v59 p294 Guyer and Linder

Craig Guyer'
and
Allan O. Linder
I)epartment of Biology
Idaho State Universiry
Pocatelio, Idaho 83201
Growthand PopulationStructureof the Short-HornedLizard
(Phrynosomadouglassr)and the SagebrushLizard
(Sceloporusgraciosus)in southeasternldaho
Abstract
A merk recxprure stud] of rhe short-hofned l\ztrd (.phry osoma dorsl4s.rl) and the sagcbrlrsh tizarcl
(Scelaparus gr"aclosrr) $-xs conductcd during t97a) and 1977 on ihe ldaho Nerionrl Engineering
Laboratory site in southeastcrn Idaho. Three age classcs rvere eviclent in both spccjesr voung-ol the
_y."ar,jul.eniles, anLi adults. ln bofi species, xdult females wefe significantly larger than adulr malcs.
This xppears to bc.1 e ro continuc.l growrh offi$t year aclnlr ftmales and cesserionofgrowrh in similarly
sized males. Both spccies meinraincd densiries ofepproximatcly I .1 incli\.ic1uelsper i h.r grid Sur\ rv.ll
of edults over wintcr r!.?s high in adults of boih species. Survivxl of juveniic S. gra.rosrs was slmrlar
to lhar of adults, !vhere?s survival ofyoung of the-ycar xncl iuvenile /,. /orglrssl $,es loiver rhrn that
o f a d u l r s f h e l a r g c r j u v e n i l c m o r r a l i r _ vs < n j i t , d o u g t a s s i r r r y b c r < t a r e d r o g r e a r c r f e p r o d u c t i v €
effort of rhis specirs when conpefed ro S grdcir.rr.r. Eorh sp€cies exhiblr the tong livcll ireroparous
reproductive s!r:rte!av. The t)ct that f€males Rcrc lxfger tltan males in both species nlav be relatecl
to this reproduclive strxtegv.
Introduction
Lizardsare important components of North American dcsert ecosystems,rivaling
mammalian ancl avi;rn speciesin ecological inlportance with respect to numbers,
biomass, and energetics (Turner el al. lc)76i). population ecolo€jists haye found
lizards to bc excellent experimental organisms for testing theorics of life historl,
strategies(Tinkle 1959a)and of niche srructure and function (pianka 1973). Un
fortunatelv, most studies of lizarclecoiog)r have been pertbrmecl on populations
inhabiting the south$.est deserts of North America. Important corollaries to theories
developecl from these populatior.rs have been proposeal for tlopical and north
temperate lizarclsbut few studies in the lafter two geographic regions have becn
performcd. Stuclies are needecl on species and genera l.hich occLlr over wlde
geographic areas.Two such speciesin North American desert ecosvstcmsare thc
short horned lizard (PlJrlnasa)ma doz-zgla.ss/)
and the sagebftLshli7,atd(Sce/oporus
graciosus). P. dauglqssi is the most widel], distributed member of thc genus
Pbrlnosoma a[d S. ,graciasus is among the most widely distributcd members of
the genus Sceloporus(Srcbbins 195,1).
The major objectivc of this study was to collect clataon selected ecoio€lical
aspects of P. daugl.rssi "nd S. grqciosus near the northern limits of their ranqes.
'Preseni aclllressi
Department of Bjolog\/. Unir.crsir\, of N1iemj, Corel cables, Fjorida l3t2.l.
291
North$.est Science,Vol. 59, No. 4, 1985
Estimatesof growth rates,population size, survivorship, :tnd sex ratios are compareclwith predictions macle by theories of reprocluctiYestrategies.
StudyArea
The study was conducted during 1976 and 1977 on the Idaho National Engineerinll Laborator)' (INEL) sire locateclin Bingham, BOnncville, Butte, Clark, and Jef
'fhe
ferson counties of southeasternIdaho.
area is located :rt an elevation of ca
1500 m. It is characterizedbv rolling hills producealby recent lava flows coverecl
by alluvial and loessial cleposits. Summers are hot (maximum and minimum
temperatures.rveraging30.5 and 10.0 C, respectively)?nd s'inters cold (maximum
anclminimum temperaturesaveraging 2.7 ?nd - 16.1 C, respectively).'I he average
yeady precipitation is 21.6 cm mostly in the forln of spring rains. Much of the
data were collected on a t ha grid systcmsthal was 100 m on each sidc and had
grial stakesplaced 10 ln apart. f his grid systenr and the surrounding area were
dominated by big sagebrush (,4rt?nlsid tridentata), rabbitbrush (C/:rysotbamnus
nauseosus), hak)€lcton (Halogeton glomeratus), xnd squirreltail gtass (SitanioLt
bJ)strix).
