roof above the nest was 2 feet thick, and there was ample
ventilation through the interlaced twigs and poles.
Two entrances led from the pond into the house, uniting at the mouth of
the bedchamber (see figs. 289, 290). One was short and direct; the other
widened out into a chamber 42 inches wide, 30 inches long, and 30 inches
l'
'"i:
-------- - - - -
high which formed a sort of hallway right at the water level, Marks of
muddy droplets on the walls indicated that the beavers used this as a place
in which to shake their coats free of water before entering the nest proper.
The relation of the beds in the lodge to the water level of the surrounding
pond is apparently important to beavers. Beds are always built a few inches,
usually from two to six, above the mean water level, and the water usually
just fills the entranceways. The beavers thus have a constant and reliable
water gauge inside their houses and within a few inches of their beds. Upon
Fig.'287. Beaver house D (of the text), which was abandoned afte'r a poacher had trapped two of
its occupants, broken their dam, and drained the pond. The entrance, originally under water, is here
shown exposed. Photographed on February 28, 1920, near Snelling, Merced County. Mus. Vert.
Zoo1., no. 3271.
entering or leaving a house they quickly become aware of any change in
the level of their home pond, such as would be caused by a break in the
dam. Water sufficient to cover the entrances of their houses is necessary to
protect the beavers from predatory enemies. This is one reason why they
are so alert to keep their dams in repair and their ponds full of water.
In early December, '922, a whole family of beavers trapped near Snelling
Was thought to be from one house. This family was made up of the 2 adult
parents, 3 young approximately twenty months old, and 3 young about
eight months old. Seven of this group were caught within one week. These
Were all taken within a stretch of one-half mile along a slough where there
were four fair-sized dams, 50 to IDa feet long and 24 to 30 inches high. Only
two seasons before, in '920, this neighborhood had been thoroughly trapped
Golden Beaver
over. The appearance of the sexual organs in the two adults indicated that
the mating season was not far off.
The golden beaver is characteristically a bank beaver. High dirt or clay.
banks are common along many of the permanent water courses inhabited
by this race. In such streams there are holes or pools deep enough to afford
a ready refuge for the beaver and to cover the entrances to their burrows.
Our observations in California lead us to believe that here beavers' breeding
. dens are usuaily secluded burrows in banks densely overgrown with brush
Fig. 288. House D (of the text), cut away to show interior. This house was composed of
2000 willow sticks, by actual connt, from Yz -inch to 4 inches in diameter and from
Yz -foot to 14 feet in length. The nest chamber was situated behind the clump 'of growing wil-
nearly
low stems near"the upper center of the photograph; the picture was taken on February 29.
near Snelling, Merced County. Mus. Vert. Zool., no. 3273.
1920,
rather than lodges, even though lodges in the vicinity may at the same time
be used for ordinary shelter' purposes. In some places a series of burrows
has been found, some of which are situated nearer the top of the bank than
others. This arrangement had been developed, apparently, to meet the fluctuating level of the water in the stream. By having a number of burrows at
different levels the beavers are enabled at any time to occupy a den that is
protected by having its entrance under water.
, On several occasions beavers have invaded a large irrigation canal near
, Mendota and have made numerous burrows in the high banks left on either
side of the ditch by the canal dredger. A number of these burrows, examined
on March 25, 1920, were found to be from 20 to 24 feet in length and from
Fur-bearing Mammals of CalifOrnia
18 to 24 inches in diameter. They led from beds of tules at the margin of the
canal to the highest parts of the adjoining levee or bank (see fig. 291), extending beneath the surface of the water until within two or three feet of
the den. One of the burrows which was dug out measured 24 feet from the
entrance to the nest cavity or den. The den was 6 feet in length, 4 feet in
width, and 2 feet in height. The top came within 14 inches of the ground
surface. Ventilation was afforded, at least during dry weather, through the
cracks in the adobe soil that formed the roof. Dry, shredded willow bark
and leaves carpeted the floor. Numerous short, peeled willow sticks, measuring from liz-inch to 2 inches in diameter and from 12 to 18 inches in
a
b
d '
Fig. 289. Diagram to scate, showing plan o~ house D (of text). The main entrance, a, connects
with a spacious hallway, b, which leads to the nest chamber, c, A second entranCe is shown at f.
The nest is protected by growing willow stems at hand i. A large alder, g, also protects the house.
A wood rat had built its nest in the wall of the beaver house among wild fose bushes at j.
length, were strewn about on the floor of the passageway and den; the sticks
had probably been taken into the den and the bark eaten off at leisure, the
peeled remnants being then discarded. Sanitary conditions appeared to be
ideal; there was no evidence that feces or urine were ever voided in the den.
The construction of canals appears to be nearly a lost art among golden
beavers. Canals that have been found in California have been few and of
small size. Here, we have not found, nor would we expect to find, any complicated system of locks and canals such as has been described of beavers
elsewhere (Mills, 1913, p. 88; Dugmore, 1914, p. II6). All the canals which
we have examined have been dug on the same general level as the adjacent
-----------,---------------------
,
---------------------------
Golden Beaver
streams or ponds with which they connected. The levelness of the beaver's
habitat in California, together with the prevailingly alluvial soil, would seem
to be favorable to the construction of beaver canals. However, the primary
purpose of the canal is to make the obtaining of food easier and safer, and
in this State beavers have no need of transporting and storing large quantities of heavy green wood for winter use; hence canals are not necessary here
I
\I
Fig. 290. Diagram to scale showing sectional view of house D (of text). Underwater entrances,
a and f, lead tei nest c. A burrow·, e, used by the beavers to duck into in emergency, extends under
and behind the nest c, ending blindly in the bank at d, high above the former watedeve1 indicated
by dotted line w.
.
. 6 Feet ,
Fig. 291. Diagram of beaver burrow in canal bank. About IS feet of the burrow, b, is under
water. The nest or den, at u, is above the water level in the higher part of the levee. A burrow such
as this may cause the levee to give way and result in damage through the flooding or "drowning
out" of the growing crops near by.
and are rarely built. Another important reason why they are not built may
be because man destroys many of the predatory natural enemies of the
beaver. Beavers are quick to learn the advantages of sanctuary, and do not
now hesitate to make long portages or to wander about overland in search
of food such as acorns, going even as far as one hundred feet from streams
and ponds in open country.
Illustrating how canals are sometimes built, the following account, which
we believe to be typical, may be given. In December, '9'9, a canal near
Hopeton had its beginning as a mere slide or landing place where a welltraveled beaver trail dropped into a pond. There was nothing at this time
to indicate the beginning of a canal, but as time passed the indentation in
the bank grew deeper and wider. In the summer of 1920 this trail was aban.doned by the beavers, but the following March it was again in use. When
examined on April 15, 1921, it was found that the beavers had recently converted a part of the trail into a ditch filled with water, thus forming a canal
10 feet long, 12 inches wide, and 10 inches deep. It was gradually extended
until at last it almost entirely replaced the 40-foot trail that connected the two ponds. In this
instance as well as in others we
have noted, the canal had its beginning at the water's edge and
was dug landward. As the mud
and roots were removed, the
water, always seeking its own
level, flowed back into the newly
dug canal. Thus it will be seen
that the level or grade of the canal
is automatically determined by
the water, and all that the beaver
Fig.' 292. Feces of beaver in one foot of water.
has to do is to follow the water.
The constituent material resembles coarse sawdust.
When the mucous envelope decays, the droppings
Examination of numerous beaver
go tq _pieces and the grains of wood float into the
canals in Alaska and British Cointerstices of the dam,-helping to make it still more
impervious to water. The white peeled stick in the
lumbia, as well as in California,
photograph was exactly 12 inches long. Photo·
graphed on PcbJ,"Uary 28, 1920, near Snelling, Mer-
ced County. Mus. Vert.
Zool., no. 3 2 77.
leads us to believe that fully 90 per
cent of the canals dug by.beavers
anywhere are built in this manner: the beavers let the water flow outward
from some body of water, stream, or spring which they use as a base. With
this fact in mind we have no ground for "marveling at the beavers' engineering skill in running levels."
Only a few beaver "forms," or beds outside of dens, have been noted by
us. Those found were on steep brush-covered banks near deep water, some
at the bank tops. This setup exists on the San Joaquin River near Mendota.
Here, slight surface beds have been found in secluded localities. In one
place, judging from the tracks, a lone beaver had crawled out of the stream,
climbed the bank, shaken his fur free of water, and, after digging or wallowing out a slight depression, had lain down to rest. Such retreats are obviously
Golden Beaver
insecure, and the occupant is always ready to escape by a short scramble
down the bank, ending with a plunge into the stream.
The beaver is a good-sized animal and a single individual consumes a
relatively large amount of food. Much of the wood eaten passes through the
body with but little change. Despite these facts, it is difficult to find droppings or feces of this animal. All that we have been able to discover had been
deposited in water. Once, feces were found in a foot of water in a small pond
just above a dam. Some of these had been deposited singly and others in
. bunches of six or eight (fig. 292), or adhering to each other end to end,
Fig. 293. Materials before and after passage through the beaver's "mill" (m.olar teeth in
action). Young beavers do not chew their food as well as old beavers do. 'this has been shown
by examination of many stomachs. In this photograph, the fine material, at upper right, was
taken from the stomach of an old beaver, and the pile of coarser material, at center, came from
the stomach of a young beaver. In both piles were represented the wood and bark of willow.
Mus. Vert. Zoo1., no. 3797.
sausage-fashion. An average individual dropping was cylindrical in shape,
slightly longer than thick, and measured I inch in length and Ys-inch in
diameter. A very thin gelatinous (mucous) covering wa.s noted on many of
the droppings. This covering disintegrated when the feces were lifted from
the water, thereby revealing that the droppings consisted almost entirely of
finely chewed pieces of clear white wood, chiefly willow and cottonwood,
which resembled clean coarse sawdust (see fig. 293). Many of the pieces of
wood were from I to 3 millimeters in diameter and from 3 to IO millimeters
in length. In this material there were numerous finer particles of wood, but
the bark had almost all disappeared. Several pieces of wood liz-inch in
-
length were noted in one large dropping. Droppings found in the large intestine of beavers, when examined proved to coincide closely with the above_
mentioned findings with regard to size, shape, structure, and composition.
It was noted that the feces deposited in the water by beavers float down
and collect just above the first beaver dam downstream, where in time they
disintegrate. Observation in some places showed that the fine white particles
of wood had lodged in the smallest openings in the upper face of the dam
and chinked them up, just as any other buoyant sediment would do.
It appears that male and female beavers, with the exception of females
nursing young, cannot dependably be distinguished by external examina-
Fig. 294. Photograph of female beaver fetus, from Snelling, Merced County, M~rch 16, 1920,
showing situation of nipples, here indicated by the four white spots. Both pairs of mammae are
on the breast. These spots are absent in males. Mus. Ver.t. Zool., no. 3248.
tion. There is a common canal (cloaca) for receiving and discharging the
products of the genital, urinary, and digestive organs. The reproductive
organs lie within the body cavity and cannot be seen except by dissection.
Both the two pairs of mammae are pectoral. The positions of the mammae
are best seen in female embryos of advanced age (see fig. 294). The first pair
is directly between the forelegs, and the second, in adults, is about four
inches farther back. All the mammae, in the adult, are concealed in the
dense underfur of the breast and have proved difficult to locate except in
females that are pregnant or are suckling young.
Females are as large as males of corresponding ages. No differences in
color or in quality or quantity of fur have been noted between the sexes.
Males are likely to be scarred as a result of combats occurring in the m'lting
season, and hence their pelt value, other things being equal, is frequently
less than that of females.
The genus Castor differs from all other genera of Californian rodents in
.its possession of two pairs of highly developed scent glands. The beaver does
not use the products of these organs as a means of defens~, as do the skunks
(Mephitis and Spilogale). Indeed, their function is of quite another sort, as
,
Golden Beaver
r
rI
will be seen from the following account. The scent glands of the beaver are
situated on the under side of the body near the cloacal orifice (vent). The
larger pair of glands constitutes the so-called "bark bags," which secrete
the castoreum. These castor bags are flattened and oval in form and of a
light yellow color. In an average breeding adult they are about 5 inches in
length and 2 Yz inches in width, but are subject to noticeable variation with
season. The walls are thin, almost membranous, and have a creased or
folded appearance. Numerous pockets inside the bags contain the castoreum, which, when fresh, is a granular, sticky, yellow substance of a rather
pleasant odor. When dried, it is much used in the preparation of certain
"animal scents" used by trappers in luring various other fur-bearing mammals, as well as beaver, to their traps. It is well known that formerly, and for
a long time, it was used in perfumery and, in certain countries, as medicine.
The oil sacs are to be found behind the "bark bags," and like them they
also open into the cloaca. These oil sacs are thick-walled and about one-half
the length of the bark bags. Because they are similar in size and form to the
testicles of the male, they are sometimes confused with them.
