sphex ichneumoneus l. - California Academy of Sciences

Anim. Behav.,1980, 28, 426--445
THE CONTROL OF NEST DEPTH IN A DIGGER WASP
(SPHEX ICHNEUMONEUS L.)
BY H. JANE B R O C K M A N N *
Department of Zoology, University of Wisconsin, Madison, WI 53702
Abstract. Golden digger wasps, Sphex ichneumoneus L. (Sphecidae), are a solitary, ground-nesting
species that dig burrows to particular depths in the soil. I develop and evaluate alternative hypotheses
about the mechanisms controlling digging behaviour. By altering the wasps' burrows as they are
digging, I show that they are not digging for some prescribed length of time nor are they digging until
they reach some suitable environmental characteristic deep in the soil. Rather, they appear to be
digging until they reach a particular tunnel length, making corrections if the tunnel is too shallow or
too deep. This distance can be altered somewhat by surface environmental conditions.
Animals that build artifacts such as burrows,
webs, or nests (without the aid of visual cues) are
faced with the difficult task of measuring precise
distances. Often the necessary measurements are
shorter than one body length, which means that
the animal can simply use an appendage as a
ruler. For example, bees use receptors on the
antennae to determine the thickness of the wax
walls of the larval brood cells (Martin &
Lindauer 1968), caddisfly larvae manipulate and
choose particles of the right size before incorporating them into their houses (Hansell 1968),
and leaf-rolling beetles bore into the leaf stem
one body length from the base of the petiole
(Daanje 1975). But many animals are faced with
a more difficult problem: how to measure distances that are greater than one body length. A
number of insects, for example, show correcting
behaviour, i.e. after being forced to turn to the
right, they turn to the left at the next choice
point so that they continue walking in a more-orless straight line (Dingle 1962, 1964; Wilson &
Hoy 1968). However, if they have walked more
than a specific distance after the forced turn, then
they continue to walk straight rather than choosing to turn at the choice point. It was found that
the distance traversed from forced turn to choice
point rather than the time taken to walk this distance was the cause of the decline in correcting
behaviour (Dingle 1965). A remarkable example
is found in the nest-site selecting behaviour of
honey bees. They are apparently capable of estimating the volume of a cavity by integrating information on the distance and directions of walking movements made inside the cavity (Seeley
1977). Animals that dig long burrows in the
ground to particular depths have a similar measur*Present address: Department of Zoology, University
of Florida, Gainesville, Florida 32611.
ing problem. This paper describes the nestbuilding behaviour of the great golden digger
wasp, Sphex iehneumoneus (Sphecidae), a solitary
ground-nesting species that digs burrows which
it then provisions with katydids as food for its
offspring. I develop a model for the control of
burrow length and experimentally evaluate a
series of hypotheses about the possible mechanisms controlling nest depth. I will show that
these wasps are probably measuring the length
of the burrow they dig and are capable of correcting this length when the burrow is artificially
altered.
The Problem
Nest construction in Sphex ichneumoneus involves many behavioural patterns influenced by
a multitude of factors. The behavioural system
can be summarized in a kinematic diagram (Fig.
1). In such a system, control is exercised through
the timing and selecting of alternative transitions.
Wherever alternative behavioural patterns occur,
the animal is making a decision (Dawkins &
Dawkins 1973) about whether to continue with
one activity or to switch to the next. In this ease I
am concerned with the factors influencing one
particular decision: the transition between digging the main tunnel and widening the main
tunnel in preparation for digging the side tunnel.
In other words, what controls the depth to which
Sphex dig ?
The control of tunnel length is a particularly
interesting problem, both because of the ease
with which one can manipulate the system experimentally and because of the importance of
choosing the optimum length for the survival of
the wasp's offspring. The soil close to the surface
becomes extremely hot (up to 53 C at 3 cm) in
the middle of the summer, conditions that almost
426
BROCKMANN: CONTROL OF NEST DEPTH
certainly are lethal to the wasp grub. Nests with
very deep chambers may be too cool for the development of the larva, too far for the emerging
wasp to dig, or simply too time-consuming for
the adult wasp to construct. (Winter conditions
probably play no role in the selection pressures
influencing tunnel length, since soils freeze regularly to 20 to 50 cm.) Natural selection must have
acted in some way to 'set' the appropriate compromise depth. But how?
There are two general approaches for establishing what factors influence a particular behavioural pattern: descriptive and experimental.
The descriptive method involves measuring every
environmental variable I can think of and correlating these with burrow length. I did this by
measuring soil and air temperature, soil moisture, air humidity, light levels at the surface and
in the burrow as it was being dug, cloud conditions, wind velocity, barometric pressure, soil
compaction, time of day, date, and age of the
wasp. Very few of these correlated with the
length of the tunnel and none showed any consistent significant correlation over the different
years of the study (Brockmann 1976). A multiple
regression analysis of all these factors revealed
that only 35 70 of the variance in the length o f the
main tunnel was accounted for by covariance
with light level near the surface (21 700) and soil
moisture (147o). Clearly, I could not base any
conclusions on this kind of descriptive study. An
experimental approach would have to be used.
When a wasp digs a nest, she performs a set of
behavioural patterns repeatedly until apparently
reaching some criterion or condition. At this
point she stops digging the main tunnel and begins to construct the side tunnel. Clearly, the
decision of whether or not to continue digging is
under closed-loop control (McFarland 1971),
where the output of the digging behavioural system, for example how far the wasp has dug, influences the input in some way. The objectives of
this study, then, are to determine experimentally
the nature of the input, output, and feedback
mechanisms involved in the control of digging
behaviour. But first it is necessary to give more
background on the methods used and the behaviour of the wasps.
Materials and Methods
Study Area
During the summers of 1972 through 1975 I
studied an aggregation of 8 to 35 nesting Sphex
ichneumoneus on the campus of the University
427
of Michigan in Dearborn, Michigan (referred to
throughout as MI). I observed the wasps daily
from the date they began to emerge until middle
August: in 1972 from 5 July to 15 August, in
1973 from 10 July to 16 August, in 1974 from 14
July to 22 August, and in 1975 from 15 July to 10
August (in 1975 my assistant, Timothy Manning,
made the observations from 22 July to 10
FLIES TO NESTING
AREA (WASP HAS
NO ACTIVE NEST)
IOO%
I00 %
T 85%~
70%B1
FLIES FROM
NESTING AREA
NTS, FLIES FROM
rING AREA AND
ER RETURNS TO
E ACTIVITY
ORIENTS, FLIES
FROM NESTING
AREA
(N=231)
Fig. 1. Kinematic diagram of the nest-contruction
behavioural system (based on 761 landings at the nesting
area by wasps that did not have active nests). The limit
of the digging system is indicated by the large box. A
block represents a behavioural sub-system, an arrow the
transition between two sub-systems (the dashed arrow
is a temporary interruption of an activity). The number
on the arrow is the percentage of the observations of a
sub-system where that transition to another sub-system
occurred. For example, wasps flew from the nesting area
during the construction of 33 70 of the main tunnels, and
107o of the main tunnels were abandoned before the
wasps began to widen the bottom. Wherever there are
alternatives in the behaviour, control is taking place.
This study concentrates on one control problem, indicated with a star.