Methods
Lizardson fhc grid were cailtureclby hand or noose, sexed, and measuredsnout
to yent (SVL) to the nearest nn. Individuals were marked permanently by toe
cl4r ancl, for field iclentit'ication, by painting legs various colors with felr-r4rped
markers. The location of each capfure site was recorded by estimating the clistance
from this site to the nearesttwo grid stakes.Thesesiteswere later plotted on maps
of the grid. Recaptures were made periodically througl]out the two yeers, particularly when color marks were lost through sl]eclding. These individuals were
remeaslucd, reweighecl, and released.Lizards retaining their color marks $'ere
relocated v!.ithin the grid system aDd thcir positiolts recorded. The pcrimeter of
the gri.l was periodically samplcd tbr a dist?nce of 50 m from each side ir an effort to find migrant individuals. Additionallv, many lizards from areas alound the
gricl were captured and markcd for other aspectsof rhis study (Guyer 1978). Sex
and SVL data from these lizards y'ere used in determining age and sex groups as
were similar data collected from lizards on the grid.
Growtlt ratcs were estimated ffom lizards neasurcd two ot more til-l]es and
by linexr rcgressionof SVL or.l time of vear. The latter method was used to tcst
for differences irr size patterns of males and females (analysisofcovariance). llecause
both speciesappeareclto lav a single clutch $tich hatched over e short period
of time (2 weeks, Guyer 1978), the slope of each regressionline should reflect
averagellrowth rates \\.ithin each age and sex group. Data for each of the two years
were treatcd scparately.Capture-recapturedata s'erc used to estimatepopulation
size via Joll]'s method 0oliy 1965).
Results
Size data for P. douglassi were based on 8J captures ol 69 individuais in 1976
an.l 161 capturesof 130 individuals in 1977 (Figure 1). Three agc grorrpsfor each
sex xrere prescnt dLlringboth yeafs: young of the ycar (YOY), juveniles (er.rtering
first tull ,vear),ancladults (entering scconcllull r'ear or oider).Juvenile malcs were
Thc Short Horned and SasebrushLizar{1iD SoutheasternIdaho
1976
1977
Males
3
r
I
T . .
40
+ . +
8
fo
3
3 i + ; t
5
6
7
a
?
+
€zo
5
Females
o80
I
9
'
s
T ++
60
40
. +
20
3
"
A-M
EI
3
* s *
+ + f t
6t
o
LJ
EJ
r
LI
EA
+
o+
+
7
LA
ES
LS
TIMEOFYEAR
Figure1. The rclationshipberweensnoutventle^gthof phttnosomadougtassiandrimeof year.
Horizontalbarsaremeans,
verticalbarsare t I sd,numbersaresamplesizes,anddotsare
singleobservations.
Timeof yearextendsfrom AprilMay(A M) to lateSeprember
(LS).
smaller than adult males from April to August after which the two groups became
indistinguishable. Little of no growth occurred in males afrer eafly August. Juvenile
females were smaller than adulr females throughout borh years, although they approached adult size by early September. Growth in iuvenile females occurred
throughout the active season.
Adult females were significantly larger than adult males for P. douglassi during both years (Table 1). There was no significant difference between the slopes
of SVL data fof juvenile or YOY males and females (Table 2). Thus, rhere was no
difference in growth rate of male and female yoy or juveniles. When adiusted
for the effect of time, juvenile females were significantly larger than males during
both yea-rs.Female YOY v/ere larger than males during 1977 but nor 1976 (Table 2).