Both pairs of scent glands are as well developed in the female as they are in
the male. In both sexes, they attain their maximum size and development in
the mating seasoh. An adult male beaver, weighing 44 pounds, taken on the
American River near the town of Fair Oaks, Februaty 17, 1920, was found
upon dissection to be at the height of sexual development; in this animal the
testes were 3 inches in length and I inch in diameter. Outside of the breeding season, their length would be scarcely more than 1Yz"inches. The vasa
deferentia were greatly enlarged and convoluted. The bark bags of this
breeding male were 6 inches long, 3 inches wide, and contained a rather
large accumulation of castoreum. The thick-walled oil sacs were about the
size of the testicles and when pressed exuded a thick, pungent, yellow oil.
The combined weight of the oil sacs and castor bags, when removed from
the animal, was 250 grams (more than 8 ounces), and 100 grams (3Yz
ounces) when thoroughly dry. At this rate it would take only 4Yz sets of
large-sized scent glands dried for marketing to weigh a pound. Ordinarily,
5 or 6 sets would be required to make a pound.
As previously intimated, these scent glands are close!y associated with
the reproductive system in function as well as in location. Their main function seems to be the distribution of sex "information." Both sexes deposit
the secretions of these glands, and are attracted by them. At Snelling, during the rutting season, traps set beside scent mounds-that is, placed where
castoreum and oil are regularly deposited by breeding beavers-caught
both males and females, in about equal numbers. The following. incident
shows that besides preventing the unduly rapid loss of scent by evaporation,
the oil from the oil sacs also apparently carries some special, subtle message.
Several old "trap-wise" male beavers were being trapped for, without any
success, for several weeks at Snelling. Castoreum alone was being used, in
renewed quantities, as a lure at the traps. Then, oil from the oil sacs of a
female was substituted and the old male beavers were promptly caught in
the very traps which they previously had shunned.
During the mating season beavers of both sexes nightly deposit
both oil and castoreum at certain
points that are prominent in their
nightly itinerary. These deposits
apparently provide a means of intercommunication among the
adult beavers of the general neighborhood. They probably give information about the stage reached
on the part of each individual in
the season's reproductive cycle.
During the mating season this
record is read through the sense
of smell, by the members of the
colony as well as by any visiting
beaver that may happen along.
Scent mounds are composed of
Fig. 295· Scent mounds are at the water's edge
mud, sand, dead leaves, dried
on certain bars and points. During the mating sea~
d lk
d
son the mounds are, it i~ indicated, regularly visited
wee sta s, an sometimes green
by beavers
of both sexes.
vegetation, -mixed with secretions
of both the oil sacs and the bark bags. They are situated on sand bars, islets,
promontories, or other points regularly visited by the beavers. (See fig. 295.)
Judging from the tracks left in the soft ground in the vicinity, each beaver,
after taking olfactory note of the sign left by his or her predecessor, deposits
oil or castoreum, or both, on top of the mound and then covers it up by
scratching together damp refuse or mud with the forefeet, so that eventually a conical mound, sometimes a foot in height and cover,ing nearly a
.square yard of ground, is accumulated. No deposition of feces or urine is
made in or about the mound; at least, none has been found in more than
ninety scent mounds examined by us. On March 5, 192o,ten scent mounds
Golden Beaver
•
were counted within a distance of 100 feet along the margin of a cattail
swamp. This row of mounds was about 200 yards from a large inhabited
lodge. One of the largest mounds, composed mainly of decayed cattails and
tules, measured 4Yz feet in length, 3 feet in breadth, and I foot in height.
On a warm damp morning a person could detect the odor of the castoreum
in this mound at a distance of 50 feet; beavers, believed to have a much
keener sense of smell than man, could probably detect the site of such a
'mound at a much greater distance.
'. Occasionally a small scent mound may be found in any month of the
'year, by careful se';"ch. It has been our observation, however, that scent
mounds are primarily associated with the mating season and reach their
highest development within that period. In the fall of '9'9, and again in
'920, a search made for scent mounds in several localities where beavers
were common resulted in the finding of only one small one, barely 3 inches
in diameter. This was discovered on the bank of the San Joaquin River
near Mendota, Fresno County, on November 10, '920. It consisted of a
bunch of dry grass that had been twisted up into a ball, upon which had
been deposited a small amount of dark-colored castoreum.
In February, '920, scent mounds were found to be numerous in the vicinity of Hopeton. Two weeks later (March 5) the mounds had attained what
was estimated to be their maximum size, although some were being added
to slightly. When the sites of these mounds were again visited five months
later (August 7) they were overgrown with a dense growth of weeds, and
the mounds had almost disappeared. At this time, no trace of recently built
scent mounds could be found, although beavers were inhabiting a lodge
close by. On February 28, '92', the locality was again visited and many
freshly made mounds were found; about half of them were situated just
where sirriilar ones had been seen a year previously, the rest from two to ten
feet from their former sites, in the same general vicinity. At this time, seven
fresh scent mounds were found on an islet 20 feet wide and 30 feet long
which had grown up to willows and cattails, in water somewhat over knee
deep. A medium-sized mound on this islet was composed of decayed roots,
dead leaves, and freshly pulled cattail stalks, all twisted up together; it
measured 18 inches in length, '4 inches in width, and 5 inches in height
(see fig. 296). The larger of two fair-sized scent mounds found at this time
measured 42 inches in length, 32 inches in width; and 10 inches in height
(see fig. 297). The odor of the castoreum in this one could be readily detected, in calm atmosphere, by a person ten feet distant. On April '5, '92',
these scent mounds were again visited and the two largest ones that had
680
been so prominent six weeks earlier had almost disappeared. Nearly all
showed signs of decay; only a few had been kept up and even these consisted
.
of but a few handfuls of material.
From the foregoing account it will be seen that scent mounds are regularly constructed by the adult beavers at appointed places. The period dur-
Fig. 296. A scent mound at an early stage of accumulation, as here shown,
is composed ot matted foots, green cattail stalks, and mud, upon which scent
(castoreum and "oil") is deposited. The function of such mounds is to con~
vey information having to do with the annual reproductive program.
Photo~
graphed on February 28, 1921, two miles southwest of Hopeton, Merced
County. Mus. Vert. Zool.,- no. 3612 ..
ing which they are under construction coincides closely with the mating
season. When the mating season is over the scent mounds are no longer
repaired or visited by the beavers.
It has been said (for example, Seton, 1909, p. 471) that the beaver is "a
strict monogainist." This statement seems not to accord with the facts.
If beavers are monogamous it is in marked contrast to the promiscuous
mating of rodents in general. The following data bear on this question.
Golden Beaver
681
In 1920, 80 per cent of the beavers trapped at Snelling in January, in the
fore part of the breeding season, were males. Probably at that season of the
year the males were traveling about more than the females. Traps set at
scent mounds were notably successful, indicating that the males were alert
to the purpose of the mounds. Many of the males trapped in the mating
season had recent incisions intheir rumps and tails, and the shape, slant,
and depth of these wounds showed that they had been made by the incisor
teeth of other beavers. Female beavers caught at the same time did not show
such scars. If the 'males are monogamous and already mated for the season,
why, throughout the mating season, should all the breeding males chase
about over miles of territory, fight viciously among themselves,. make
numerous scent mounds, and otherwise spend a great amount of energy,
unless they were seeking further opportunities for breeding? If the males
are strictly monogamous, why didn't these stay at home, each with his
chosen female? Yet many of the females trapped were found to be pregnant,
and at the same tirne almost all the males were traveling about as if seeking
still other females.
In 1920, a certain male beaver, identified by the absence 'of two toes of, a
hind foot previously lost in some trap, was noted living in an isolatedpopd
where also was a pregnant feinale. Both beavers were subsequently ttapped
and their identity, as well as the pregnancy of the female; established. The
tracks of the male showed that at that time, which wa";n the mating season, he made nightly excursions, heaping up scent mounds as far distant
as two miles from his headquarters. The actions of this male, which were
characteristic, indicate that although he already had.one mate. he was looking for another.
In 1920 and 1921, the monogamy theory of beavers was giveu'a crucial
. test, and was found wanting. Between JanuarY1sand March't5, 1920, II4
beavers were trapped undet permit in a region near Snelling 6 miles long
and 2Yz miles wide. Careful checking before and after trapping showed
that the uumber caught equaled about'one-half of the estunated total beaver
population (228). All the beavers caught at this time were skinned by one
man, who, when fresh specimens were available, repeatedly proved that
he was able correctly to tell the sex of a beaver by dissection after the animal
had been skinned. of the II4 beavers trapped, 77 individuals (70 per cent)
were males and 37 (30 per cent) were females.
'i The follo~ing data and consequent reasoning have a bearing lipon the
monogamy theory with regard to beavers .. We have found, from examinations of embryos, that at birth the sexes are about equally represented. Adult
,
males might number somewhat less than females in a given locality, since
in the mating season the males are known sometimes to fight to the death
among themselves. Granting an equal representation of the two sexes before trapping began, there were 'some II4 pairs of beavers at Snelling previous to the trapping period. Since 77 males and 37 females were known to
have been trapped in '920, 37 males were left to mate with 77 females in
'921. Under such circumstances, either one of two things might happen.
Fig. 297. As the rutting season advances, the scent mounds increase in size. When the mating
season wanes they are neglected by the beavers and soon de.cay, or the materials composing them
become scattered. Photographed on February 28, 1921, two miles southwest of Hopeton, Merced County. Mus. Vert. Zool., no. 36rr.
If beavers were strictly monogamous, then there would be 40 beaver widows
left unable to secure 'mates and to breed. As a result, the crop of young bea- .
vers for that season (1921) should have been greatly reduced. However, if
mating were promiscuous, then almost all the 77 females should have bred.
During '92', the year following the trapping, a special watch was kept
to see if there existed any large number of nonbreeding females; in vain.
Most of the old dens and houses revisited were found to be occupied. The
females had all bred as usual, as was shown by the litters of young known
to be in the dens. This would not have been possible if the relict males had
each been constant to one female.
There is little doubt that a female is far more constant to a given male
than a male to a given female. The female beaver is much the more closely
identified with the home locality, because there fall to her lot the care and
Golden Beaver
safety of the young. Having once settled upon a favorable homesite, she is
likely to stay in that place for life; whereas, the male is not closely tied down
to one locality by duty to offspring and can wander about without jeopar,dizing the interests of the race. The chances are, then, that a female, once
established, will mate year after year with one male. He adheres to her
established homesite as a sort of headquarters; at the same time there is a
strong probability, supported by the data just given, that he continually.
casts about in the mating season for chances to mate with any other females
agreeable to his advances. From the facts given above we conclude that in
one sense the females of the species may in many instances be monogamous,
the males at the same time polygamous. The species, as a whole, is not in
any strict sense "monogamous."
We do not believe, as has been suggested (Morgan, 1868, p. 137), that
"beaver outcasts" are individuals that have lost their mates and refused to
mate again. We think that such outcasts are simply old males that have bee~
"licked" or "driven out of the herd" by the younger, more vigorous males
and that they obtain new mates when opportunity arises.
Beavers are generally believed to reach maturity and to breed in the third
year of their lives. In two instances known to us, beavers which, judged by
.skull characters, were barely two years old, were found mating. Judging
from the reproductive organs of twenty-eight females examined between
January 9 and March 15, the mating season for some individuals begins the'
first of January and for others continues to as late as March 5. For the entire
population it thus extends over a period of two months. Beavers in the Sacramento Valley appear to breed slightly earlier than those of the San Joaquin
(see table, p. 634). It has been our observation that the early breeders are
old females and that most of the late breeders are small young females.
We believe that the period of gestation in the beaver lasts about three
. months. This is figured from the time of the beginning of the mating season
to the time of birth of the earliest litters,
It is difficult to determine exactly when young beavers are born, since the
females retire to the secluded breeding dens for this event. Our most trustworthy information on this point is furnished by embryos found in trapped
females and by small young beavers found in breeding dens. A set of four
well-furred and fully developed fetuses, 13 inches in length, weighing a
pound (more exactly, 485 gm.) each, was taken from a beaver trapped
" March I I, 1920. This female would probably have given birth to her young
in less than a week. Judging from the stages of development in the various
sets of embryos examined (see table, p. 634), the time of birth varies from
L
-.
------------------
the middle of March to the last of May. The majority of young are probably
born in April. Our information on this topic is meager.
On April 15, 1921, a week-old baby beaver was found in a lodge near
Hopeton, Merced County. The den (see fig. 283), which had previously
been visited, was originally a burrow in the bank. It had caved in, and had
then been roofed over with willow poles by the beaver and made into a'
lodge or house. When visited, at midday on March I; 1921, it was found to
be untenanted. On April 15, as the observer (D.) approachedthe lodge at
dusk, he saw that the outside of the structure had recently been repaired.