428
ANIMAL
BEHAVIOUR,
August). In 1975 1 studied a second aggregation
of wasps from 24 July to 22 August in a sunny
section of grassy lawn in Exeter, New Hampshire (referred to throughout as NH). In 1976 I
observed a third aggregation of these solitary
wasps along the east side of a building on the
campus of Carleton College in Northfield,
Minnesota (referred to throughout as MN), from
8 July to 6 August 1976.
Marking Wasps
At each nesting area I individually marked all
wasps with colour-coded dots of Testor's Pla
enamel paint. When a wasp began to dig a burrow for the first time, I captured her by placing a
test tube over the entrance of her burrow while
she was inside. As she backed out, she walked
up into the test tube, which I then plugged with
cotton. I anaesthetized her lightly with ether
(1972-1974) or CO2 (1975-1976), measuring her
length and weighing her on a balance as well as
marking her with dots of paint.
Measurements Taken While the Wasp Was
Digging
I marked the location of each new tunnel and
recorded the following information while the
burrow was being dug: (a) which individual was
digging; (b) when she started and ended the digging; (c) the soil surface temperature (measured
with a thermocouple, see Brockmann 1976) and
the temperature at the entrance and at the bottom of the main tunnel (when the wasp started on
the side tunnel); (d) the percentage soil moisture
at the bottom (by taking soil samples or by estimating from a nearby tensiometer); (e) the surface humidity (from a nearby humidity probe);
and (f) the light level at the surface (using a
pyrheliometer) and at 2 cm intervals down the
tunnel (using a light meter probe, see Brockmann
1976). With an otoscope I frequently observed
the wasp while she was digging. I measured the
length of the straight main tunnel (or 'main
burrow' of Evans 1957, 1966b) when the wasp
began digging the side tunnel (or 'cell burrow'
of Evans) (Fig. 2). Other measurements of the
burrow were taken when the wasp had completed the side tunnel and chamber. Occasionally
the wasp first dug some distance beyond the
point where she ultimately dug her side tunnel,
thus creating a 'spur' (as shown in Fig. 2). All
measurements taken while the wasp was digging
were accomplished within a minute. During this
time the wasp merely stood aside, walked, bit at
the instruments, or groomed. Occasionally an
28,
2
individual would fly up to a nearby tree, returning within a few minutes.
The Wasps and Their Burrows
Adult male and female Sphex ichneumoneus dig
their way out of the ground in early to mid-July.
After emerging the wasps immediately fly to trees
and open fields, where they feed on the nectar of
flowers and presumably copulate (although this
has never been observed). After a day or two, the
females return to nest in the area from which
they emerged. Each female digs several burrows
during her short (about six-week) lifetime, each
with several brood chambers. She provisions
these chambers one at a time with several paralysed katydids as food for the single egg she lays
in each. After laying the egg, she fills in the side
tunnel with soil and begins to dig a new chamber
or she fills in both side and main tunnels and
digs a new nest.
Digging Behaviour
After returning to her natal nesting area, the
female wasp goes through an active process of
searching before beginning to dig. She chooses
to nest in the warmest and driest areas available
that are not too near the nests of other wasps
(Brockmann 1979). She begins digging by biting at the ground with her mandibles and scraping away surface debris (Fig. 3A, 3B). The biting
and scraping soon result in an indentation in the
surface soil. After she has progressed a centimetre or so, she begins to carry the loosened soil
out of the burrow (Fig. 3C). Repeatedly she
enters, bites at the bottom, picks up the loosened
soil, carries it out, and drops it on the mound,
and then re-enters the tunnel for another load.
When the wasp is on the mound, this pattern is
broken by scraping (Fig. 3D; occurring on average once per minute), trampling (Fig. 3E; ocr, length ,~
ht.=~§
~ hole diam.
mound
i~{!}~ main
(~)
~ i tu' nnel~
5cm"
sp
side
angle of
hole, ~-metel
skewer
(~
tunnel
Fig. 2. The method used for measuring the dimensions
of the wasps' nests. (A) The names used for the different
parts of the nest. (B) The length of the main tunnel was
measured from the soil surface to the bottom of the spur
(if present).
BROCKMANN: CONTROL OF NEST DEPTH
429
or in bouts of 2 to 10 (generally 2 or 3). (E) Trampling.
When on the mound, the wasp flattens and spreads the
dirt by trampling. She flexes the coxal and femoral
joints of the hind legs, drawing the leg anteriorly and
dorsally so that the tarsi are under the anterior part of the
abdomen but still touching the ground. Then with a
quick extension, she moves the hind leg posteriorly
along the ground in line with the body axis. Often this is
accompanied by a simultaneous lateral motion of the
leg to almost a right angle with the body. She may
trample while stationary or while moving backwards.
Trampling occurs in bouts of 2 to 15 (generally 2 to 5),
being performed alternately or simultaneously by the
two hind legs. (Size of wasps: 2.0 to 3.0 era). Drawings
by Cheryl Hughes.
|
Fig. 3. Digging behaviour of Sphex ichneumoneus. (A)
Digging walk. When a wasp digs, the prothoracic tarsi
are curled medially and dorsally so that she walks on the
distal end of the first tarsal segment and the dorsal surface
of the second and third (and sometimes fourth) segments,
with the remaining two (or one) tarsal segments raised
up off the substrate. The wasp walks on either the ventral
or dorsal surfaces of the mesothoracic tarsi and on the
ventral surface of the metathoracic tarsi while digging.
B. In contrast, during normal walking, the front tarsi
are not curled and the wasp walks on the ventral side of
the distal two to three tarsal segments of all three
pairs of legs. (C) Carrying. In the digging walk position,
the wasp gathers up and consolidates loosened soil with
a series of short, quick anterior to posterior and medial
movements of the prothoracic legs, rather like a person
gathering up loose sand between two hands. With her
front legs on either side of the accumulated pile of dirt,
she clasps it. The top of the gathered pile of dirt is supported against the ventral side of the head and the ventral
anterior part of the prothorax. (Wasps that carry dirt
in this manner are described as 'pullers' by Olberg 1959.)
Bearing her load of dirt, the wasp walks backwards up
the hole on four legs. Emerging at the surface, she walks
backwards 1 to 5 cm before depositing her load of dirt
on the accumulating mound. (D) Scraping. The wasp
levels the mound with scrapes ('raking' of Olberg 1959
and Evans 1966a), a simultaneous quick, posterior movement of the prothoraeic legs. Scraping occurs singly
curring on average once every 1.5 rain), a n d
g r o o m i n g (average once each 2 min). She digs
quickly, t a k i n g 5 to 11 trips p e r m i n u t e a n d p r o gressing at a rate o f 0.1 to 0.7 e m / m i n (Table I),
r e m o v i n g 0.01 to 0.09 g per trip. Digging is often
a very t i m e - c o n s u m i n g process. T h e w a s p spends
12 to 68 min digging the m a i n tunnel, a n d c o m pleting the nest (all the time spent digging to the
first provisioning o r inspection) takes f r o m 30 to
225 m i n (Table I).
W h e n the w a s p begins to dig the side tunnel,
her b e h a v i o u r changes from biting at the b o t t o m
o f the tunnel to biting at the sides, thereby
widening the end o f the m a i n tunnel. I n the early
stages o f constructing a side tunnel, the w a s p
brings each l o a d o f dirt to the surface. Later,
however, she deposits dirt at the b o t t o m o f the
m a i n tunnel. She digs for 30 s to 5 min in the side
tunnel, then clears out the a c c u m u l a t e d soil f r o m
the b o t t o m o f the m a i n tunnel in b o u t s o f three
to five carries, a n d then returns to digging the
side tunnel a n d chamber. I f there is a ' s p u r ' on
the m a i n tunnel, the w a s p m a y fill it in with soil
while digging the side tunnel.
A l t h o u g h Sphex ichneumoneus b o t h dug holes
t h a t they later a b a n d o n e d a n d dug in the a b a n d o n e d nests o f other wasps ( B r o c k m a n n et al.