TABLE
1. Sizes of ad!lt P. .touglassi
^n
S. graciosus.
Year
M
1.0
5
1.6
o.7
l0
1976
F
69.8
M
52.O
r976
S. graciosus
r977
F
6a.2
M
57.4
1.3
0.3
F
59.8
o.4
22
46
4.6'
'significaflt at
0.05 level.
296
9
Guyer and Linder
TABLE 2. Results of analysis of covariance for young-of-ihe'year (YOY) and juvenile lizard sizes on
time ofyear. N refe6 to sample size and F refe$ to F-restsofequafity ofslopes rnd equaliry
of edjusted mean snout-vent-length (SVL), respecrively.
Species,
Age ancl Year
P. douSlassi
YOY 1976
AdtusredX
SVL I se
SIope
M
F
t2
t7
0.11
0.15
2.O3
27.2 ! 0.1
27.2! O.2
0.001
YOY 1977
M
F
l8
0.15
o.l7
0.68
24.2 .r O.3
29.6 ! O.3
I l aa'
Juv. 1976
M
F
o.r4
o.20
t.42
19
42.4t 0.7
4a.3 ! O.1
1977
JLJV.
M
F
39
13
o.20
o.25
M
F
49
0.01
0.09
2.r5
28.4 .t
M
F
22
18
0.14
|.52
51.5I
5 1 . 1i
S, graclosus
YOv 1977
Juv. 1971
r9
r.)
o.t]
43.01 0.6
45.1 ! O.5
9.29'
o.J
o.4
0.3
o.4
6.44'
0.54
'significant at
0.05 level
Table 3 presents the known growth rares of adult female P. douglassi. Growth
rates of two females known to have been juveniles in 1976 were significantly
greater than other adult females (one-tailed Mann-Whitney U-test, p < 0.05). Thus,
females entering their first year ofadulthood grew faster than older adult females.
Insufficient data were collected to compare growth in adult males in a similar
fashion. However, the size data (Figure 1) suggest that all adult males grow at the
same slow rate.
TABLE 3. Growth rates of female lizerds duting 1976 a'ld 1977. Stars (*) indicare animals knowo to
have been iuveniles in 1976.
Species
Animal
Number
ASVL
(mm)
49'
3
2l
27
16
95
t40
6.0
14.0
o.0
3.0
5.O
3.0
2.O
1.0
0.0
6.
t267*
8
32
J7
a4
r14
4.O
5.0
J.o
1.0
0.0
2.O
2.O
0.0
ATime
(days)
21
83
4u
89
113
16
35
13
42
r26
30
aa
20
59
23
t5
The Short-Horned and Sasebrush Lizard in Southeastern ldaho
Growth Rate
(mm/day)
0.14
o.I7
0.00
0.06
o.03
o.03
0.03
0.03
0.00
0.09
o.04
0.ll
0.01
0.00
o.03
o.08
0.00
297
S. graciosus size dara are based on 276 captures of 256 individuals in 1977
(Figure 2). Three age groups were evidentr yoy, juveniles, and adults.Juveniles
smaller than 40 mm were too light to be noosed and were difficrrlt to hand caplure. Also, this group was not commonll, seen until late in the year. Consequcntly,
few juveniles were captured early in tlte year. Juvenile males formed a clistinct
Males
80
3 g
a
o
f t
26
Females
tn
6
1
.
4
,
3
1
3 1
+ + + + ++
5
11
6
3
t2
+1 1
4
9
+
EJ
LJ
5
LA ES L S
TIME OF YEAR
Figure 2. The relationship berween snout venr length of .t elopo,"rs g,,a.l.)srr and timc of year. Sec
Figurc 1 fof exphnation of graph.
group, based on SVL, until early August when they entered ?dult size. Little or
no growth occurred in this group after carly August (Figure 2). Juvenile females
y'ere distinguishablefrom adults throughout the active season(Figure 2).
As tn P. .louglassi, aciult female S. grrclosrs were sillnificantly larger than males
(Table 1). There was no sillnificant difference in the sbpes of SVL dara for iuvenile
males versusfemalesnor was there a diflerence in adjustedmcan size. Howevef,
YOY females were significanrly larger than males even though these lizards did
not appear to grox' during the short time period over whiclt they were captured
in 1977 (Table 2).