Fig. 298. At birth, a baby golden beaver weighs about a pound. The individual here shown,
when one week old on April 15, was 14 inches long and weighed 25 ounces. The average number of young in a-litter is 2.7, with I and 5 as extremes. These figures are given on the basis of
14 litters exa11?-ined. Mus. Vert. Zool., no. 3615 ..
Upon a closer approach, when he was within six feet of the lodge, he heard
a loud whimpering coming from near the back of the den. The cry reminded the listener very much of the softer whimperings of a very small
puppy dog. After listening for several minutes to this cry, he became fairly
certain that all the noise was made by one baby beaver. When the observer
crawled right up onto tl,e house, the mother beaver, which until then had
remained quietly in the nest with her lone young, nursing it, as was shown
by its milk-wet muzzle, plunged noisily out through the main underwater
exit of the lodge. The baby beaver at once ceased whimpering.
When the visitor made an opening through the roof of the house he
found the youngster squatting motionless near the back of the bed that so
recently had been occupied by its mother. The interior of this lodge, which
--------- - - - -
Golden Beaver
685
had been examined previously on August 7, '920, was now found to have
been recently remodeled, as was the exterior, so that it was decidedly
smaller than it had been before. There was but one small bed, used by the
one adult beaver and her baby. There was no indication that the male parent
or any other adult occupied the den or shared in any way the care of the
young. The bed, which measured 24 inches in length, 18 inches in width,
and 3Yz inches in depth, was too small to be shared by two adult beavers.
The lining, unusually deep and soft, was composed of clean, slightly damp,
shredded tule stalks. The baby beaver showed no fear of man, and instead
Fig. 299. When sought out in its home, the baby beaver made no effort to escape by taking to
the water, though given repeated opportunity to do so. However, a week later (April 23, 1921),
when placed near a stream, it proceeded to swim and dive. In the picture it is shown drinking.
Mus. Vert. Zool., no. 3618.
of fleeing, snuggled up to the hand that was thrust into the interior of the
lodge. When captured, the young beaver was judged to be not more than
a week old. It measured 14 inches from end of nose to tip of tail and tipped
the scales at 25 ounces (see fig. 298). It was well protected from cold and
dampness by the thick, soft pelage, the underfur and overhair of which
were well developed. The overhair on the middle of the back was 23 millimeters long; the adjacent underfur was 15 millimeters long. When the
young beaver was first found, its fur was perfectly dry.· The youngster,
though given repeated opportunities, made no effort to escape by entering
the water.
This young beaver was brought to Berkeley and an attempt was made to
rear it. When it was about two weeks old it was left at the edge of a shallow
686
pool (see fig. 299)' At first the beaver did nat seem to know what the water
was, but after taking the first smell it plunged boldly in and swam about
and dived with evident enjoyment. When first obtained, it had just cut its
upper incisor teeth. The lower incisors were long and sharp, yet the youngster made no effort to bite one's finger even when this member was placed
between its teeth. The eyes were dark brown and small, the part showing
between the lids measuring 3 millimeters in diameter; yet the animal was
able in dim light to distinguish readily objects (children) in motion across
a 20-foot room. Whenever possible, the beaver avoided direct sunlight and
other strong light. The scaly, fully developed tail was much used as a prop
when the animal sat up. Even at this early age both of the scent glands, the
bark bags and the oil sacs, functioned whenever the animal was irritated, at
which times a musky fluid was emitted.
.
The young beaver was sociable and appeared to enjoy being held in a person's lap. It responded readily to the human voice and soon learned to come·
when called.
Its whimpering or crying was so similar to that of a very small child that
the two were frequently mnfused .. When the baby beaver and a human
baby were crying at the same time, it was difficult for the child's father to
distinguish between them when he was in an adjoining room.
rt was quite difficult at first to getthe young beaver to nurse from a bottle.
Various sizes and forms of rubber nipples were tried and finally a short
stubby one was obtained which was similar in shape to the nipple of a
feinale beaver. The baby beaver took readily to this nipple and consumed
an ounce of warmed cow's milk fout times a day. When nursing, the diminutive beaver sat up on its hind feet and used its scaly tail as a brace behind.
The nipple was held habitually in one side or the other of the mouth so
that it was not injured as it would have been if it had been held in the
middle, between the sharp incisor teeth. While the beaver was nursing, the
forepaws were used continually as hands to work and massage the rubber
base of the nipple.
When the animal ambled slowly along over a smooth surface, the flat
tail, held slightly curved upward just off the ground, swayed from side to
side. The young beaver never, even when in a hurry, galloped, or hit the
ground with the tail, as adult beavers do.
Many full-grown beaver parasites were found on this young beaver,
doubtless immigrants from its mother's coat. Under favorable circumstances, as many as five or six of these aberrant beetles (Platypsylla castoris)
were seen at one time about the head of the animal. These parasitic insects
-
,
Golde1t Beaver
are 2 to 3 millimeters in length, yellowish brown in color, oval in form, and
flattened like a louse. We have found them to abandon the beaver host
within a few hours after the animal dies. This probably explains why we
have only once been able to find them on trapped and drowned beavers.
After birth, baby beavers remain in the house or burrow about three
,weeks before they begin to venture out much. Their tiny tracks indicate that
at first they do not wander far from home. It has been our observation that
. for many subsequent weeks the mother and her small young go about together, unaccompanied by the male parent or by any other adult beaver.
From personal observation, W. L. Ayres, of Mendota, describes the actions
. of a family of beavers adollows. On an island, one morning late in June,
the observer saw an old beaver accompanied by three little ones, about the
size of large cottontail rabbits. The young were romping about in the rank
.grass twenty feet back from the water's edge. He saw the mother pick up
the youngsters one after another and turn them over and over, meanwhile
combing out their fur with the claws of her fore paws and apparently trying
to find and dislodge the parasites that infested her young ones.
By late summer or early fall, baby beavers are weaned and the old and
young of both sexes are again united under one roof. Of many houses exam, ined at various seasons of the year, we have never found any in which more
than eight beavers were living together at one time~ The young and old
6ccupy the lodge or den together during the winter, but the following
spring the yearling beavers start to shift for themselves while the parents
begin to center their interests in the activities of a new breeding season. Following the mating season, the pregnant females assume exclusive rights to
the old breeding dens or else they retire to new dens to give birth to their
young, while the adult males travel restlessly about, often far from the
home pond. By this time, the second summer of their lives, the young are
able to look out for themselves with entire independence; and they usually
hunt up burrows of their own.
In the East, muskrats have been commonly known to live in beaver dams
and in the outer parts of beaver houses. In California, there are no muskrats
native to the San Joaquin and Sacramento valleys and these smaller rodents
are therefore unknown in the habitat of the golden beaver. The only mammal that we have found associating itself closely with the beaver has been a
species of wood rat (Neotoma fuscipes streatori). At Snelling, these rats
were found utilizing the stick house of the beaver for the outer covering of
"their own nests, instead of build~ng a separate stick house..
As would be expected, the formation of ponds and sloughs by the beavers
688
very intimately affects the vital interests of quite a number of other animals.
Among these are frogs, fishes, turtles, various insects, and other essentially
aquatic types. Among birds, favoring conditions are afforded for kingfish_
ers, mudhens, herons, and several kinds of fresh-water ducks. Among the
latter, of especial interest, is the wood duck .
. . In the bottom lands around Snelling, wood ducks are restricted almost
entirely to beaver ponds. On November I, 1921,51 wood ducks were counted
as they flushed from one beaver pond; 28 flew by overhead in one flock at
one time. W. L. Means, of Hopeton, counted 65 of these ducks in one flock
on another beaver pond. On the ranch of Mr. Will Ferrell there is a third
beaver pond full of dead trees and brush that have been drowned out by
the rising water, where wood ducks are (or were) to be found the year
round. At least 150 were present on these three beaver ponds on November
I, 1921. The species was entirely absent from other open ponds and sloughs
in the region.
.
However, the spreading of the water areas and the rising of the water
table beneath the adjacent lands undoubtedly drive or drown out a number
of nonaquatic animals. Pocket gophers and ground squirrels, for example,
disappear from s\)ch territory.
Beavers in California are now just about free from any natural enemy.
Bears, mountain lions, and river otters may have disturbed them formerly.
A valley coyote near Snelling was seen in February, 1920, "snooping" about
a beaver lodge, thereby suggesting the possibility that these carnivores may
occasionally capture beavers, especially 'Young ones. But we really have no
definite occurrence of this sort to report. Man, at the present time, is the
chief enemy of the beaver.
For food and building materials, beavers levy upon certain kinds of plants.
At the same time, these same plants-willows and cattails, for instancethrive because beaver ponds are present, where otherwise they would not
be able to grow at all. There is thus in evidence a certain compensation by
which the coexistence of the beavers and plants is mutually beneficial. Of
course, the "drowning out" of nonaquatic vegetation, such as oak trees, at
the same time often does occur. "
The beaver is strictly a vegetarian (see fig. 293). Of 20 beaver stomachs
examined (D.) not one contained even a trace of animal matter. Castor as
a group exhibits maximum activity in a northern or boreal habitat. The
form subauratus is a southern, indolent race of beaver. Because the climate is
mild, the growing season of aquatic vegetation is relatively long in the habitat of the golden beaver; and having a co~stant supply of green vegetation
Golde" Beaver
689
ready at hand for a large part of the year (see fig. 300), this beaver leads a
relatively carefree existence. The ponds about his house are rarely covered
by even a skim of ice, so that all winter long he is able to come and go at
will in search of food. Here, there is no great need for storage of food such·
as exists among beavers that inhabit colder regions. Only occasionally are
willow saplings cut and stored under the water, where they form a woodpile near the den or lodge. Although storage proclivities are thus sometimes
apparent, their manifestation is very weak. This mild manifestation is the
Fig. 300. Bulrushes and a yellow-flowered composite formed' the principal food of the golden
beaver at one place during the summer season, The tips of such vegetation had not yet been
touched by frost when this photograph was taken, November 2, 1921, near Hopeton, Merced
County. Mus. Vert. Zoo!', no. 4064.
result of a habit inherent in the parent stock, name!y, storage of food for
winter use, modified to meet changed conditions in a special habitat.
California beavers cut comparatively few trees in summer. Occasionally
an observer will find a place where a thrifty green willow sprout Yz-inch
in diameter has been smoothly sliced off by a single nip of the beaver's powerful incisors. At other places the larger limbs of willow trees that overhang
or dip into the water will show white patches where the bark has been chiseled off. Beaver cuttings can, of course, always be recognized by the peculiar
contour of the tooth marks left in the wood: each incisor tooth of the beaver
leaves a shallow groove, about 14-inch in width, and since the incisor teeth
are slightly separated, but in pairs, the grooves left by them will likewise
be found in pairs, with a little ridge of wood, which has escaped the slightly
separated teeth, sticking up between each pair. (See fig. 293.) A certain
amount of gnawing seems to be necessary to keep the rapidly growing incisor teeth sharp and worn down toa proper level of emergence. Apparently
Fig. 301. In 1919, a colony of heavers ncar Hopeton, Merced County, cut
down a number of valley oaks from 6 to 12 inches in diameter. The largest
oak attacked (here illustrated) was 18 inches in diameter, but this one was
not cut through. The other oak trees were left where they fell and were not
cut up and used in building dams Of houses. Neither was oak bark or twigs
used for food. Photographed on February 22,
1920,
Mus. Vert. Z06l., no. 3260.
for this reason, dry fence posts, bridge timbers, and even oak trees are sometimes extensively gnawed by beavers that happen to feed largely on roots
and stems of tule, cattails, and other soft aquatic vegetation, and to live in
a locality where there is a scarcity of young cottonwood, willow, or other
trees which are suitable for food.
Golden Beaver
The golden beaver does the greatest amount of tree felling in November
and December. Willow, cottonwood, and ash are the principal trees cut by
beaver in California, here named in order of estimated importance as food
trees. Quite a bit of alder is cut, but this wood is used chiefly or solely in
building dams and houses. The golden beaver is the only race of beaver,
to our knowledge, that cuts down oak trees. (See fig. 301.) This is done regularly along the lower Merced River, though for no obvious purpose other
than to "exercise" the gnawing mechanism. Such trees are left where they
fall until some person comes along and cuts them up for wood.
It has been noted that trees growing along the banks of streams, when
cut by beavers, usually fall toward the water. It is to the beavers' advantage
that the trees fall streamward, for once the tree is in the water the branches
and bark are easily cut off by these chisel-toothed rodents; the beaver has
therefore been credited with the ability to fell trees in any desired direction.
In felling trees, however, the beaver is merely profiting by favorable natural
conditions, and is not to be credited with any special engineering skill.