1979; B r o c k m a n n & D a w k i n s in press), the
following descriptions a n d experimental study
a p p l y only to new nests dug entirely b y one wasp,
a n d to nests t h a t the wasp c o n t i n u e d to occupy
for at least one full day. The m e a s u r e m e n t s were
t a k e n either at the time the wasp reached the
p o i n t where she dug the side tunnel o r directly
after she c o m p l e t e d the side tunnel a n d c h a m b e r
Length of the Main Tunnel
The length o f the newly dug m a i n tunnels
varied m a r k e d l y f r o m one locality to another. In
M I tunnels dug in a flower p l a n t e r (Site P1) were
shorter t h a n those dug on a n e a r b y knoll (Site
430
ANIMAL
BEHAVIOUR,
28,
2
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BROCKMANN: CONTROL OF NEST DEPTH
K). The N H wasps generally dug longer tunnels
than most of the MI wasps (Table I). Literature
accounts reveal even more variation in burrow
length. Ristich (1953) observed that tunnels were
dug to a depth of 10 to 33 cm (mean 17.0 cm)
in cinder-fill while being dug from 10 to 17.5 cm
(mean 14.5 cm) in compacted soil. Similarly
other observers have found tunnels varying in
length from 7 to 20 cm depending on whether
they were dug in hard-packed clay or sandy soils
(Packard 1869; Peekham & Peckham 1898;
Davis 1911; Hancock 1911; Rau & Rau 1918;
Abbott 1931; Frisch 1937; Fernald 1945;
Mendoza 1969). Rau (1933) observed much
shorter tunnels (3.5 to 5.5 cm) in hard-packed
ground on Barro Colorado Island in Panama,
where the wasps dug between the daily rainfalls
in July and August.
Individually, the wasps also showed considerable variation in the length of the several (newly
dug) main tunnels they constructed over a season
at one nesting area. However, the individual variance was not significantly different from the
population variance (one-way analysis of variance, F ratio : 1.2, P > 0.10, Table I). This
means that some individuals are not consistently
digging shorter or longer tunnels than other individuals at one site.
My observations of tunnel lengths combined
with those from literature reports make it clear
that wasps are digging to a prescribed depth that
differs between sites. Are they perhaps responding to some external cue such as soil moisture
that always occurs at a certain depth at one site ?
Or, is it possible that they are simply digging for
a prescribed length of time, which means that
given a certain soil type their holes are all about
the same length and different sites differ in soil
type ? Or, could they be actually measuring out
the length of the tunnel ? Natural selection may
have acted in such a way as to programme individuals to respond adaptably to local conditions, or individuals at different sites may be programmed to dig to different depths as a result of
generations of selection. But in either case it remains to be shown what they are responding to
or how they do the measuring.
Experimental Design
Figure 4 is a block diagram of the hypothesized
mechanism controlling burrow length. At any
given moment the state of the digging system is
defined by the values of all the variables of the
block diagram. Alternative hypotheses for the
nature of the feedback mechanism are evaluated
431
in field tests by interrupting the closed-loop
control.
In each experiment I compared the lengths of
two different tunnels dug by each individual
wasp. One was designated the 'experimental' and
the other the 'reference' burrow (the order of
these was randomized so that the experimental
burrow was dug first about half the time). I f an
undisturbed wasp dug two consecutive tunnels,
one arbitrarily designated the experimental and
the other the reference, one would expect that on
average the length of the experimental tunnel (E)
would be equal to the length o f the reference
tunnel (R). The descriptive results discussed
earlier show some individual variability in tunnel
length. Therefore, although the length of a particular experimental tunnel may not be exactly
equal to the length of a particular reference tunnel, a population of experimental tunnels should
not be significantly different from a population
of reference tunnels dug by the same wasps.
Alternative hypotheses were evaluated by artificially altering experimental and reference tunnels. As the wasp dug I not only implemented
the conditions for the experiment but I also observed her behaviour and took the measurements
described earlier. I allowed her to continue digging until she had dug one body length in the
side tunnel at the bottom. At this point, I measured the length of the main tunnel. Since wasps
were easily disturbed during the early stages of
....
x -~,~
...... ,o.o, _I o,oo,.olo~
w
-I BEHAWOURJ Y
~ense
siqnal
Z
_.__] SiN S I N G ,[
DEVICE
stimulus
Fig. 4. Block diagram illustrating the functional relationships between or the flow of information carried by the
variables of the digging system. The arrows are variables
and the blocks are transfer functions betweenindependent
(input to box) and dependent (output from box) variables.
The decision to continue digging, or alternatively to cease
digging, is controlled by the values of four variables, in
this simplest representation of the system. The wasp
senses the value of some stimulus variable (II) that results
from digging, and converts this to an internal sensory
signal (Z) that is compared (symbolizedby the quartered
circle) with an internal reference value (set-point variable
X). If the two match, no error-signal is generated and no
further digging occurs, but if the two do not match,
an error-signal (W) results, and the digging continues,
which results in a further change in the value of Y.
ANIMAL
432
BEHAVIOUR,
digging, I always allowed them to dig at least one
body length before introducing the experimental
conditions. The alternative hypotheses were
evaluated statistically by comparing the actual
length of the experimental tunnel with that
predicted by each hypothesis. For each comparison I performed two non-parametric statistical tests for related samples: the sign test and
the Wilcoxon matched-pairs signed-ranks test
(Siegel 1956).
Internal or External Cues?
The first problem is to determine whether the information affecting tunnel length originates from
sources external or internal to the wasp. This
dichotomy is illustrated in Fig. 5. I f the wasp is
measuring a cue originating from within herself,
then I can alter her experimental tunnel as she is
digging by coring out a few extra centimetres of
s0il and she should continue to dig as she does
in her uncored reference tunnel (internal cues
hypothesis). At completion, then, her experimental tunnel should be longer than her reference tunnel. On the other hand, if she is using an
external cue, then coring should not affect the
tunnel length. At completion, the experimentally
cored tunnel should be the same length as the unaltered reference tunnel (external cues hypothesis).
X
~
W
L
Y: TUNNEL LENGTH
ble
[SENSING
DEVICE
~ ,
u.
WASP
Fig. 5. Block diagram illustrating the two alternative
hypotheses for the control of tunnel length (the dashed
line indicates the boundary of the wasp; see Fig. 4
for symbols). (a) Internal cues hypothesis: digging
results in a change in some internal variable Yi, such as
hunger level. This variable is measured by a sensing
device and compared with an internal set-point. The
wasp continues to dig until she reaches a set value of the
internal variable when digging ceases. (b) External cues
hypothesis: digging results in the increasing length of the
main tunnel, which in turn results in a correlated change
in some variable Ye, for example decreasing temperature
in the soil. The box H is the transfer function that relates
tunnel length to the possible controlling variables such as
temperature, soil moisture, and light level. The external
variable is measured by some sensing device and then
compared with an internal set-point. The wasp continues
to dig until reaching a set value of the external variable.