From known growth rates of aclult female S. graciosus (Tablc 3), females known
to have been juveniles in 1976 grew faster tl.ranother females(one-tailedMannWhitney U test, p < 0.05). Again, insufficient data were available to analyze growth
ratesin m?les. However, siocc juvenile males reach adult size bv the end of their
298
Guler end Lindef
juvenile year (Figure 3) it seems unlikely thar first year adult males grow ar a fasrcr
rate than older males.
14.3
Jolly's estinute of population bizetX t I S E) fot P. d()ugtctssr'averaged
:t 2.1 during 1976 a.Ld 14.6 + 2.1 during 1977. Survival of P. douglassi over
winter (percentageof animals marked in 1976 that were recaptured in 1977) was
low in YOY and juveniles ancl high in adults (Table 4). Of 124 p. (lougtassi captured and marked outside the grid, three were emillrants from the llrid. All were
juveniles. One established a home range approximately 60 m north of rhe grid
and two were recaptured approximately 500 m south ofthe grid. Two other lizards,
both juveniles first captured outside the grid, were later recaptured within the grid's
boundary. One of these establisheda home range.
Sex rafios for P. douglassi dicl not differ significantly from a one-to onc ratio
lbr YOY or juveniics (27 males: 35 fcmales for YOY and 46:46 for juveniles; chisquare goodness of fit, p < 0.05 in both cascs).The sex ratio for adulrs was
significanfly different from a one-to-one r,ttho(11:2); p > 0.05). Since there was
no significant difference (chi-square rcst ofhomogeneity, p > 0.05) berween years .
for any age and sex group, data for both years were pooled.
Population size estim?tesfor S. graciosus averaged 14.0 I 5.1 for 1976 and
12.0 ! 2.2 for 1977. Over-winrcr survival of juvcniles was slighrlv greater than
that of ?dults (Table 4). Too few YOY were caprured ro esrir.n?tetheir over-winter
suryival. Sex r2tios of .t. graciosus did not diffcr significantly from a one-to one
ratio for YOY or adults (22:16 for YOY and 49:58 for adults; chisquare goodness
of fit, p < 0.05 in both cases).Significanrlymore juvenile males s.ere caprured
than juvenile females(67:42; p < 0.05). Ot 249 S. graciosus marked outside rhe
grid, none elnigfated off the grid and none immigrateal onto it.
TABLE.i. Survivorship ovcr winrer (1976 1977)fot P daugltlssi ^nd S..qracrbsrr-in southcrstem ldeho.
Spcci€s
Age
# Ceptufed
in 1976
# Cxptured
in i977
YOY
Juv.
Adult
19
13
6
2
YOY
Jur.
Adult
0
5
t1
I
0.tI
0.15
0.67
o :lo
o.61
Discussion
A common feature of the two species was a sexual dimorphism in size. pianka
and Parker (1975) noted a similar dimorphis|o in P. douglassi ffom Urah. Sexual
dimorpbisnr of size has been rcported for S. grqciosus in northern Califbrnia (Steb
bins 19,14),l0yoming (Kerfoot 1968), and Utah (Burkholder and Tanner 1974)
This size difference could result from a differential in size at birth, growrh rates
of YOY or juveniles, or length of growth period. P. douglassl -]rOy females were
larger than YOY males durir]g 1977 but not 1976. Thus, my resultson dilTerenrial
size at birth in this species are equivocal. Howevcr, the small size difference bet
ween YOY males and females in 1977 is not enough to explain the large difference
The Short-Horned and Sagebrush Liz?ld in Soutlteastern Idaho
?oo
in adults. For S gra.cic)sus,
YOY femaieswere significantly larger than ntales,but
no such difference v!.asfound in iuvenilcs (born the previous year). Again, rcsults
for diffefential size ar birth are ecluivocal.Gro$'th rates betwccn sexesfor yoy
or iuvcniles werc nor statisticallydiffercnt in either species.In contfast, pianka
and Parkcr (1975) reported tltat juvenile femalcs gror,' fastcr than juvcnile males.
In this study, coDtinued growth in femalescntering thcir first l,ear of adr-rlthood
occurred in borh species while similarly aged malcs epiteared to ceasegrowth.