Plants and trees naturally grow toward the light. Trees growing nearest a
stream often come into keen competition for light with other trees growing to the landward side of them; the stream or pond side of the tree receives
an unusually large amount of light, which falls unobstructedly on that side;
the leaves and branches are most numerous and heaviest on the side of the
tree which gets the most light. Therefore trees which grow on the margins
of streams or ponds are heaviest on the side toward the water. Irrespective
of how they are notched when cut off at the base, in calm weather such trees
fall inevitably toward the water. Streamside trees also, and for the same
reason, nearly always fall clear, rarely lodging in thickets, as sometimes do
trees elsewhere. (See fig. 302.)
Stumps of former streamside trees often show that the trees fell into the
water when gnawed only one-half or two-thirds through, which proves conclusively that the tree leaned strongly. In densely wooded areas, trees are
often so evenly balanced that they do not fall until completely severed, and
then they often lodge against neighboring trees and fail to come down even
when the trunk is cut through at two or three points. In cutting streamside
trees, beavers at Snelling usually made the first deep cut or notch on the
landward side of the tree (see fig. 303). A woodsman would have made the
first and deepest cut on the opposite side, that is, on the side on which he
wished the tree to fall. Ten cottonwood and oak trees from ro to 28 inches
in diameter, all growing beside streams or ponds, were examined closely on
different days while they were being cut down by beavers. In eight of the
--_.
_ _. . - - - - - - - - - - - - - - - - - -
ten trees, the first and deepest gnawing was made on the side of the tree
away from the water. It is difficult to see what advantage might arise from
cutting on the landward side of the tree first; if safety were the factor considered by the beaver, then the first cutting should be done on the streamward side (namely, on the side toward which the tree was bound to fall,
anyway) and the last cutting on the opposite side so that the beaver would
be well out of the way of the falling tree.
Fig. 302. Beavers have been credited ~ith the ability to fell a tree in any desired direction. Streamside trees that when cut by beavers have fallen into the water are often cited as proof that they were
so "intended" to fall. It is true that trees growing beside streams or ponds usually do fall into the
water when cut by beavers; but this direction is the result of gravity and not of any planning on the
part of the animals. As will be seen from the diagram, s,treamside trees such as c, d, and e have their
largest and heaviest branches on the side that receives the most light. They often lean toward the
lighted open space above the stream or pond, and when cut they naturally fall in that direction.
Experience has shown that trees, such as a, that lean away from the water fall away from the water
when cut. A perfectly balanced tree; like b, may fall in any direction.
Perhaps, after all, the direction in which the tree WIll fall is a circumstance
not conditioned by any feature of the beavers' mental make-up.
When cutting trees, the beaver stimds upright on his hind feet, braced
behind by his broad tail, which serves as a third leg or prop. The animal
then reaches up as high as is convenient, from 16 to 32 inches in ascertained
instances, and starts cutting. The direction of the tooth marks left on the
wood shows that, as a rule, when the cut was made, the beaver's head was
held at nearly right angles to the fiber of the tree trunk. The beaver bites
above and below the center of the cut and then forces out the chip by driving his teeth (lower ones chiefly) behind the piece cut off (see fig. 304).
The size of the chips is dependent upon the hardness and toughness of the
tree that is being cut. In soft, straight-grained woods, such as willow and
cottonwood, the chips are ordinarily from 1 to 3 inches long, liz-inch to 2
inches wide, and )Is-inch to \r.4-inch thick. Oak-tree chips are much smaller,
-
Golden Beaver'
averaging ry' inches in length, %-inch in breadth, and \i-inch in thickness.
Chips cut by the beaver for bedding are usually cut from straight-grained
willow shoots. These slivers are long and slender, measuring in ascertained
-;-.
,\
Fig. 303. A woodsman in felling a tree makes the first cut Of notch
in the side on which he wishes the tree to fall. In 8 out of
10
con~
secutive cuttings by beavers, examined by liS, the first and deepest
cut was made by beavers on the side away from the stream toward
which the tree leaned and finally- fell. Photographed on De6:niber
23. 1919, three miles east of Snelling, Merced County. Mus. Vert.
Zoo1., no. 3267.
instances 9 inches in length, %-inch in width, and \i-inch in thickness. Such
splinters should not be confused with ordinary chips (see fig. 305).
It has been our observation that usually a small tree is felled by one beaver,
working alone. After a tree is down, several beavers may join in trimming
up the branches andremoving the bark from the larger limbs. After the
tree has been stripped of its branches and bark, the main trunk is rarely
cut up, but is usually left just where it fell (see fig. 306).
Beavers sometimes "show poor judgment" and attempt to cut down trees
that prove too much for them. This tendency has been noted many times
by us, and three of the most striking examples are here given. On March
4, 1920, near Snelling, marks indicated that several beavers (the tooth marks
were of different sizes) had been working off and on for weeks gnawing
into the trunk of a large cottonwood tree that measured 62 inches in diam-
Fig. 304. Willow chips, showing how they are cut off at each end and then pried out by the
chisel~like front teeth of the beaver, X X.
eter at the point where the beavers were working (see fig. 307). This cut
was never finished. Although plenty of small cottonwoods were available,
this particular colony of beavers had notched several cottonwoods more
than 36 inches in diameter. They had not actually cut down any tree more
than 30 inches through; therefore much of their work had been wasted.
In December, 1919, a valley oak that measured IS inches in diameter at the
cut was gnawed one-third of the way through (see fig. 301), and this cut
was never finished, though none of the beavers was trapped or molested in
any way to prevent the finishing, and other trees within a few feet of the
oak tree were subsequently cut down.
On the Tuolumne River near Lagrange, on March I, i:92I, a cottonwood
tree was found which had recently been double-cut by beavers. This tree
Golden Beaver
was 13 inches in diameter and 40 feet high. The center of the first or upper
. cut was 35 inches above the ground. This first gnawing had seemingly been
done while the beaver was standing on a pile of driftwood that had lodged
about the trunk of the tree in a time of flood. For some reason not apparent
to us, the beaver had given up the gnawing and quit before it was half finished. Some weeks later the same beaver, or another with teeth of exactly
the same width, had trimmed off all the driftwood within 16 inches of the
tree trunk and had then started a new cut below the first one (see fig. 308).
This new cut, the center of which was 21 .inches above the ground, was
Fig. 305" The beaver makes a clean, sanitary bed-out of shredded, straight~grained wood of
willow or cottonwood. Average slivers used for bedding are here shown, three-fourths natural
size, in comparison with an ordinary small, 2-inch-long chip. ~us. Vert. Zoo!', no. 3796.
never completed. On April 12 the tree blew down in a windstorm. In the
three nights following, the beavers stripped the upper part of the tree of
its smaller branches and bark. Also, one of the largest branches had been
stripped of bark for more than 20 feet.
Although the beavers mentioned above showed poor responses, other beavers in the same general region were found carrying on their work efficiently and consistently. One example of such work is cited in the following
account. Several willow and cottonwood trees grew in a row within twenty
feet of a slough inhabited by beavers (see fig. 309). These trees were from
II to 14 inches in diameter and from 30 to 50 feet high. In a period covering
about two months, beavers had cut seven out of ten of these trees. The trees
(see fig. 310) had been cut off from 16 to IS inches above the ground. The
broken-topped stumps showed that most of the trees had leaned strongly
and that five of the seven cut would have fallen into the water, no matter
how they were notched or cut. The two trees that fell away from the water
were stripped by the beavers of their limbs and bark and then left where
they fell (see fig. 306). Trees which would fall landward were tackled and
cut clear down and trimmed. Other trees which leaned strongly toward
the water Were left standing The beavers apparently exercised no selective
choice in this particular matter, since the trees were cut in the approximate
order in which they stood along the slough.
Fig. 306. This 4o~foot cottonwood tree when cut down by beavers feIt away from the water.
Even so, the animals at once cut off and carried away the branches and some of the smooth bark.
At Hopeton, in the fall of 1920, beavers systematically cut off nearly all
the willows that grew in an isolated pond (see fig. 3II). The willows were
cut at an average height of 24 inches above the ground or shallow water,
extreme heights of the remaining stubs being 12 and 32 inches, respectively ..
This severe pruning did not kill the stumps but merely caused them to send
out sucker sprouts which eventually served as further food for the beavers.
A typical cutting of alder, used chiefly in dam building, is shown in figure
312. These alders were 6 inches in diameter and were cut 16 inches above
the ground. Such cut stumps usually rot .off at the ground and disappear
within a feW years, leaving few or no traces of beaver activity.
As a result of our observations upon beaver-cutting we are led to conclude
--------------~-"----~----------
Golden Beaver
that there is great individual variation among beaver workers. Mistakes and
apparent miscalculations are often made by certain individuals, whereas
others do not attempt tasks that are beyond their powers to fulfill. Some
Fig. 307:"In I919.:i beaver started to gnaW dO-.yll this large ~otton'Yo6d:.
tree, which was 5 feet, :i inches in diameter, even though smaller arid bettcrsituated cottonwoods 'Yete plentiful in the-vicinity. The 'cut was n~ver CO~~ ,
. pleted; the tree was still standing in 1922", Photographed on March 4; f920~'>:
. three miles wes~ of Snelling, Merced Count?_ 'Mus,.Vert.·Zool., tio. 3264_
.
beavers work spasmodically;· others carryon their work· to its c~mpletion
in a businesslike way. At least they keep continuously at it.
The beaver's main purpose in gnawing down trees is obviously, as a rule,
to procure the bark for food. Sometimes a good deal of the wood just be-
~~~~~~~--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.-~~~~~~~~--.--,-.------------~~-----
Fig. 308. On this double~cut cottonwood a beaver did quite a bit of wo~k need·
le5s1y. The upper cut at its cente! is 35 inches above the groun~. The direction of
the tooth marks, across the tree trunk, indicates that the beaver must have stood on
a pile,of driftwood that had lodged against the base of the tree. When this cut was
half finished, the work was abandoned. Several months later the same beaver. judging from the width of the tooth marks, returned, gnawed off the drift sticks about
• the base of the tree, and, then standing on the ground, began the second cut 25
inches above the ground. Before the beaver finished this second cut, a hard wind
blew the tree over. Photographed on 'March I, 1921, on the TUolumne River, four
miles southwest of Lagrange, Stanislaus County. Mus. Vert. ZoOl., no. 3619.
-------
-----~~----
Golden. Beaver
neath the bark is eaten, but this may be done more or less accidentally, just
as a hungry man is likely to take in some of the cob along with the corn in
eating a roasting ear. Bark that is old, with the surface checkered or scaly,
is rarely taken, a decided preference being shown for smooth bark of recent
growth. Although bark may be said to be the beavers' staff of life, many
other food materials are eaten at various seasons and in different localities.
In December, 1919, beavers were foraging about for acorns that littered the
ground under valley oaks that grew near sloughs at Snelling. Beaver tracks
Fig. 309. A typical habitat of the golden beaver on a slough paralleling the Merced River is
here shown. The cottonwood in the foreground had 'recently been felled by beavers au9. stripped
of its upper l,imbs and b?rk.
found at this time and stomach contents examined in January proved conclusively that acorns form a staple article of food for the beavers of that
locality during the late fall and winter months. At Cache Slough, Solano
County, on January 4,1921, beavers were digging up and eating the underground root stalks of the tule. On March I, 1921, at Snelling, beavers were
scratching up the ground under clumps of budding willows in search of
sprouting willow roots, which at that time were about the size of toothpicks,
white and tender, "and seemingly much relished. In the summer months the
white underwater st~lks of the cattail are eaten extensively, as are also the
bulbous roots of pond lilies and the stems of other aquatic plants such as
the yellow waterweed (Jussiaea). Feeding places are frequently found at
the water's edge where the beavers "haul out," to eat leisurely the white
-
--------------------------
--~----~,-----
~
700
~
-------~
IIII
!
I,
Fig. 310. Out of ten cottonwood and willow trees that stood in a row beside a
slough near the railroad bridge over the Merced River below Snelling, seven were cut
down by beavers in the fall and winter of 1919. These trees ranged from I I to 14
inches in diameter and -were cut off at a height of 16 to 18 inches above the ground.
The stumps show that the trees fell when slightly more than half severed. This fact
indicates that they leaned strongly, some toward and others away from the water.
Investigation showed that the trees feU in the direction in -which they leaned, irre~
spective of how they were notched by the beaver. No selectiv.e choice was shown for
trees leaning toward the water. The next to the last tree in the row had been deeply
notched on the landward side when this photograph was taken, March 3, 1920. Mus.
Vert. Zoo!., no. 3265.
--.~---
Golden Beaver
basal parts of cattail stalks which they obtain in the water near by (see. fig .
.3r3). Once, in August, beavers were found eating green stems of alfalfa.
The bark of apple, pear, and cherry trees is relished, and such fruit trees
growing near beaver ponds are subject to being cutfdown. We have no
record showing that beavers have bothered vegetable gardens, although in
some places known to us, they have had excellent opportunities'to do so,
and surely would have, if their food preferences had so pres~r1bed.
The beaver is a hearty eater. Because the food eaten is relatively low in
I
I
I
j
II
I!