28,
2
Method for Coring
When the wasp had dug a specified distance, I
artificially increased the length of the main tunnel
by coring. After she backed out, I twisted and
pushed a hollow metal rod a specified distance
into the bottom of the tunnel at the same angle
and with the same diameter as the original tunnel. I removed the soil and saved it for soil moisture analysis (the soil was not placed on the
mound). Three separate coring experiments were
completed. (1) Tunnels cored early in digging: I
cored the experimental tunnel to 6 to 8 cm after
the wasp had reached 3 to 4 cm, i.e I lengthened
the tunnel by 3 to 4 cm. In the reference tunnel I
cored less than 1 cm of soil from the bottom
when the wasp had dug to 3 to 4 cm (conducted
1974 MI). (2) Tunnels cored late in digging: I
cored the experimental tunnel to 11.0 to 11.5 cm
after the wasp had dug to 5.5 to 7.0 cm, i.e. I
lengthened the tunnel by 4.5 to 5.5 cm. In the
reference tunnel I removed less than 1 cm after
the wasp had reached 5.5 to 7.0 cm (conducted
1973 MI). (3) Tunnels cored very late in digging:
I cored the experimental tunnel to 13 to 14 cm
after the wasp had reached 8 to 9 cm, i.e. I
lengthened the tunnel by 5.0 to 6.0 cm. In the
reference tunnel I removed less than 1 cm after
the wasp dug to 8 to 9 cm (conducted 1973 MI).
Analysis of Data
I f the experimental tunnel is lengthened by
coring, then the final length of the experimental
tunnel, E, depends upon whether the wasp is
monitoring an internal cue (int) or an external
cue (ex0 to control digging. I f she is using an
internal cue, then she should continue to dig as
though no coring had occurred, resulting in a
longer tunnel than usual. Under the internal cues
hypothesis, then, the predicted length of the experimental tunnel (Eint) is the length of the
reference tunnel (R) plus the amount I cored
(CE). However, the actual experimental situation
is somewhat more complicated because in each
case the reference tunnel was cored slightly as a
control for any factors associated with the act of
coring (as opposed to the length of the coring).
Therefore, the amount of the reference coring
(C~) must be subtracted from the amount of the
experimental coring (C~). Hence,
E~n~ = R -t- CE -- Cu.
(1)
On the other hand, if the wasp is using an external cue, then she should detect and respond to
the alteration I made by coring her tunnel. The
external cues hypothesis predicts that the length
BROCKMANN: CONTROL OF NEST DEPTH
of the experimental tunnel (Eext) will be, on
average, the same as the length of the reference
tunnel (R). However, when I cored the experimental tunnel late in digging, the amount that
the wasp dug before I cored (B~) plus the amount
I cored (CE) was greater than R. When this occurred one would not necessarily expect any
further digging. Under the external cues hypothesis, then, the predicted length of the experimental tunnel depends on when I did the coring:
If BE + CE < R, then E e x t = R and
if BE + CE > R, then Eext = BE -}- CE.
(2)
(3)
There is one further complication. When I cored
very late in digging, the wasp sometimes filled or
shortened the reference tunnel after I had done
the coring. When this occurred, the length of the
experimental tunnel must be considered undefined. Therefore, there is one further restriction
placed on equations (2) and (3):
If B E + C ~ < R
and R ~ > B R + C m
E e x t = R and
if B E - k C E > ~ R and R ~ > B ~ - b C m
Eext : BE @ CE.
then
(4)
then
(5)
Results
(1) Tunnels cored early in digging. When 12
tunnels, already dug 3 to 4 cm, were lengthened
by artificial coring to 6 to 8 cm, all wasps continued to dig from 1.2 to 8.0 cm farther. (Two of
these wasps dug 2.1 and 5.7 cm beyond the point
where they ultimately constructed side tunnels.)
There were no significant differences between the
lengths of experimental and reference tunnels,
i.e. there were no differences between the actual
results and the predicted values under the external cues hypothesis (Fig. 6A). The lengths of
the tunnels were significantly shorter than the
predicted lengths under the internal cues hypothesis (Fig. 6A; sign test P = 0.02, Wilcoxon test
P < 0.01). In addition, there were no significant
differences between reference and experimental
tunnels when the nests were first provisioned, or
in the lengths of the tunnels on the following day.
(2) Tunnels cored late in digging. When 11
tunnels already 5.5 to 7.0 cm long were cored to
11.0 to 11.5 cm, two wasps stopped digging and
nine continued to dig. Both the wasps that stopped
at the cored length moved up 1.0 c m from
the cored bottom to dig their side tunnels. The
other nine wasps continued to dig 1.0 to 2.8 cm
beyond the cored length where they constructed
side tunnels. The lengths of experimental tunnels
433
were significantly shorter than the predicted
lengths if the~wasps had continued to dig disregarding the coring, i.e. shorter than predicted
by the internal cues hypothesis (Fig. 6B; sign test
P < 0.006, Wilcoxon P < 0.01). The cored experimental tunnels were, however, slightly longer
than the reference tunnels (Fig. 6B; sign test P
= 0.03, Wilcoxon P < 0.01). The time spent digging the experimental tunnels was not different
from the time spent digging the reference tunnels.
The lengths of the experimental tunnels remained
greater than the reference tunnels when the nests
were first provisioned (sign test P = 0 . 0 1 ,
Wilcoxon P < 0.05). However, by the following
day, the lengths of experimental tunnels were the
same as the reference tunnels, due to the fact that
by then wasps had constructed side tunnels at
shallower depths, filling in the main tunnels below. There was no difference in the number of
prey provisioned or in the number of days experimental and reference nests remained active.
(3) Tunnels cored very late in digging. When 11
tunnels already 8 to 9 cm long were artificially
lengthened by coring to 13 to 14 cm, eight wasps
stopped digging within 0.5 cm and three continued to dig from 1.5 to 7.5 cm farther. Of those
that stopped digging immediately, two dug a side
tunnel within 0.5 cm of the cored length, whereas
six wasps moved 1.0 to 4.0 cm u p the main tunnel
to dig the side tunnel. Of those that continued to
dig after the coring, two moved up from the distance they had dug (by 3.2 and 5.5 cm) to dig the
side tunnel and one dug a side tunnel 1.0 cm
below the cored length.
When tunnels were cored very late in digging,
most wasps partially filled their experimental tunnels and four also partially filled the reference
tunnels. For these four, the predicted value for
the experimental tunnels is undefined, thus reducing the sample size by four. Experimental
tunnels were significantly shorter than predicted
by the internal cues hypothesis (Fig. 6C; sign test
P < 0.03, Wilcoxon P < 0.02). The length of the
experimental tunnels was the same as predicted
by the external cues hypothesis (Fig. 6C; sign
test P = 0.227, Wilcoxon P > 0.05). There: was
no significant difference in the time spent digging
experimental and reference tunnels. By the time
the nests were provisioned, usually on the following day, the lengths of the experimental tunnels were no longer greater than the lengths of
the reference tunnels. There was also no significant difference in the number of days experimental and reference nests remained active nor
in the number of prey provisioned in each.
434
ANIMAL
~qo;,,
_
uJlo
_J
LU
Z
Z
BEHAVIOUR,
9
28,
2
/
/
|
~BE+C E
~,>i v
~8
p-
~oe,/.C
12
.
rr
LU
--I
9
I0
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~
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i
Eext = B E + C E
--I
9 t
_,,~/"
~
~6
:E
,~.e-
14
[
.."'f~l
wasp
b,J
9
//e
t
,/
/
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9
, Eext if wasp fills
experimental
compleeely
fills
9
ill/'
reference,
Eext undefined
!~cR
CR
LU
i,
0
"r 2
p-
CE
CR
E
z.
BR _,t,,,.q
~ o':7~tl
o
ill
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t
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oR
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6
8
tO
LENGTH OF REFERENCE TUNNEL, R
6
t
CR
_l~,
I-i fi
t
8
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12
14
I
R
Fig. 6(C)
Fig. 6(A)
/
/
/
/
?