Similaf resuitss'ere reported for S. gracictsusby Srebbins(194,1)and Burkholder
and Tarlner (1974). Coniinued growrh of young adult felrlalesis rhe nost iikely
ncchanism causing sexual climorphism of size in the two spccies.
Another common gfov'lh ttature of the ts-o speciese/as the number of age
classcs.Similar classess'.erefound b_vPianka and parker (1975) fot p. tJouglqssi
and by Stebbins (194i4),Mueller and Moor.e (1969), ancl Goldberg (1975) for S.
graciosus. Tinkle (1973) and Burkholdcr and Tanncr (197,1)I'ere able to disrirguish
lour agc gro f5 [or \ gr.,1r'1osi/\.
Horned lizards rypically maintain 1(lw population densitics when compared
to other iguanids (Pianka and Parker 1975). Densiriesof 1 .6/ha for p. cornutunl
in New trlexico 0x/orthingron 1972), 1.O/haand 5.Olhaf..f;p. plotyrtl ros in Nevada
(Medrcaet al. 1973, Tanner and Krogh 1973) have been reporred. However, in
this study densities of 14.3/ha and 14.5/ha occurred in 1976 and 1977. resDec
tively. The high observed density of P. dr.tuglassi may have becn due to seleciion
of the study site based on the relativcly large numbers of horned lizards seen rhere
bv previous investigators (Sehman and Linder 1978). Densities ofS. graciosus dur
ing the two years were lower rhan rhe 206 to 216lha reported by Tinkle (1973)
or the 6l to 73/ha by Burkholcler and Tanner (1974).
Sur\.ivalpatterns for both speciesindicated that they are long,liveclas adults.
This conclusion s'as also Leached by Medica, et al. (1973), Tanner and Kro€ih
(.1973),and Piankaand Parker (1975) tor P. dctugitassiand l.:v Stebbins (1948) and
Tinklc (1973) for .t. graciosus. J\Nenile over winrer surviv?l differcd between rhe
two species.Lo$ sulviv?l of juvenile P. dauglassl occurred during this study.
Similar results werc reporrecl by lted lclr et al. (197 3) .rnd pianka and parker ( 1975).
High suryivorship, evcn exceeding that of adults, was found in juvenile .t.
graciosus. Burkholder' ?ncl Tannef (197,i) showed similar sun'ival fbf iuvenile S.
Sraci.rsusand Tinkle (1973) found juvenile survival to be greatcr than adult survival. The diftcrence in juvenile sufvival bet\\.een the two species mav bc impor
tant in explaining ditTcfencesin rcproductive output between them. The largc
clufch sizeofP. douglassi (Goldber€j1971)ascomparedto S.gr.tciosus(Goldberg
1975) mav be recluifed to overcone the compararivelv krw juvenilc survival.
Sex ratios of YOY did not cliffer from one to one for eithcr species.Assuminll
equai r]umbcrs of males end femalcs are born cach year, an_vdeviations from a
one-to onc ratio in older age groups should rcsult from survir.alor behavioral dif
ferences between sexes or to random chance. In p. douglassi, sex ratios deviatecl
from one to-one in the edult age group only. During both years more ?clult fcmales
were seen than males.This may indicate that juvenile malesiDcur greatcr mortali
tv while over wiltteriDll or that nortalitv is greater in adult males than females.
Territorial displavsby adult male igu2nids are thought to increasetheir risk of be
1ng prevcd upon as corrparcd to females (Tinkle 1973). This is unlikely in
-l0(l
Guyer and Linclcr
Phrlnosama since these displays are greatl_vreduced and may serve no social func
tion (Lynn 1965).Aclult S. greciosus had a one-to one sex rario while for iuvcniles
more malcs were captureclthan females.I suspectthat this is a behavioral artifact
since most juvenile males occupied highly visible locations, particula.ly in rocky
areas.Thus, the.v may have been located and captured norc frequently tltan
females.