I
Fig. 3II. Beavers regularly pruned this w.il1ow patch near Hopeton. The willows had been cut
off from 12 to 32 inches above the ground and responded to this rigorous pruning by sending
out a new growth of sprouts which were harvested by the beavers the following year. Photographed on February 27. 1921. Mus. Vert. Zool., no. 3608.
nutritive value, a large amount has to be taken. Data obtained by us from
. the examination of beavers' stomachs, on the quantity and kinds of food
eaten, are shown in the table on page 705.
The stomach of an average adult beaver is, roughly speaking, bean-shaped
and measures 6 by 6 by 3 inches. Exclusive of contents, three stomachs examined weighed 4, 4'15, and 5 ounces, respectively. One quart of finely
chewed material has been found to constitute a square meal for the average
beaver. The weight of the stomach and contents in ascertained instances
constituted Ys2 to '124 of the total body weight. If arSo-pound man should
eat asheartily as a beaver he would consume six pounds of food at a meal.
~~~~---""---------
-----
.
702
The food particles found in the stomachs of young beavers are not as finely
chewed as is the food found in the stomachs of adults. By the same token,
the young beavers also eat more wood with their bark than do the old ones
(see fig. 293). This may result from differing tooth structures.
Fig. 312, These alder stumps, which are 6 inches in diameter and 16 inches
-high; show typically how the beaver operates. Alder is used primarily for
building dams and 'houses, being rarely eaten. Photographed on March 3.
1920, near Snelling. Merced County. Mus. Vert. Zool., no. 3268.
We have found that people commonly overestimate the number of beavers in a given locality. On January I, 1920, it was "conservatively estimated"
by several persons well acquainted with the territory that there were rooo
beavers inhabiting IS square miles of river bottom in the vicinity of Snell-
Golden Beaver
.
ing. Subsequent trapping indicated that there were not more than 250 beavers in that area, and this was then admitted by the same people who had
first believed that there were 1000. Furthermore, the estimate of a trained
observer for the saine area proved 100 per cent too high. At least threefourths of the 15 square miles of river-bottom land at Snelling is cultivated
fields and pastures, separated by old river channels and sloughs inhabited
by beavers. If we include the total area, taking the fields and sloughs together, there is an average of '5 beavers per square mile or I beaver to each
42 acres. In one 400-acre section of dredged-over river channel, extensive
trapping showed that there was an average of I beaver to ,each 10 acres. In
'920, on the San Joaquin River near Mendota, trapping showed beavers to
be present at the rate of 12 per square mile of river bottom.
Figuring the density of the beaver population on the basis of average
number per mile, we have the following data. In 1921, on a favorable old
river channel near Snelling, there were present about '5 beavers on 5 miles
of slough, or an average of 3 per mile. Along the San Joaquin River near
Mendota, the beavers would average about 5 per linear mile, with 10 per
mile as a maximum along the most favorable parts of the river.
On January I, '92', the total population of golden beavers in the State
was, in our estimation, about 1000. We have visited"most of the known
colonies of beavers in the State, and some of our figures are substantiated by
known numbers of beavers caught, so that our estimates are thought to be
in truth conservative.
The recuperative powers of the beaver, as a species, are well illustrated by
the beavers whose habitat includes the Merced River bottom in the vicinity
of Snelling. Before '9II, when the State law protecting beavers was enacted,
the beavers of this region had been so reduced by continued trapping that
only a few individuals, probably not more than five pairs, were left in an
, area of IS square miles. Eight years later (1919) the beavers had become
conspicuously numerous, and in that year they caused ,much trouble by
their activities in irrigation canals. Beaver remains found in abandoned steel
traps at this time showed that some beavers were being trapped illegally.
In '920, out of a total estimated population of 228 in the territory, II4 beavers were trapped under permit. In March, '92', beavers were found in
small numbers in all places where they had been found in '920, before legal
trapping began. This was true in one place where a determined effort had
been made to remove every beaver from the vicinity of a certain dam. Nineteen had been caught in '920 on this one dam, and the locality was declared
free from the animals. Yet, just one year later, fresh sign showed that an
I
1
Fig. 313. A beaver's feeding station is usually near the water's edge on some
point or islet. The remains of food, in this figure, show that the beaver had been
eating the white underwater parts of cattail stalks. In summer this sort -of food
forms a large part of the golden heaver's diet, Photographed on June 4, 19X8, near
Mendota, Fresno County. Mus. Vert. Zo01., no. 2764.
-----------------------------------------
Golden Beaver
attempt had been made by a surviving beaver to repair the break that the
rancher had made in the dam in efforts to drain the "waterlogged" land
above it. Beavers come persistently to favorable places.
QUANTITY AND KINDS OF FOOD EATEN BY GOLDEN BEAVERS, AS ASCERTAINED BY EXAMINATION OF
STOMACHS IN NEIGHBORHOOD OF SNELLlNG,- MERCED COUNTY
Date
Total
weight of
beaver,
in pounds
Weight of
stomach
Condition
a!ld c<?,ntents, of stomach
In
1920
Feb. 28
32
16
% full
Mar. 4
22
16
% full
Mar. 5
33
20
% full
Feb. 28
34
22
%fu11
Feb, 29
42;;
25;;
% full
5
Dec. 5
Dec. 6
Dec. 7
31
36
. .....
37;;
16;;
28;;
21
...
... .. -
35
15%
~
Dec.
Dec.
34
5M
~full
19
18
35
41
20
25
48
8;;
5
20
% full
1922
Dec.
,
8
8
D~. 12
Dec. 14
D,~
Dec. 14
Dec. 15
D,~
17
Dec.
Dec.
Dec.
Dec.
Dec.
17
17
20
20
23
1923
Jan. 3
Jan. 5
Jan. 6
Jan. 6
Jan. 7
Jan. 7
Jan. 8
12%
50
6j{
7j{
42
15
7
8
38Va
lOVa
36;;
13
20
4)4
20
22
48
13 Va
31
46
35
47
Food material in stomach
ounces
7
17%
17
19
21%
7;;
Many small willow roots 1 mm. in diameter and 25 rom.
long; wood coarsely chewed
Willow wood and bark coarsely chewed, chips aVeraging
h3xl mm.; many bud scales
Chiefly willow bark and wood; many pieces coarse,S mm.
long and 2 mm. thick; willow roots
Hundreds of pieces of cattail, 10 mm. long, 2 mm. wide;
some willow bark, and one willow root 20 mm.long
Cattail; Bermuda grass root stalk 3 incncs long; willow hark
and dark-colored roots
... ...
.. ....
Acorns 10 per cent; cattai120 per cent; willow 70 per Cent
Cattail 10 per cent; willow 90 per cent
Cattail IS per cent; willow 85 per cent
Acorns 10 per cent; cattail IS per cent; willow 75 per cent
CattailS per cent; willow 9S per cent
Cattai1S0 per cent; willow 20 per cent
Cattail 50 per cent; willow SO per cent
Acorns 5 per cent; wiJIow 95 per ceut
Willow 100 per cent
Cottonwood bark and wood 100 per cent
Willow SO per cent; roots 10 per cent; cattail 10 per cent
Cattail 10 per cent; cottonwood 90 per cent
Cattail 100 p~r cent
Cattail 100 per cent
... ...
... ...
.. ....
... ..
_. .. . .
... ...
.... ..
Cattail 100 per cent
Willow 100 per cent
Ca ttail40 per cent; willow.60 per Cent
.,
full
Va full
......
......
......
.... ..
......
......
......
.....
Observation of these rodents elsewhere in the United States and Canada
indicates that the beaver has the capacity to increase in numbers rapidly
when it has adequate protection.
.
From December, 1919, to sometime in 1925 one of us (D.) was called upon
several times each year to investigate reports of alleged depredations of beaver, and, if the damage proved real, to suggest ways and means of preventingfurther impairment. In the course of this work it has been a common
dis~overy that prospective trappers have been the first and sometimes the
only persons to report that harm was being done. Reports of such depredations thus fall readily into two classes. One class of report is made by persons
who seek an excuse to obtain the valuable pelt of the fur bearer, the other
by persons who are not interested in the pelt but seek relief from real damage. The second class has the better claim to attention.
The damage done by beavers in California is varied and, at times, of rather
large extent. The following are the commoner kinds of depredation done
by beavers in this State: (r) Burrowing into levees or dikes of reclamation
works; (2) burrowing into banks of irrigation canals; (3) obstruction of
Fig. '314. Beaver burrows in this reclamation levee caused a 4o~foot section to collapse, thereby
endangering 1000 acres of growing grain. Photographed on January 4, 1921, on Cache Slough,
near Dozier, Solano County. Mus. Vert. Zo81., no. 3454.
.
drainage canals by dams; (4) flooding and waterlogging of land; (5) gnawing at head gates of irrigation canals; (6) cutting down of fence posts and
fruit trees. The. first two kinds of depredation are attributable to the beaver's burrowing habits, the third and fourth to his propensity for building
dams and creating ponds, and the fifth and sixth to his proneness to gnaw.
A large part ·of the former habitat of the golden beaver lay in the delta
region at the mouths of the San Joaquin and Sacramento rivers, and much
of this territory has recently been reclaimed. Thousands of acres which formerly were flooded at high tide have been surrounded by levees and changed
from tidal marshes into productive farm lands. But here the safety of the
crops depends primarily upon the dikes which keep back the floods.
Golden Beaver
70']
At certain points along the banks of Cache Slough in Solano County the
levees are 100 feet wide at their bases and rise abruptly 20 feet above the
low-water mark in the adjacent slough. In such places the-tops of the levees
are from 12 to 14 feet above the level fields of growing grain. Ten thousand
acres are often included in a single reclamation project. At the points mentioned above, Cache Slough has a width of from 200 to 300 feet and a depth
of 20 feet. Its banks support a good growth of willows which furnish abundant food for the beavers. The high levees, composed of fine silt and day
dredged from the bottom of the slough, are easily burrowed into and afford
ideal sites for beaver dens; in fact, they constitute the only land areas that
remain above water in the event of general flood. The.water in the slough
adjoining the levee is deep at all times, so that the combination of living
conditions is ideal for the beaver. When the water in the Sacramento River
is low, the danger from the beaver burrows in the levees is slight; but after
heavy rains or when the snows in the Sierra Nevada melt rapidly, especially
if there are extra-high tides at the same time, the water from the river is
backed up at the river's mouth by the tidewaters o{ Suisun Bay until it rises
many feet above the lowland fields that are protected only by dikes. At such
critical times, beaver burrows in the levees become a real menace because
they may cause widespread disaster. Cdntinued rain and rising waters soften
the levees where they have been undermined by the burrows so that they
collapse, and the fields are flooded through the breaks thus started.
On January 4,1921, an incipient break in a levee was visited (D.). The
first sign of trouble had been indicated by cracks in the earth where a section of the levee began to sink in above a previously unnoticed beaver burrow. This burrow, similar in plan to the letter Y, forked so that a section of
the levee 40 feet in length was affected. A dredger near by was- moved up to
a place suitable for action, arid a section of the levee 40 by IDO feet in extent
was torn up piecemeal by it, thus completely destroying the burrow (see
fig. 314). However, the clay soil of the levee had been so softened by the
water's penetrating along the burrow that it would not stand firmly under
the additional weight of earth piled on top of it, but kept oozing out so that
streams of mud ran like lava into the adjoining barley field. Several days
passed before the levee became firm enough to permit the completion of
the work of rebuilding. During this time the water stood within four feet
of the top of the break, and a sudden rise in the river would have meant the
destruction of 1000 acres of growing barley.
If beaver burrows in levees are discovered in tiroe, the greatest danger
may be averted, since they can then be uncovered and filled in before any
I
l.1 •.
-.,
dangerous stage of high water is reached; but because the entrances to such
burrows are usually under water, it is difficult to locate them and they may
therefore remain undiscovered until high water comes and the undermined
levee gives way.
The territory comprised in the delta region of California, where beavers were originally abundant, includes some 268,000 acres of marshland,
·of which more than two-thirds had been reclaimed before 1921. As more
and more of the marshland has been reclaimed, the beaver's habitat has
become gradually restricted to the deeper sloughs and the adjacent high
Fig. 315. Beavers have invaded many irrigation canals in the San Joaquin Valley. Tn such
places, as here illustrated, they cut off considerable aquatic vegetation, which "floats down the
canal, dogs the head gates, and causes the.canal to overflow. They also cause breaks by burrow~
ing into the canal banks. Photographed on November II, 1920, ten miles northwest of Firebaugh, Fresno County. Mus. Vert. Zoo1., no. 3453.
levees. In 19II, a State law was passed protecting beaver at all times; provision was made, however, for the issuance of permits "to catch or kill any
beavers that are endangering or destroying the levees or other protective
works of any reclamation district, levee district, or swamp land district."