//
/ C.,~,
|
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14
Fig. 6. A comparison between the final lengths in centimetres of experimentally cored tunnels and the predicted
final lengths (based on the lengths of uncored reference
tunnels) under two alternative hypotheses. (See text for
symbols.) Each data point represents the intersection
between the lengths of the experimental (E) and reference
(R) tunnels dug by each individual wasp. A. Tunnels
cored early in digging, i.e. after the wasp had dug 3 to 4
cm I lengthened the experimental tunnels 3 to 4 cm (12
individuals tested). B. Tunnels cored late in digging, i.e.
after the wasps had reached 8 to 9 cm I lengthened the
experimental tunnels by 4.5 to 5.5. cm (11 wasps tested).
C. Tunnels cored very late in digging, i.e. after the wasps
had reached 8 to 9 cm I lengthened the tunnels by 5.0
to 6.0 cm (11 wasps tested). All reference tunnels were
lengthened by less than 1 era.
12
Conclusions
BE+C
Eexf :BE + C E / /
E
I0
/
bJ
/
:_-F_cR
8
CE
/
6
BR
4~-1
I
/
/
CR
I I
6
I
I
8
I
R
I
I0
Fig. 6(B)
I
I
12
I
14-
and Discussion
The result o f coring tunnels clearly shows that
the wasps were taking the alterations into acc o u n t : the cored tunnels were significantly
shorter than the predicted length by the internal
cues hypothesis and the same length as predicted
by the external cues hypothesis. Thus it appears
that some external cue is controlling the length
o f the wasp's tunnels. I n the model presented in
Fig. 5, the variable controlling tunnel length is
dearly measured external to the animal, i.e. outside the dashed line.
Wasps were capable o f shortening the lengths
o f their burrows when they were cored close to
the time o f completion. After coring, the wasps
tended to continue digging for a short while and
BROCKMANN: CONTROL OF NEST DEPTH
PLATE
I
O
o~
o=
o~
0~
oh
V~
Brockmann, Anita. Behav., 28, 2
BROCKMANN: CONTROL OF NEST DEPTH
then moved back up the main tunnel to construct the side tunnel, leaving a spur. The lengths
of these experimental tunnels remained longer
than the lengths of the reference tunnels by a
mean of 2.5 cm. When I measured the length of
the main tunnels, I always included the length
of the spur that was not filled with soil. In excavations of tunnels, the spur was found to be 1
to 3 cm long. The presence of the spur is sufficient to explain the observed differences in the
lengths of experimental and reference tunnels
cored late in digging. Therefore, all the results of
these experiments seem consistent with the hypothesis that tunnel length is controlled by some
external variable. The next question, then, is how
the cue is measured.
Depth of Soil or Distance from Entrance?
There are two general kinds of variables associated with tunnel length: the distance from the
tunnel entrance and the depth in the soil (ordinarily not the same as tunnel length because the
tunnel is dug at an angle with respect to the surface; see Table I). The model in Fig. 7 is a
modification of Fig. 5. It illustrates the dichotomy between the two possible external cues
the wasp might be using to measure tunnel
length. The wasp may be digging, for example,
until she reaches 29 C in the soil. The depth at
which 29 C occurs is a function of her distance
from the surface (given a particular temperature
profile and soil conditions). On the other hand,
=/
3
-88
OlOth
, s'?~
Yc
I.A=.
Fig. 7. Block diagram illustrating two alternative hypotheses concerning the nature of external feedback in the
control of digging. The length of the tunnel affects two
variables: distance from the tunnel entrance at the surface
and depth in the soil. Even in normal tunnels, these two
are not quite the same: the transfer function Heist is the
identical transform, but the function Hdo~this not because
the tunnel is dug at a slight angle (75~ with respect to
the vertical. The wasp may be sensing either a variable
(Va) that is a function (Ha) of the distance from the
entrance or a variable (Vb) that is a function (Hb) of
the depth in the soil.
435
she may be digging, for example, until the light
in her tunnel drops to a certain level. Since light
enters at the entrance, the light level at any given
point is a function of her distance from the entrance of the tunnel (and surface lighting conditions). The wasp could be cueing on any variable
associated with either distance or depth, so a
series of experiments was conducted in which the
two variables were dissociated.
I f the wasp is measuring a variable deep in the
soil, then it is possible to raise the level of the
entrance without affecting the digging (prediction
of depth in soil hypothesis). I f she is digging as a
function o f her distance from the entrance, then
extending the entrance above the ground level
would decrease the length o f the tunnel (prediction of distance from entrance hypothesis).
Method for Extending the Tunnel
I extended the length of the wasp's main tunnel by raising the entrance above ground level
with an 'extension block'. Extension blocks are
7- • -5-cm wooden blocks o f various heights.
Extending through the block is a hole at a 75 ~
angle (with the surface of the block). The hole is
lined with a glass tube (1-cm diameter) that can
be adjusted so that there is a good fit between
the tunnel in the block and the wasp's own tunnel in the soil. The glass tube is lined with a thin
layer of soil so that the wasp can climb more
easily.
After the wasp had dug 3.5 to 4.8 cm (far
enough so that her hind tarsi were in the tunnel),
I placed the extension block over the entrance
while she was inside her tunnel. I picked up most
of the dirt on the small mound that had accumulated and placed it on top of the block in the
appropriate orientation to the new tunnel entrance. Invariably the wasp simply backed out
and dropped her next load of dirt on the new
mound at the top of the block (Plate I, Fig. 8).
Once on the mound, only occasionally did she
seem bothered by the new surroundings; and
soon she was digging as rapidly as before the
block was added. Sometimes (about once per
tunnel) a wasp fell off the block and was not able
to find the entrance to her tunnel. I then removed
the block, allowed the wasp to enter the tunnel,
and then replaced the block immediately, before
she backed out again. After a wasp had spontaneously left the block once, going through the
orientation flight, she returned directly to the entrance in the bIock. I generally removed the
block after the wasp was well into (more than one
ANIMAL
436
BEHAVIOUR,
2
from the actual soil surface to the bottom of the
main tunnel.
body length) the side tunnel, although twice the
wasp provisioned the nest with the block in place.
Two separate tunnel extension experiments
were conducted: (1) the experimental extension
block extended the tunnel 7 cm and the reference
block extended the tunnel 2 cm (conducted MI
1974); (2) the experimental extension block extended the tunnel 4 cm and the reference block 1
cm (also conducted MI 1974). Naturally, placing
the block over the tunnel changed the temperature and moisture gradients through which the
wasp walked while she was in the block, but once
below the surface of the soil, there were no differences between experimental and reference tunnels (Brockmann 1976). The definition for the
length of the tunnel remains the same for these
experiments as that used previously: the distance
Analysis of Data
Under the depth in soil hypothesis the length
of tunnels dug under the experimental extension
block (Eaepth) should be the same length as the
tunnels dug under the reference block (R):
(6)
Edepth = R
(Fig. 9A). On the other hand, if the wasps are
measuring some variable as a function of their
distance from the entrance of their tunnels, then
the extension block should affect the length of the
tunnels they dig. Under the distance from entrance hypothesis, then, the lengths of the experimental tunnels (Edis0 should be shorter than the
A. Depth in Soil
EXPERIMENTAL
TUNNEL
Experimental J f / /
Extension I X /] ]
28,
Hypothesis
PREDICTION : Edept h
REFERENCE
TUNNEL
Reference
=R
BE
I
BR
EXPERIMENTAL
TUNNEL
^E///J_
so,,
B. Distance
from E n t r a n c e
R
Hypothesis
TUNNEL
/ ........... I"': ...... Ix l
I
PREDICTION: Edist
Edist = R-D
/
/
~ /,,
R
Fig. 9. Experimental predictions of the depth in soil and distance from
entrance hypotheses (see text for symbols). A. If the wasp is measuring her
depth in the soil, then the distance she digs should not be affected by
the presence of extension blocks. B. If the wasp is measuring her distance
from the entrance of her tunnel (at the top of the extension block), then
the length of the tunnel she digs should be affected by the presence of
the blocks.