Seyeralprcdictions have been dcvclopeal felatinll latitudinal changes with shifts
in life histo4' strategies.Tinkle (1969a,b) denote.l rwo iife hisrory srrrrcgies rmong
lizard populations. C)ne stratcgy involves high reproductive effort (largc clutch
sizeor high clutch frequencv), semelparity,short adult life span, ancl territoriality
whereas thc olher strategy involves low reproductivc cffort, iteroparitv, long adult
litu span,and clominancchierarchies.Hirshfield and Tinkle ( 1975)specularedthar
environmental predictability may determine which strategy evol\'es in a lliven
population. In highlv unpredictable enviroltmen$, where sunit'al of young may
be negligible dufing certain )'ears,individuals rhar devote relarively lirrle effort
to anJ.onc reprodlrctive seasonand heve long life spans would be at a sclcctive
aalyantagcover individuals which devote most or all ctf theil reprocluctivc eflbrt
to one season.The reverse$'ould be tfue in areas$'herc environments favorable
to juvenilcs are preclictable. Pianka (1967) has shown that the nortltern Gre?t Basin
desert has the most unpredictable environment of any North Ar.nericandcsert.
Tl.tereforc, lizalds present there should belong to the long iived. iteroparous group.
Both speciesin this stud,vconformed to tltesepredictions. Adults of both specjes
commonly survived througir t\vo or more reproductive seasonsand should thus
be terlned longlived. The,v clearl,v did nor exhibir rhe high mortaliry of adulrs
characleristicof sl.rortlived species.Tlte smali variation in SVL data at each sam
ple cletefor YOY P. dougl.lssi drtring each ycar suggestss,vnchronous production
of a single clutch in this species.The gestation period of P. dougldssi probably
restrictsthis slleciesto a singleclutch throughout its €ieographicrange.Much more
variation in SVL occurred in .S.graciosus YOy which could indicate grcater variation in developnent time for this egg-layingspeciesor that t$ro clutches ere laid.
Thc fbrmer mechanism is probably more likelv since a single clutch is t,vpicalfor
northern populatiorls of S. gracioslrs (Fitch 1970, Burkholder and Tanner 197,1).
Thus, both species s,'ould appear to be iteroparous.
Fitch (1978) suggestedthat a shift in sexual dimorphism of size should occur
in these ts'o reprocluctive strategies.In populations thet are highlv territorial, nales
should be larger. In populations selected for long life a1-Ici
iterated brecding, females
should be largef. Both speciesin this stuclvfit the latter prediction. A shift occufs
in scxual dimorphism of sizc in S. gracioslts for s()Llthernpopulations as males
are largcr than females in southcrn Utah (Tinkle 1971). This shift fits rlte gcographic
change in reproductive strategypredicred by Tinkle (1969a,b).An appafent con
tradictior] to tl.rispaftern ofsize alimorphism and reproductivc strategy $'as rep(xtcd
by Mueller and Moore (1969) \.ho found no difference in rhe rorel length ofadulr
males and fenales for S. graciosus from Wyoming. Since tails are generally longer
in males than in tenales (Kerfoot 1968). sexual alinloryrhisn of size may have bcen
masked. A ke1' factor in tesring predictions made by theories of reprocluctive
strategiesand sexual dimorphism of size is social organization.This aspectof the
ecology of the t$-o lizards $'as not determined. This appears to be a fruitful line
The SIrort Horned and SaqebrushLizard in SoutheastcrnIdaho
301
of researclr especiaily for S. grqclosus slnce I]necurrent informalion would predict
a change fiom territorial behavior in southern popul?tions to dominance hierachies
in northeIn populatior.rs.
Acknowledgments
This researchis a contribution fron the Idaho Nationai Engineering Laboratory
Ecologicai Studics Program and was funded by a grant frorn the Office of Healrh
and Environmenral Research,U. S. Dcpartment of Energy (#Ey 76,5-07 1529) to
Allan D. Linder. l e are indebted to O. D. Markham, Director of Envi.onmcntal
Studieson the INEL, for his assistanccand encouragementthroughout tltis study.
V/e thank T. D. Reynolds and R. \X/. Sehman for assisrancein the field and J. M.
Savage,M. P. Hayes, M. A. Donnelly, c. L. powell, and S. D. rverman for sug
gesting improvements in the ntanusctipt.
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Receiued 24 March 1984
Accepted Jor publication ll
Nouember 1984
The Short Horned and Sasebrush Llzard in Southeastern Idaho
303