In 19II relatively few beavers were left in the delta region, but eight years
later, in 1919, they had bred up and become numerous enough to endanger
the levees along Cache Slough, and in that year seventeen beavers were
trapped there, under special permit.
In the winter of 1919-20, noticeable damage was done to irrigation canals
l;ii
d
II
I
11
Golden Beaver
in the vicinity of Mendota, Fresno County (see fig. 315). In one place, two
large irrigation canals 50 feet wide and 6 feet deep ran parallel to each other
for a short distance. They were only roo feet apart and the beavers, by burrowing into the bank that separated the two canals, caused a 60-foot section
of it to give way; a bad break resulted; and much valuable irrigation water
was wasted because it went where it was not needed.
At another point in the same vicinity the beavers dammed up one of the
canal gates with freshly cut willow brush, cattails, and rubbish and then
diverted the water from the canal back into the river through a series of
burrows, thereby causing a serious washout or break in the irrigation canal
at a season of the year when the water was greatly needed for growing crops.
The winter of 1919-20 was extremely dry throughout northern and central California. As a result of the lack of snow and rain, the flow of water
even in the petmanent watercourses became unusually low. Where water
was available, irrigation of alfalfa fields was resorted to until February,
when the long-delayed rains arrived. In the Merced River bottom in the
region about Snelling, beavers had increased greatly in numbers, having
been protected by certain ranchers even before the State protective law went
into force-in 1911. As the season advanced, the drying up of certain beaver
ponds and the low stage of water in the irrigation canals as well as in the
river itself caused evident uneasiness throughout the beaver tribe. Every
effort was made by_ the rodents to conserve as much of the water as was
possible within their ponds. Dams were tightly chinked to reduce seepage.
Irrigation ditches that diverted water from these ponds were effectively
closed by temporary dams of brush, stones, decayed tule stalks, and mud.
The ranchers of the region were likewise anxious to obtain all available
water for their parched alfalfa fields, since the chief industry of this region
is dairying. As the season progressed, the riparian water rights became objects of contention between the beavers and the ranchers. The beavers were
on the ground first and were backing up their claim of prior right and usage
by nightly damming up the intakes of the irrigation ditches. In the morning this work had to be undone by the ranchers. One rancher, repeatedly,
would go out in the morning to find his irrigation ditches empty and his
fields dry. Walking four miles to the intake of the canal, he would find the
new-made beaver dam across the irrigation canal. With the aid of a pitch~
fork; he would remove the obstruction, and throw the material out on the
bank of the canal, where it formed a pile of rubbish. The water then flowed
unobstructed until evening, when the beavers gathered up the same material that the rancher had thrown out of the ditch and built another dam
across the irrigation canal. This procedure went on for several days and
nights, until the rancher hit upon the scheme of frightening the beavers
away from the canal by hanging a lighted lantern on a stake at the point
where they were wont to start their work. This ruse was successful for several nights; but the beavers soon lost their fear of the lighted lantern and
within ten days' were back again, building a dam across the canal a few
yards below the lantern. The rancher then made an effort to drain water
from one of the beaver ponds by inserting a I2-inch pipe through the beaver
dam near the bottom of the pond. The pip~ was extended 10 feet into the
Fig. 316. A pair of beavers plugged this culvert every night with sticks and mud, causing the
water to back up and flood the road. Each morning, a man removed the obstruction. In three
weeks he had heaped up'two piles of rubbish, each 4 feet high and 6 feet long. As building
ma~
terial grew scarce, the beavers started to levy on these rubbish heaps, and the m~n grew impa~
tient! Photographed on March 4, I920, near Hopeto.q, Merced County. Mus. Vert. Zoot., no. 3258.
clear water of the pond so that the open end would be clear of the dam. The
first night after the water level of the pond was lowered, the beavers found
the open end of the pipe and plugged it with sticks, decayed vegetation and
mud. The rancher made repeated efforts to keep the end of the pipe open,
but the beavers were likewise determined that it should remain closed, and
by perseverance they carried their point.
Steel traps were then set in the shallow water of the irrigation canal where
the beavers had a dam, and several of the animals were caught. These
trapped beavers were soundly spanked by the rancher, who used a good
stout board for this purpose. He had protected them on his ranch for years
,
Golden Beaver
7I I
and still did not want to kill them. After they had been spanked and turned
loose they stayed away for a while, but a few weeks later some of the same
beavers, identified by the trap marks on their toes, were again caught in
this canal.
In another instance a mated pair of beavers insisted on nightly damming
a culvert and flooding the fi,eld and adjacent road so that trucks had difficulty in crossing the soft gtound. The rancher removed the rubbish that
choked the culvert eachdayfor three weeks until he had stacked up two
piles of sticks 4 feet high and 6 feet in diameter (see fig. 316).
Another source of damage traceable to beavers lies in their building dams
across drainage channels which have been dredged to carry off flood waters. At Snelling, in 1920, four ~)'ch drainage canals were repeatedly obstructed in this way (see P.793).
Many acres of good farm land inthe',Merced River bottom have been
flooded because beavers have built dams in efforts to form ponds. In many
places where the soil was heavy, the general raising of the water level
"waterlogged" the land for appreciable distances around the beaver ponds.
In certain places the water level was thus raised 3 or 4 feet until, over large
areas, it stood within 18 inches of the surface of the ground. This undue raising of the water table caused the taproots of the alfalfa to decay. Relief was
not obtained until both the beavers and their dams were done away with.
In December, 1919, beavers did quite a bit of damage near Mendota by
gnawing through the braces and weir boards of head gates of irrigation
canals. This locality was visited on December 26, 1919, and investigation
showed that the beavers had gnawed through several 2 by 6-inch redwood
weir boards and, further, had endangered the entire head gate by gnawing
off several pairs of 2 by 6-inch redwood braces at or below the surface of
the water (sedig. 317)' The gnawing above the water was easily located
and repaired, but the gnawing off of the timbers beneath the water was
more serious because such damage was difficult to find and hard to repair.
Furthermore, such cutting was likely to result in the unexpected collapse
of the entire head gate, which was 50 feet long, IO feet high and, at that
time, served to raise the water level 4 feet.
On December 23,1919, near Hopeton, Merced County, an observer noted
damage done to the posts of a barbed wire fence that had recently been
built beside a ditch inhabited by beavers. The posts had been cut from Lombardy poplar trees and had been put in the ground while still green. Of 14
consecutive posts in this fence 4 had been cut off at the ground and stripped
of bark by the beavers (see fig. 318). One post had been cut into three sec-
7I2
tions in the efforts of the animals to get it loose from the wires. Failing in
this, they proceeded to chisel off all the bark together with some of the wood.
In this same locality on August 7, I920, some beavers were feeding exten-
Fig. 317. Near Mendota, beavers gnawed through several 2 by
6~inch redwood braces, thereby endangering the entire head gate of
a large irrigation canal. Photographed on December 26, 1919.
Mus. Vert. Z061., no. 3461.
sively on green stems of alfalfa which grew in a field at the margin of their
home pond. A quarter-acre of alfalfa, then just in blossom, had been close
cropped by these harvesters. In the early morning, freshly nibbled stalks
of alfalfa were found strewn along the beavers' much-tracked path that led
from the pond out into the alfalfa field.
Golden Beaver
Several old apple' and pear trees which grew about fifty feet away from
this same beaver pond were cut down by the beavers (see fig. 3I9). One of
the tree trunks about 8 inches in diameter was sawed off, after the beavers had nearly severed it, and shipped to the California Fish and Game
Commission, which for a tinte put it on exhibition. Trees such as willow;
cottonwood, and alder, which form the usual food source of the beaver,
are short-lived woods of little commercial value except for firewood. Loss
to man from such activities of beavers in California may occur locally, but
elsewhere is inconsequential.
The common benefits to be derived from the beaver in California may
be listed as follows: (I) Educational or esthetic value; (2) value of the
pelts; (3) value of beaver ponds used as pumping basins; (4) value of beaver dams used as diverting dams for irrigation canals; (5) value of subirrigation of sandy soil by the raising of the water table. These benefits will
now be considered in the order in which they are given above.
The beaver has a distinct educational and recreational value. In the majority of people, curiosity and then interest are quickly aroused by the unusual character and amount of the work done by beavers. Because night is
the tinte of their activity, wild beavers themselves are seen by relatively few
persons. It is usually the UJork of these rodents rather than the animals themselves which first attracts attention. Where beavers live in the vicinity of
human habitations and have been left long unmolested, they become relatively tame and are then likely to be seen abroad in the early morning and
late evening.
At Belle Island, near Mendota, Fresno County, the beavers, after seven
years of protection, became so accustomed to man's presence that they continued to occupy three lodges from IDa to 200 feet distant from an inhabited
human dwelling. In the summer months, Mr. Wm. L. Ayres, the owner
of this house, frequently observed the beavers, sometimes three or four of
them in sight at one time, swimming about within a stone's throw. In his
unfenced garden, patches of onions, potatoes, tomatoes, and beans were
growing within a few feet of one of the beaver lodges; yet the vegetables
were never eaten by the rodents. The only difficulty experienced was when
the beavers attempted to dig a canal and started to burrow under the potato
patch. Mr. Ayres dumped a lot of empty tin cans into this burrow, and this
discouraged the beavers so that they abandoned work at that point.
One of us (D.) was strongly impressed with the ease of observing the
Belle Island beavers when, at 5:20 o'clock on the morning of June 2, I9I8,
he approached in a boat to within twenty feet of a.large adult beaver. This
I
l
particular animal was unconcernedly floating about, nipping off the green
succulent stalks of an aquatic plant known locally as "kelp." A sudden
movement of the observer in reaching for a camera caused the beaver to
dive, after "popping" his tail and splashing water all over the boat and
its ·occupant.
The beaver is one of our most popularly appealing native mammals and
should be conserved and encouraged for this reason alone, when other considerations permit. It might be advisable, even, in certain districts where
Fig. 318. This green poplar fence post was cut off and stripped of its bark by a hungry beaver.
Several posts in succession were similarly treated. Photographed on December 23, 1919, one mile
west of Hopet~n, Merced County. Mus. Vert. Zool., n,o. 3270.
they are somewhat detrimental to the works of man, to condone their shortcomings on the score of their esthetic and educational value-to maintain
them so·that they will be accessible for observation by the public at large,
both adultsartd children. The latter, especially, find in beavers and their
works a keen stimulus to wholesome enquiry.
In the region about Hopeton,Merced County, when this animal was numerous there, beaver ponds were frequently utilized by ranchers as sources
of irrigation water- When the crops began to dry up during extremely dry
seaSons, large centrifugal pumps were installed at the beaver ponds and the
crops were saved. Because the soil is sandy, a large volume of water is necessary to flood the alfalfa fields. Ordinarily the farmer would have gone to
quite a bit of expense in excavating a suitable pumping basin to provide the
volume of water ·needed.
Golden Beaver
I
j
Ii
At the homestead of W. L. Means, near Snelling, there is a large beaver
dam known (I930) by old residents to have been built by beavers more than:
sixty years ago. This dam (see fig. 280) in I920 served as a diverting dam
for two large irrigation ditches that supplied water to several hundred acres
of alfalfa. Other beaver dams in this same region were used to divert water
into irrigation ditches. Not only did this save the. rancher the expense of
constructing the dam but the beavers also took charge of the maintenance
of the dam, keeping it in constant repair.
.
Fig. 3I9. Beavers cut down and then trimmed off and carried away the branches of this ~id
pear tree. Cherry and apple trees growing near beaver ponds have met a like fate. Such damage
can be prevented by putting heavy galvanized wire nettipg around the trunks of the trees. Photo~
graphed on February 27. 1921, one mile west of Hopeton, Merced County. Mus. Vert. Zo01.,
no. 36°7.
.
As has been pointed out previously, there are certain localities in Cali.'
fornia where the obstruction 6f streams by beaver dams has raised the water level and has done injury by flooding or "waterlogging" rather large
areas. On other ranches, however, perhaps along the same streams, the
building and maintenance of dams by beavers and the resultant raising of
the water table has been of decided benefit.
The acreage affected in this
.
way is sandy river-bottom land that must be irrigated during the summer
months if maximum crops are to be produced. In certain neighborhoods
the streams have cut down their beds until they are several feet below the
surfacedf the·fields. In the summer, the top soil, being hot and sandy, dries
.
-~
out quickly, and the moisture below is too deep down to get to the surface
in an adequate amount. Then the beavers come along and build a series of
dams across the channel; this raises the water level three or four feet; the
moisture can then work up to the roots of the growing corn, which is thus
enabled to mature without surface irrigation.
Many ranchers on sandy farms in the Merced River bottom have thus
been able by the help of the beavers to raise good summer crops on ground
that previously produced but little. One rancher stated that the beavers have
Fig. 320. The barren, desolate nature of bottom land recently dredged for gold is here shown.