437
BROCKMANN: CONTROL OF NEST DEPTH
lengths of the reference tunnles (R). How much
shorter should they be ? According to this hypothesis, the wasps are measuring their distance
from the tunnel entrance at the top of the extension block. Therefore, the length of the experimental tunnel (E) plus the length of the experimental extension (X~) should equal the length of
the reference tunnel (R) plus the length of the
short extension block on the reference tunnel
(XR). Hence,
E d i s t = R @ XI~ - - X E .
(7)
If D is the difference in length between the
reference and experimental extensions, then
Eaist = R
-
-
D
(8)
(Fig. 9B). However, there is a complication. With
the 7-era-high block, it was not always possible
to place it over the tunnel early enough, i.e. the
length of the tunnel when I added the block was
already greater than the predicted length, BE >
E a r n . The coring experiments showed that the
wasps were capable of shortening tunnels by filling. However, I decided not to assume that the
wasps could shorten tunnels under the different
protocol of this experiment. Therefore, I decided
to use the following predicted values for the
lengths of the experimental tunnels under the
distance from entrance hypothesis:
if R -- D > B~, then
and
if R -- D ~< B~, then
Eaist = R - - D
Earn = B~.
(9)
(10)
Results
When the 7-cm experimental extension was
used, the lengths of the experimental tunnels
were shorter than predicted by the depth in soil
hypothesis. Experimental tunnels dug with the
7-cm extension were shorter than those dug with
the 2-cm reference extension (Fig. 10A; N -~ 12:
sign test P < 0.003, Wilcoxon P < 0.01). However, when the 4-cm experimental extension was
used, experimental tunnels were not significantly
different from those dug with the 1-cm reference
extension (Fig. 10B; N = 8: sign test P = 0.36).
Tunnels dug with the 4-cm extension were also
no different in length from those dug with no
extension block (N = 11 : sign test P ----0.15).
In both experiments the experimental tunnels
were significantly longer than the predicted
values by the distance from entrance hypothesis.
Experimental tunnels dug with the 7-cm extension were significantly longer than predicted
from the length of those dug under the 2-cm
reference extension (Fig. 10A; N ~ 12: sign test
P < 0.02, Wilcoxon P < 0.01). Tunnels dug
with the 4-cm experimental extension were also
shorter than the values predicted from the 1-cm
reference extension (Fig. 10B; N = 8: sign test
P = 0.04, Wilcoxon P < 0.01). In summary,
then, the lengths of the experimental tunnels
generally fell between the values predicted by the
two alternative hypotheses.
Eight of 12 experimental tunnels dug with the
7-era extension were abandoned after a side
tunnel was dug (and the block was removed),
whereas only four were abandoned when dug
with the 1-cm and three when dug with the 2-cm
reference extensions. Of the nests that were not
abandoned, the lengths of experimental and
reference tunnels on the following day were not
significantly different, i.e. by then the wasps had
made the experimental tunnels longer.
Condusions and Discussion
When I used the 7-cm extension over the experimental tunnels, the wasps dug tunnels that were
shorter than the reference tunnels. When I used
the 4-cm extension over the experimental tunnels,
the wasps dug burrows that were the same length
as the reference tunnels. Such a result suggests
that the wasps were measuring some factor as a
function of their depth in the soil as well as their
distance from the entrance of their tunnel. When
the extension was very high (7 cm above soil
i5
|
/
/
/
/
/
/
10
//
///
9 "
/
-B E
Edisl = B E
9
,,
BR
5
I0
R
Fig. 10(a)
o
/
/
/
/
438
ANIMAL
BEHAVIOUR,
|
/
/
9
/
/
/
/
/
/
IO
9//// /
8
b3
/11~
9 4, ("
//// ~@
6
-B E
4
Edist = B E
2
BR
0
0
I
21
I
41 ~ I
61
I
81
~
II0
I
IL2 I
R
Fig. 10(b)
Fig. 10. A comparison between the final lengths in centimetres of artificially extended main tunnels and the
predicted lengths under the depth in soil and distance
from entrance hypotheses (see Fig. 9 for explanation).
A. The tunnels were extended by adding a 7-cm extension
block to the experimental tunnel and a 2-cm extension to
the reference tunnel (12 wasps tested). B. The experimental tunnel was extended by adding a 4-cm extension
block and the reference tunnel was extended by adding a
1-em extension block (eight wasps tested).
surface), the wasps' distance from the entrance
affected the lengths of their tunnels. When the
extensions were lower (4 cm), this distance apparently did not affect the lengths of the tunnels
they dug.
What might account for the different results
from the 7-cm and 4-cm extension experiments ?
I know of no physical differences between the
two kinds of extension blocks nor any difference
in the procedure used in the two experiments.
There were three differences that conceivably
could have had an effect. When I added the 7-cm
extension to the experimental tunnels, the distance the wasps had already dug plus the length
of the extension either usually exceeded or was
within 1 cm of exceeding the length of the reference tunnel (18 of 22 tunnels). When I added the
4-cm extension, however, all but two of the wasps
still had more than 1 cm left to dig (2 of 16
tunnels). If the wasps measured the length of the
main tunnel only periodically, for example, this
could influence tunnel length differently in the
28,
2
two experiments. Another difference between the
two experiments was that the 4-cm experiment
was conducted later in the season in 1974 than
the 7-cm experiment. Any physical factor that
correlated with date might influence tunnel
length differently under the two extension experiments. However, there were no significant correlations between any environmental variable that
I measured and the date in 1974. Another difference was that all but two of the wasps that dug
in the 4-cm extension experiment had also dug
with a 7-cm extension block earlier in the season.
But once again I do not know how this could
differentially influence tunnel length under experimental and reference blocks. I am left, then,
with the somewhat contradictory results of the
extension experiments, indicating that the factors
influencing tunnel length were measured both as
a function of the wasp's distance from the entrance of the tunnel and as a function of her
depth in the soil. Looking now at the distance
from the entrance component, how does a wasp
measure how far she is from the entrance of her
tunnel ?
Optical Distance or Path Length?
There are at least two kinds of distance from
entrance cues that a wasp might use: what might
be called the 'optical distance' and the actual
physical path traversed by the animal. The wasp
might measure the optical distance, for example,
by assessing the light level at the bottom of the
tunnel (e.g. 'dig until it gets dark') or by judging
the diameter of the tunnel opening as seen from
the bottom (e.g. 'looking over her shoulder' at the
tunnel entrance). Alternatively, she might measure the physical distance, for example, by counting the number of steps to the bottom or by
sensing the concentration of some air-borne
chemical that must follow that same path length.
Ordinarily these two distances would be the same,
of course, but they can be experimentally dissociated by allowing the wasp to walk through a
clear plastic block while digging, thereby lengthening the physical path of the tunnel without
influencing the optical distance.
Method for Increasing Path Length
I increased the tunnel length relative to the
optical distance by using 'lengthening blocks',
transparent blocks cast from clear acrylic plastic
through which there extended either a straight or
a curved tunnel. I added the block after the wasp
had dug 3.0 to 4.3 cm, far enough so that her
hind tarsi were below the ground surface. The
BROCKMANN: CONTROL OF NEST DEPTH
lengthening b l o c k p l a c e d over the reference tunnel was similar to the 4-cm extension b l o c k - - a
7-cm • 8-cm plastic b l o c k with a 1-cm-diameter
hole t h r o u g h it at a 75 ~ angle so t h a t the wasp
w a l k e d 4.5 c m t h r o u g h the block. The experim e n t a l lengthening b l o c k was also 4 c m high,
b u t the tunnel t h r o u g h this 7-cm • 13-cm b l o c k
was curved so t h a t the distance the wasp actually
w a l k e d while in the b l o c k was 8.5 cm. The curved
tunnel in the experimental b l o c k was constructed
so t h a t the wasp entered at a 75 ~ angle regardless
o f whether she entered the b l o c k f r o m the m o u n d
at the t o p o r f r o m her b u r r o w at the b o t t o m . The
A. Opticol
Distance
angle o f the tunnel t h r o u g h the r e m a i n d e r o f the
b l o c k was 13 ~ with s m o o t h l y r o u n d e d corners at
the turns (Fig. 11). T h e inside o f b o t h experim e n t a l a n d reference lengthening blocks was
lined with a thin layer o f q u a r t z sand (the wasps
need r o u g h walls in o r d e r to climb u p the tunnel).
There was little difference in the light, t e m p e r a ture a n d h u m i d i t y o f the tunnels d u g b e l o w the
two kinds o f blocks.
The curved tunnel was a fairly artificial situation, as wasps rarely have curved m a i n tunnels
to their n e s t s q a n d when they do it is for a couple
o f centimetres at most, to get a r o u n d a stone o r
Hypothesis
EXPERIMENTAL
TUNNEL.
E,perimenta,
L.o0,
.o,o0
439
e~
I PREDtCTION: Eopt
REFERENCE
TUNNEL
Re'ereocel ~
L.o0,.
=R
._~~I
Optical
distance
R
Path
length
R
B.
Path Length Hypothesis
EX PERIMENTAL
TUNNEL
PREDICTION :
REFERENCE
TUNNEL
/
r /
/
BE-
Epalh : BE
B'R
R
Fig. 11. Experimental predictions of the optical distance and path length
hypotheses (see text for symbols). A. The optical distance in the experimental and reference tunnels was increased by placing a 4-cm-high clear
plastic block over the wasps' tunnels. If the wasps were measuring their
optical distance from the tunnel entrance, then experimental and reference
tunnels should be the same length. B. The physical path length was increased by 8.5 cm in the experimental tunnels and by only 4.5 cm in the
reference tunnels. If the wasps were measuring the length of the tunnel from
the entrance at the top of the block, then tunnels dug under the experimental block should be 4 cm shorter than those dug under the reference
block.
Epath
/
/
/
/.'~
Z,~"
440
ANIMAL
BEHAVIOUR,
root they have encountered while digging. Nonetheless, after 2 to 7 min, the wasps walked
through the experimental lengthening block as
they walked through the reference block. The
only difference was that when the experimental
block was first introduced, the wasps backed up
the first 75 ~ section and deposited their load of
soil in the section with the 13 ~ slope. After 5 min
or so, the accumulating dirt and hence the narrowing of the tunnel forced them to walk out on
top of the block, which they continued to do
from then on. I kept the lengthening block over
the tunnel for one full day after digging or until
the wasp had provisioned her nest with one
katydid. In the analysis I did not use tunnels that
were abandoned while the block was in place.
This was necessary since I did not have the otoscope during this study ( N H 1975), so I could
not see when a side tunnel was constructed.
Analysis of Data
Under the optical distance hypothesis, the predicted length of the experimental tunnel (EopO
is the same as the length of the reference tunnel
(Fig. l l A ) :
Eopt = R.
(11)
Alternatively, under the path length hypothesis,
the predicted length of the experimental tunnel
(Ep ath) is shorter than the length of the reference
15r-
/
o
/
/
/
/
/
9
9
,.,,,"""
u
/ I1~"
5
/
-BE
/
/I
I
/
"i/
~
i
t/~<~"
Epoth = B E
BR
0 _ _ 3 _ _
0
~
I
5
10
_
I
15
28,
2
tunnel (R) by the difference in the distances
walked while in the two blocks, or 4 cm. However, as in the extension experiments, when the
predicted length of the tunnel (Epath) is less than
the length of the tunnel when I added the block
(BE), then the predicted length is B~ (Fig. 11B):
i f R -- 4 > BE then Epat~ = R -- 4 and (12)
i f R -- 4 < BE then Epath = BE.
(13)
Results
All 10 wasps continued to dig when the experimental and reference lengthening blocks were
added. The lengths of tunnels dug with the experimental block were shorter than those dug
with the reference block (Fig. 12; sign test P =
0.011, Wilcoxon P < 0.01). The lengths of experimental tunnels were not significantly different
from the values predicted by the path length
hypothesis. The lengths of the experimental tunnels remained somewhat shorter than the lengths
of the reference tunnels on succeeding days (sign
test P = 0.03, Wilcoxon P = 0.03).
Since the reference lengthening block was similar to the 4-cm extension block discussed in the
previous section (except that the former was
made of clear plastic and the latter of wood), it
was possible to make a comparison similar to
those made in the preceding section and thereby
add more data to the problems discussed under
the extension experiments. Tunnels dug with the
4-cm plastic block were significantly shorter than
tunnels dug with no block (Fig. 13; N = 10: sign
test P = 0.01, Wilcoxon P = 0.02). Tunnels dug
with the 4-cm plastic block were not significantly different from the values predicted by the
distance from entrance hypothesis (Fig. 13; sign
test P = 0.09). Therefore, the results using the
4-cm plastic block and N H 1975 wasps were
different from the results obtained using the 4cm wooden extension block and the M I 1974
wasps, where the tunnel length data were far
more variable (Fig. 10). These results provide
further support for the hypothesis that the wasps
are measuring tunnel length as a function of their
distance from the entrance of their tunnel rather
than their depth in the soil. It seems possible that
for some reason the wasps were not measuring
the full distance of the 4-cm wooden extension
blocks in the preceding experiment.
R
Fig. 12. A comparison between the final lengths in
centimetres of artificially lengthened main tunnels and
the predicted lengths under the optical distance and path
length hypotheses (see Fig. 11 for explanation; 10 wasps
tested).
Conclusions and Discussion
The results of the tunnel-lengthening experiment are consistent with the hypothesis that the
lengths of tunnels dug by wasps are measured as
B R O C K M A N N : CONTROL OF NEST D E P T H
/
9
/
/
/
//
9
/
/
/////
~//////
[0
//~.
/ t~
BE
9 9
Edlsl = B E
BR
O0
5
.....
]kO
ll5
R
Fig. 13. A comparison between the final lengths in centimetres of artificially extended main tunnels and the
predicted lengths under the depth in soil and distance
from entrance hypotheses (see Fig. 9 for explanation).
The experimental tunnels were lengthened by adding a
4-cm plastic extension block. No extension was added
to the reference tunnels (10 wasps tested).
a function of the wasps' physical distance from
the entrance of their tunnels. Somehow they are
apparently measuring the physical path length
of the tunnel as they walk up and down during
digging. As was mentioned earlier, however, the
distance from the entrance probably only partially explains the mechanisms controlling tunnel
length. At least under some circumstances, the
wasps' depth in the soil may also influence the
lengths of their tunnels.
There is one problem with this experiment: it
is possible that the light level in the bottom of
the experimental tunnels may have been slightly
lower than in the bottom of the reference tunnels.
This is because sunlight could (although usually
did not) shine directly down the straight tunnel
through the reference block, whereas in the
curved tunnel of the experimental block this was
not possible. Therefore, I thought it important
to control for differences in lighting conditions
in a separate setZof experiments.
Does Light Affect Tunnel Length ?