There are thousands of acres of such wasteland along the larger streams of California. Photo~
graphed on March I, 1921, near Lagrange, Stanislaus County. Mus. Vert. Z081., no. 3620.
saved him several hundred dollars each season. He has found that these ani- .
mals will assist in the productiol). of crops, by building dams and maintaining the water level, just as faithfully as does the horse that draws the plow.
If anyone wishes to raise this man's ire, just let him try trapping beavers
on his ranch!
The beaver's pelt has gained worldwide esteem. The fur is one of the most
durable known. It has been given by experts a durability rating of from
8S to 90 points out of a possible 100. Beaver fur easily surpasses the fur of the
famed fur seal in durability, and is only slightly less durable than the fur
of the sea otter, river otter,and wolverine, which are rated at 100.
In 1<)20 the beaver ranked tenth, in point of value, among the fur bearers
trapped in California. Between January IS and March IS of that year more
---------------------------
Golden Beaver
than 100 beavers were trapped, under permit, where it had been declared
they were a nuisance in continually obstructing irrigation ditches and canals.
One lot of 75 raw beaver skins, an average run, was sold in March, 1920, to
a San Francisco firm for $2,492.50, an average price of a little more than
$33 per pelt. This probably represents the maximum average price ever paid
for California beaver pelts. Although the ordinary price of beaver skins has
been much less than $33, the beauty, durability; and increasing scarceness
:Fig.
321.
I:'onds in dredged-over lands soon become fringed with cattail and willow and then
furnish ideal homes for beavers. Where beavers are endangering reclamation levees and irrigation canais, they could be caught alive and transferred to near-by suitable wastelands where they
would become an advantage rather than a disadvantage to the people of the State. Photographed on April.15. 1921, four miles southwest of Lagrange, Stanislaus County. Mus. Vert.
Zool., no. 3623.
of this fur seems likely to ensure for prime beaver pelts even higher prices
in the years to come.
In several known instances, beavers have lived in st~eams and ponds 'of
pasture lands among cattle and hogs. These beavers have been "raised" with
much less expense and trouble to the owner than an equal number of cattle or hogs in the same fields. Some beavers have actually worked for the
farmer's benefit, as has been previously shown. And furthermore, with regard to their food, there was little or no competition with his stock. There
appears to be no good reason why beavers, through their fur value, could
not be made to yield a regular income on such ranches.
Along the larger rivers of this State, there are thousands of acres of
dredged-over lands where, as a result of gold-mining operations, little is
---------------_.
left but huge piles of cobblestones (see fig. 320). Over a period of years,
vegetation on these areas is so scant that it will hardly support cattle or even
sheep. Numerous depressions that extend below the general water level of
the waste areas have become filled with seepage water, creating small ponds
soon fringed with cattails and willows (see fig. 32I). These rock-pile ponds
form ideal homes for beavers. In I920, twenty beavers were trapped, and
at least that many were left, on a 400-acre "rock pile" near Snelling. Those
left provided a "breeding stock" of at least twenty beavers, or ten pairs. If
each pair raised only two young each year there would be supplied an
annual harvest of twenty beaver pelts. The only cost connected with raising
and marketing this crop would be the work of trapping, which in such
localities is simple. If we consider prime beaver skins worth $25 each, the
beavers on the area specified would yield a yearly income of $500 froll) otherwise useless property. We believe that much of the waste, dredged-over land
along the Merced, Tuolumne, and American rivers, wherever these animals
are not already reestablished, could be used to support beavers and thus be
made an asset to the entire State.
We have shown that at certain times and places in California beavers have
become so destructive as to outweigh their values, both educational and
commercial. In such instances, where the danger is acute, outright destruction of the beavers is justifiable. Trapping with steel traps has proved the
most effective way of destroying them. When beavers must be trapped, this
should be done as surely and at the same time as humanelyas possible. Some
of the trapping methods, those that have been found best adapted to conditions in California, are here given.
The NO.4 Newhouse steel trap is generally regarded as "the" beaver trap.
It is advisable always to set the trap in such a manner that the beaver when
caught will plunge into deep water and be held there until drowned. No
trap should be set so that the victim will remain on land or in shallow water,
because in slich event the trapped animal thrashes about, suffers needless
pain, and often escapes by breaking the bones of the foreleg and then pulling out, leaving its foot in the trap. The advantages of drowning the beaver
quickly are: the animal's sufferings are short; the danger of the animal's
escaping, especially in a crippled condition, is less; the pelt is not liable to
mutilation; and, being under water, the pelt of the trapped animal does not
readily spoil and is safe from theft.
Where beavers build dams, the customary way to trap them is to make a
slight rift or .break in the dam, and then "bed" the trap on the upper face
of the dam, upstream from the break, in about five inches of water. When
--------
-------
--------
Golden Beaver
. the beaver comes to repair the break he will spring the trap and be caught
by a hind foot-provided the trap isn't sprung by the brush or mud that he
carries to repair the break. In this, as well as in other beaver sets to be described, the trap should be anchored as follows. Fill a barley or grain sack
one-fourth full of sand or rocks. Tie the neck of the sack securely with a
wire. A "drowning wire" consisting of a I2- or I5-foot piece of heavy galvanized telephone or fence wire should be fastened to the sack of rocks.
The ring at the end of the trap chain should be slipped over the drowning
wire, which then should be made fast to a stake driven into the dam until
the top of the stake is below the water level. Twist a couple of kinks near the
weighted sack into the drowning wire (which may be a section of barbed
wire with the barbs all bent downward in one direction); then throw the
sack of rocks out into deep water.
As soon as a beaver is caught, he seeks to escape into deep water. The ring
in the trap chain slides freely down along the wire, but will not slide back
up again, because of the kinks or the barbs on the wire. The beaver is thus
held under the water by the weighted sack and quickly drowns. The drown"
ing wire is preferable to a "drowning pole" because it cannot be severed
'by the beaver's teeth, catches less driftwood, is lighter and easier to handle,
and is surer in action.
Where the ponds are shallow, it has been found feasible to drown beavers,
even in so little as eighteen inches of water, by wiring a plowshare or other
heavy flat weight directly onto the under side of the trap, which is then
attached to a drowning wire, as previously described. The trap is then set
in two or three inches of water, and in a position to catch the beaver by a
front leg. As soon as the trapped beaver seeks to escape into the deeper water,
his head is pulled under by the weighted trap, and he is drowned.
Beavers living in banks frequently leave little sign, and it is sometimes
difficult to find places to set traps for them. Such individuals are often the
ones that cause the most trouble by burrowing into dikes and levees, and
their removal therefore becomes important. The best way of trapping them
is to work from a boat, setting each trap in a hole or imitation beaver slide
made by the trapper in the bank.
The golden beaver is the last of all our fur bearers to "prime up" in the
fall. Beaver pelts should not be sought before December I, and trapping has
shown that the best pelts are to be had in January and the first part of February. By March ro, a few skins show dark spots on the flesh side, indicating
that such pelts are becoming unprime. December 15 to February 15 is the
proper time of the year to get the best returns on beaver trapping.
720
Beaver skins should be taken off flat. The opening cut should be made
from the chin down the belly to the base of the tail. Lateral cuts are then
made from the midventral cut down each fore and hind leg. Beavers are
skinned with difficulty because the skin adheres closely to the layer of fatty
tissue that surrounds the body and has to be cut free. If an experienced man
skins a beaver and "fleshes" the hide in an hour, he is doing well. The
skinning should be done with a sharp knife so that the fatty tissue will be
left on the body and not on the hide. The fleshed skin should be stretched
by lacing it inside of a suitable hoop. This can be readily done by using a
sacking needle threaded with coarse
twine and lacing the edges of the
pelt evenly to the inside of an old
buggy tire. Old tires can be cut and
easily adjusted to various sizes of
hides (see fig. 322). Any surplus
flesh or fat that may cling to the skin
can be removed after the skin is
stretched. After being stretched, the
pelt should be dried in an airy, shady
place. When dried, the skins should
preferably be kept flat, and when
stacked up, should be placed in pairs
back to back so that the fur sides will
come together. Then the fur will not
become greasy and soiled.
In many respects the beaver situaFig. 3-22. The stretched pelt of a 62~pound
tion in California from 19" to 1925
beaver. Note the shape of the skin and the
was ideal. The law prescribed commethod used in stretching it. In March, 1920,
75 California beaver skins taken under permit
plete protection for the beavers, yet
sold for $3Y50 each.
at the same time provided for their
removal without unnecessary "red tape" when it was shown that real damage was being done by the animals. Furthermore, people who trapped beavers illegally found that the law protecting beaver "had teeth in it:' One
offender was fined $100 and lost all the beaver skins that he had trapped
and shipped. The law provided that before California-taken beaver skins
could be sold or disposed of legally they had to be reported to the State Fish
and Game Commission, which then issued tags authorizing their sale. The
trapping permit thus had to be supplemented by special tags, if the skins
were to be marketed.
-
Golden Beaver
I
I
l
721
The pelts of beavers have been of sufficient value to be eagerly sought for
by trappers whenever a permit was obtainable. Therefore, whenever it became necessary to reduce the number of beavers in a certain section, their
removal could be promptly accomplished at no expense to the taxpayers.
It may be that one factor in the success of beaver protection in California
is the prevalent attitude of respect or consideration on the part of people
generally toward this animal. This attitude may be linked in people's minds
with their idea of the beaver as an ingenious and industrious animal, wellnigh human, according to the concept. This of course is the feeling of the
more intelligent class of people; for there remains the less thoughtful man
whose tendencies to "poach" are only repressed by fear of the law. The
heavy fine imposed in known cases operates potently with such men. It is
our opinion that the beaver received from 19II to 1925 the best legal protecti~n of any fur-bearing mammal of the State; and this is proved by the increase of beavers which wok place during that period, wherever natural
conditions were favorable.
By legal action in I925 protection was removed from the entire range of
the race, golden beaver. During the first two years of this open season the
beaver population in this area was severely reduced. From that time until
the present writing (1931), the decrease has continued until it seems possible that eventually this species may be eliminated entirely from the Great
Valley of California. This will surely occur if protection is not restored.
Mr. Herbert Reimche of Lodi has trapped for seven years on a 40-mile
line between King Island and Antioch. In the 1930-31 season he caught 7
beavers; the year before he got 4. In the second year of the open season
(1926-2'7) he trapped 37, and another year he caught about 20. The species
is scare now in his vicinity. A few are left in sloughs within the "islands,"
where trappers do not go. No damage is reported now; for all the remaining
individuals, having no fixed homes, move about continuously, according
to Reimche. Some of the trappers now want the open beaver season closed
again, as do also the fur buyers, in order to permit the species to regain its
former position in the general fur output.
Mr. W. L. Means told us (L.) in 1931 that since the first great reduction
along the Merced River near Snelling, when more than 200 beavers were
taken out of that vicinity under permit, before the 'season on, them was
opened, they have kept on declining. He thought that a few were still about.
Mr. Means said that even though the animal is interesting, it seriously damaged his alfalfa fields so that he could not afford to have it present in any
numbers. He wanted to see the beavers kept down, but not exterminated.
SHASTA BEAVER
Castor canadensis shastensis Taylor
NAMEs.-Casto1' canadensis, part; Castor canadensis pacificus, part; Castor subauratus shastensis; American Beaver, part; Pacific Beaver, part; Beaver, part.
General characters.-External ones presumed to be practically the same as for the
OTHER
golden beaver. The characters ascribed pertain to the skull.
.
Comparative description.-We have had but meager opportunity to examine specimens representing this race, but from the description of it given by Taylor (1916, p.
433) we have selected the following points. In C. c. shastensis the nasals do not taper
so regularly or rapidly posteriorly as in C. c . .subauratus. The lateral nasal outline is, in
shastensis, invaded by the posteromedial part of the backward-extending tongue of the
premaxilla. The nasals tend to maintain their full breadth farther posteriorly in shastensis than in subauratus. The temporal ridges in shastensis tend to form a distinct
sagittal crest posteriorly and to show a higher degree of approximation anteriorly. This
holds in specimens of the age which in subauratus exhibits a weak development of the
crest posteriorly, and a lyrate arrangement of the ridges anteriorly.
The interorbital constriction is broader in shastcnst"s, and this difference holds
throughout the series studied by Taylor in specimens of comparable age. The frontomaxillary suture, situated dorsally on the skull be~ween the backward-extending tongue
of the premaxilla on each side and the malar, is longer in shastensis than in subauratus.
This holds for all specimens irrespective of age. The interparietal is somewhat broader
in all specimens of shastensis, old and young, than it is in any specimen of subauratus.
Type locality.-Cassel [= Hat Creek], near Pit River, Shasta County, California
(Taylor, loco cit.).
Later, the series of specimens of this beaver in the National Museum was studied
(L.) and the items in the description cited above were verified. Measurements, in millimeters, of the skulls of two of these specimens, a male, no. 5I477, and a female, no.
50979, are as follows: Basilar length, I2I.3, 123.7; zygomatic breadth, 98.2, 99.8; interorbital width, 29.6, '30.1; mastoid breadth, 70.6, 69.6; greatest length of nasals, 48.1,
49.3; width of nasals,_ 26.1, 27.7; vertical diameter of foramen magnum, 13.2, 10.1;
transverse diameter of foramen magnum, 19.9, 18.2; alveolar length of upper molar
series, 31.8, 3I.6.
Distribution area in California.-The Pit River and Klamath River drainages in the
northeastern corner of the State, altogether east and northeast of the UNarrows" of the
Pit River, and therefore at altitudes above 3000 ft. (see fig. 259).
Specimens examined from California.-None in the Museum of Vertebrate Zoology;
those described by Taylor, 5 skulls-only, are in the United States National Museum,
from Cassel, Shasta County.
Aside from the fact of occurrence of beavers within the Modoc region
of northeastern California from earliest historical times (they were trapped
there for fur as early as the winter of 1826), we have been able to gather
but meager information about the race of Castor peculiar to that region.
Doubtless in all matters of life history it is essentially like the races of the
interior and northern parts of the West where the winters are cold, rather
than like the golden beaver native to the warm lowlands of the great central
valley of California.
Shasta Beaver
I
:1
Several localities where beavers occur in Modoc Co,-\nty have been reported to us within recent years. Mr. W. S. Brown, of the Forest Service
office in Alturas, wrote to us in 1920 that he knew of a colony on the north
end of the Modoc National Forest, on one of the heads of Willow Creek,
about ten miles northwest of Goose Lake Ranger Station. Beavers had also
been seen on Willow Creek just west of Boles Meadow. Farther down on
this stream, west from Steele Meadow, on September 30, 1922, old beaver
cuttings were found (G.) in masses of drift left at high water. These were
sections of peeled willow as much as 4 inches in diameter. Also, some fresh
sign was found, which consisted of cut rushes with the white bases eaten
off, freshly peeled willow twigs in the water of a slough, and freshly cut
willow stubs. A mere suggestion of a dam was seen in a mass of tules and
willow across the end of a small slough. The entire structure was not more
than 6 feet wide and 6 inches high. It was estimated that there were about
twenty-five beavers then -living along this part of Willow Creek and the
tributary sloughs.
In October, 1922, George W. Courtright, then a fish and game deputy
stationed at Alturas, told us that there were beavers on the Pit River, sixteen
miles north of Alturas, that is, just below Goose Lake.
Mr. Hubert L. Person, of the United States Forest Service, has told us that
in 1929 he saw the work of a colony of beavers at the head of Lassen Creek,
between altitudes of 6000 and 7000 feet, east of Goose Lake. The animals
there were feeding upon aspens. There were several dams, the largest about
100 feet long. Also, he knew of some beaver work on Lassen Creek between
the Alturas-Lake View road and Goose Lake.
The status of the beaver in Modoc County in 1931 is here given in detail
as it was supplied by Deputy Fish and Game Commissioner A. A. Jordan, of
Alturas, through Mr. James Moffitt. In September, 1931, there were, according to Jordan's statement, three colonies of beaver on Fletcher Creek. One
consisted of about 6 animals living right at the Willow Creek Ranch. Another, composed of 10 or 12 individuals, covered about a half-mile of territory farther up Fletcher Creek; and the largest colony in the entire section,
numbering between 20 and 25 individuals, was living near the Mulkey place.
There was a colony consisting of some 15 or more animals on South Willow Creek about two miles northwest of Boles Spring Ranger Station. The
colony formerly on the North Fork of Willow Creek near Steele Swamp·
had apparently been destroyed by trapping.
There were about 6 animals in a colony in Jack's Swamp about eight miles
north of Canby.
!
I
J
-
The largest colony in the county was on Lassen Creek and extended from
a point about two miles above where that creek enters Goose Lake, for a
distance of two and one-half miles. Jordan estimates that there were 50
beavers in this colony. There was a small colony at the head of Lassen Creek
in Fandango Valley. About 4 or '5 beavers were living on Davis Creek where
it runs into Goose Lake. There were 4 or 5 individuals on the North Fork
of the Pit River about sixteen miles north of Alturas.
Mr. Jordan states that in some places the beavers caused noticeable damage and that they were an annoyance to ranchers because their work interfered with irrigation operations. He received numerous complaints and
sometimes requests to trap and remove the animals.
Beavers have also been reported from the Shasta River in 1898 (Merriam,
1899, p. 92), and from Scott River near Callahan, trapped out previous to
19" (A. M. Alexander, MS); these two streams, in Siskiyou County, flow
north into the Klamath River. Mr. H. S. Prescott, then Fish and Game Deputy at Requa, Del Norte County, wrote us in a letter of January 29, 1922,
that he had evidence (from sign seen and reports of killing of the animals)
that beavers were present in the Klamath River above Requa at least during
the years 1915, 1916, and. 1917. Because Willow Creek, Modoc COl,Ulty, is
tributary to the Klamath River Basin, it may be assumed (though this is
merely assumption) that the reports from far down the Klamath River,
even nearly to the sea, pertain to the same subspecies as that which occupies
the upper part of the Klamath Basin, in .south-central Oregon and northeastern California. That the Klamath beavers likely belong to the same race,
shastensis, as the Pit River animals can then be argued from the facts that
the elevation of land was low and the distance (until very recent years) was
short between the extreme head of Willow Creek and the western shore of
Goose Lake. The latter flowed out to form the North Fork of the Pit.
SONORA BEAVER
Castor canadensis frondator Mearns
OTHER NAMEs.-Castor fibe1'~ part; Castor canadensis, part; Casto1' canadensis repentinus[?]; Broad-tailed Beaver; Colorado River Beaver; Beaver, part.
General characters.-Presumed to be practically the same as f?f the golden beaver.
Comparative description.-Our material representing this race in California consists (in 1931) of skulls-only from young individuals. Therefore it has been impossible
to verify the systematic characters of this race as they are exhibited in California. From
other sources, especially Taylor (I9I2, p. I67), we learn that this race, Castor c. fronda~
tor, approaches closely Castor c. subauratus in characters. In color of pelt, fr'ondator is
lighter than subauratus. Dorsally, the coloration of overhair is much the same, "hazel
to clay color" in both races. The underfur in frondator approximates Prout brown; in
II,
Sonora Beaver
subauratus it is between chocolate and seal brown, nearer the former. The ventral
underfur in fondator is gray, varying about drab gray, ecru drab, and drab; in subauratus it is sepia. Castor c. fondator has less overhair than has subauratus.
The most important difference in the skulls of these two races detected by Taylor
(1912, p. 168) was in the form of the nasals. He found the nasals in {Yondator to be
less expanded than those in subauratus. In a later review of the beavers, Taylor (1916,
p. 457) noted that the "nasal outline, condition of median process in the interpterygoid
fossa, and certain skull dimensions, show the affinities of (fondato1' to be with canadensis rather than with either subauratus or pacificus."
Type locality.-San Pedro River, near United States boundary, Sonora, Mexico
(1fearns, r897,pp.502-503).
Later (1932), we were able to examine the skin of a specimen (no. 4494) in the
collection of the San Diego Society of Natural History. This individual was a young
male collected on November I, 192,3; by Laurence M. Huey, at Bard, Imperial County.
External measurements, in millimeters, were as follows: Total length, 1018; tail vertebrae, 310; hind foot, 180; ear, 35. Comparison of the flat skin with pelts of Castor canadensis_ subauratus, also winter-taken, reveals a sti-iking difference in coloration. This
skin exhibits the same general pale rusty tone of color that distinguishes muskrats of
this region from the darker, -more horthern races. Although both the overhair and the
underfur show the paler color, the contrast seems to be more maiked in the overhair.
The Colorado River specimen is notably pale about the head and face, on the lower
surface, and on the rump. Depth of pelage and density of fur cannot be judged accurately, but the specimen of [i-ondato1' seems to have a thinner coat of fur than does
subauratus.
The tail in this specimen is conspicw:msly broad. The scaly part measures in greatest
dimensions 138 rnm. by 235 mm. This provides a ratio of 54-5 per cent, which is 6 per
cent higher than in any specimen of subauratus measured, and decidedly above the
average of all specimens of that race measured. For discussion, see page 641.
Distribution area-in California.-Along the Colorado River from the Nevada line
to the Mexican line (see fig. 259)' Also, of recent years, the large canals, and the sloughs
of the old river channels, north of the Mexican line in the Imperial Valley.
Specimens examined from California.-Skulls-only, contained in the Museum of
Vertebrate Zoology: Imperial County: Colorado River near Palo Verde, 3; near Manganese Wells, 1. Total, 4.
It was the quest of beaver pelts that led to the first crossing of the southwestern deserts by white men. For this reason, Jededi;ili S. Smith in 1826
and James Ohio Pattie in 1827 both made their way over parts of the course
of the Colorado River, incidentally reaching also the coast of southern California. On December 10, 1827, Pattie's party of eight men trapped as many
as thirty-six beaver in forty traps in one night, on the Colorado River somewhere in the vicinity of Yuma. Ever since then, this has been a famous trapping ground. At intervals it has been "trapped out" and deserted by the
trappers. With the lapse of a few years, the beaver have then bred up to a
point where they have again become profitable to trap. In 1930, one trapper,
Robert P. Bolam, reported taking ninety-seven beaver pelts along the Colorado River, for which he received $1300.
J. C. Draper, who lived at Picacho, Imperial County, wrote to us in 1922
that beaver had been rare when he first came to the Colorado River in 1898.
At the time his letter was written, beavers had become fairly plentiful and
many of them were being trapped.
The expedition from the Museum of Vertebrate Zoology which explored
the lower Colorado Valley in the spring of 1910 (Grinnell, 1914a) found
signs of beaver at many places ,ilong the river from near Riverside Mountain
to Pilot Knob. At that time it was thought beavers were scarce because professional trappers had recently pursued them too energetically.
Near the base of Riverside Mountain, sign of beaver was seen on March
16. Many young cottonwoods had been felled at the edge of the main river
channel, or within twelve feet of it, where its bank sloped steeply into deep
water. The trees had been cut off about one foot above the ground and all
had fallen downstream, diagonally and toward the river. The largest cottonwood was 14 inches in diameter where it was cut; the next two, 7 inches
in diameter. Other cuttings ranged down to )-i-inch willow shoots.
A stick house found at the edge of the river a short distance below these
cuttings resembled a mass of drift caught among the snags that form as
a result of the undercutting of the timbered bank. This house, measuring
3Yz feet in height by 14 and 16 feet in two diameters, consisted of small
branches and broken saplings from the driftwood of the river and some
brush dragged in from the land side. A little earth and dead leaves had been
worked into the interstices. Fewer than 12 beaver-cut sticks were in evidence
in the entire structure. There were no fresh signs in or around this house.
Probably the beavers did not need to cut trees, because the river provided
them with so much suitable food by continually undercutting its banks,
thereby precipitating many green trees into the water. Also, the continued
shifting of the river channels may have discouraged them in their habit of
building dams.
On April 3, we cut into an occupied house at the margin of a slough near
Palo Verde. The structure, measuring 3 feet in height by 12 feet across at the
base, was composed of branches and small saplings cut when in full leaf and
laid compactly. The mass was 8 inches thick over the nest cavity. The interior space was 2Yz feet high by 4 or 5 feet across. The house was near the
top of a sloping bank on~ side of which had been dug away, making the
floor of the house comparatively level. One side of the lodge had an underwater entrance and an opening through the wall aboveground. A fresh willow sapling had been hauled, butt first, through this opening. The beds
were merely hollows in the earth floor.
,
Sonora Beaver
Near Pilot Knob there was considerable beaver sign, which consisted of
well-beaten trails up the bank, footprints, tail marks, and cutwillowsaplings.
Mr. Frank Stephens has informed us (letter, 192I) that before the settlement of Imperial Valley and its development for agriculture there were no
places in it suitable for beaver, In those days there was no wet land between
Salton Sea and the Lower California boundary. There, the two river channels were shallow-ro to 20 feet deep-and the lakes left by the overflow
usually went dry at some season of the year, Stephens never saw a beaver
north of the boundary, but he knew of the occurrence of that species south
of the line, in the delta region of Lower California.
Beavers began to invade the Imperial Valley about 19II along two different routes: by the Alamo River and main Imperial Canal on the east side
of the delta, and by way of the Black Butte Canal on the west side. In 1921
it was estimated (D.) that there were IOO beavers in the Imperial Valley
north of the International Boundary. Nearly half of these were in the Alamo
River, and the others were scattered about in the larger canals. No beaver
sign was noted along New River on the American side. A thriving colony
had been established on the Alamo within half a mile of the center of the
town of Holtville. Serious breaks in the canal banks, have been caused by
beavers burrowing into' ,them.

Fur-Bearing Mammals of California

Transcription

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