Although the wasps are apparently measuring
their physical distance from the entrance of their
tunnels, it is still possible that they may be affected by the amount of light in the tunnel. I
tested for this by having the wasps walk through
441
a false-tunnel-entrance block as they were digging. The false tunnel entrance is similar to the
1-cm wooden reference extension block of the
tunnel extension experiment. The false-entrance
block differs in that it has a 1-cm-high rim
around the hole and mound except where the
wasp actually walks. By resting a piece of wood
on this rim, it is possible to shade either the
tunnel entrance and the mound or just the
mound or just the entrance (not done because
the small area involved made this more difficult)
or neither. The procedure is the same as in the
previous experiments: each wasp digs one experimental and one reference tunnel (in random order)
and the experimental and reference tunnels differ
in the amount o f shading given during digging.
If light at the bottom affects the length of the
tunnel, then experimental tunnels dug with false
tunnel entrances where the entrance and mound
are shaded should be shorter than reference tunnels dug through false tunnel entrances where
there is no shading. If light at the bottom is not
important, then the lengths of these experimental
and reference tunnels should be the same. But it
is also possible that light at the surface may affect
tunnel length. Therefore, it is necessary to make
a second comparison. If light at the surface is
affecting tunnel length, then experimental tunnels
dug with false entrances where only the mound is
shaded should be shorter than reference tunnels
dug without the mound being shaded. On the
other hand, if light at the bottom affects tunnel
length, then experimental tunnels dug with the
entrance and mound shaded should be shorter
than reference tunnels dug with only the mound
shaded. If light is of no importance, then all
these experimental and reference tunnels should
be the same length.
Results
(1) Lighting in bottom of tunnel. Experimental
tunnels where the entrance and mound were
covered during digging were significantly shorter
than reference tunnels where there was no shading (N = 10: sign test P = 0.06, Wilcoxon P <
0.01). However, there is no significant difference
between experimental tunnels dug with the entrance and mound covered and reference tunnels
dug with only the mound covered (N == 10: sign
test P = 0.62). As discussed above, this result
supports the hypothesis that lighting in the bottom of the tunnel is not affecting the length of
the tunnel. It suggests instead that lighting at the
surface may be important.
442
ANIMAL
BEHAVIOUR,
(2) Lighting at the surface. Experimental tunnels dug when just the mound is covered are
significantly shorter than reference tunnels dug
with no shading on the mound (N = 10: sign
test P = 0.06, Wilcoxon P < 0.02). Tunnels dug
under the two conditions differed in length by
a mean of 3.4 cm. Apparently, then, neither the
optical path length, i.e. the distance from the
tunnel entrance as determined by the wasp 'looking over her shoulder' at the entrance, nor the
amount of light at the bottom is affecting tunnel
length. Rather, the amount of light falling on the
mound at the surface is in some way affecting the
length of the burrow.
Discussion
A New Model for the Control of Nest Depth
To this point the proposed model for the control of tunnel length included only one behavioural component: digging. According to the
models in Figs. 4, 5 and 7, the wasps dig until
they reach the value of some variable, when they
cease digging and begin to construct a side tunnel. However, the descriptive and experimental
results of this study reveal that there are really
two behavioural components involved in the
control of tunnel length: digging and filling.
First, if I core tunnels beyond the point where the
28,
2
wasp digs her side tunnel, she moves back up the
tunnel to the appropriate level, filling the main
tunnel below the level of the side tunnel with soil.
Second, in the artificially extended tunnels where
the wasps dig short burrows, they lengthen them
after the extensions are removed. The ability to
adjust tunnel length is almost certainly not an
experimental artifact. Wasps frequently alter
existing burrows while the owner is away
(Brockmann & Dawkins in press). A female who
is digging a new burrow may return after an hour
(feeding .9) to find that another wasp has dug it
deeper or filled it in. The wasps, then, are not
simply digging until they reach the value of some
variable as was originally proposed. Rather, they
assess a variable, and then dig or move up the
tunnel, whichever is appropriate. Clearly a new
model that incorporates all the results must inelude filling.
The block diagram in Fig. 14 is one representation that accounts for the known facts concerning the control of tunnel length. The results
of the extension experiments suggest that the cues
influencing tunnel length are measured primarily
as a function of the wasp's distance from her
tunnel entrance. However, there may be other
factors measured deep in the soil that, at least
under some circumstances, might also influence
...........................................
wl
L,j ~
-- ngth~
)~
~
Y:TUNNEL
LENiTy
DIG
light
stimulus
z
path
~
WASP
..................................
deiPnth
~ Vp:stimulus
Vb: stimulus
Fig. 14. Block diagram illustrating the simplest scheme that accounts for
known facts concerning the control of tunnel length in Sphex ichneumoneus.
(See Fig. 4 and text for symbols.) The error signals Wand W' activate motor
activities of filling or digging, which together with the present length of the
tunnel determine its subsequent length. When Z = X no error signal is
produced and the animal begins the next behavioural subsystem (digging
the side tunnel). The results of this study suggest that surface lighting
conditions may affect the value of the set-point (X).
BROCKMANN: CONTROL OF NEST DEPTH
tunnel length (Fig. 14). Therefore, either there is
more than one variable controlling tunnel length
through one or more feedback mechanism(s), or
some factor encountered during digging may influence the value of the set-point. In addition, the
results of the tunnel lengthening experiments suggest that the wasp is in some unknown way
measuring the physical path length of her tunnel
(Fig. 14). For example, she might be pacing off
the distance or measuring how long it takes her
to walk the length of the tunnel. Only further
experiments can shed light on these intriguing
problems.
It should be noted that other, more complex
controlling schemes are possible. For example,
other external stimuli associated with tunnel
length could affect the set-point (X) in a manner
similar to the effect of surface lighting. Different
set-points might control filling and digging; in
fact, these activities might be controlled by
separate feedback loops acting through different
senses. Other complexities are also possible, but
present data do not necessitate involvement of
any components or connections beyond those
diagrammed in Fig. 14. This model represents a
hypothesis that could be tested by further
experiments.
Additional Variables Influencing Tunnel Length
There appear to be factors other than path
length affecting the depth to which wasps dig.
Shading the mound and tunnel entrance or just
the mound shortens the burrow by about one
body length. But what is the specific variable
involved ? Is it the change in light level per se or
some other factor highly correlated with surface
lighting conditions ? Are there other factors also
influencing tunnel length ?
The descriptive results mentioned at the outset
indicate that some of the observed variability in
tunnel length may be accounted for by covarianee
with surface light levels and soil moisture. However, the experimental results suggest that the
wasps are not digging until they reach some
specific value of these variables, but rather that
they dig to a preset length that may be influenced
by variables encountered during digging. Further
evidence for this comes from the fact that the
depth at which a particular temperature, light, or
moisture level occurs differs markedly over the
day (Brockmann 1976). If they were digging to
specific values of these variables, there would be
large differences in the lengths of burrows dug
early in the morning and late in the afternoon.
These descriptive results, then, are consistent
443
with the experimental finding that shading
influences burrow length but does not determine the specific depth to which the wasps dig.
Most authors who have studied golden digger
wasps in detail agree that the compaction of the
soil has something to do with tunnel length (Rau
& Rau 1918; Frisch 1937; Ristich 1953). However, I feel that soil moisture would be an equally
good explanation for their observations. Sandy,
uncompacted soils are capable of far greater drying than are packed soils with some clay content
(Miller 1973). The interesting results of Rau
(1933), that wasps dig very short tunnels in hardpacked but very wet soils in Panama, would tend
to confirm this suspicion (although, of course, it
is possible that Panamanian Sphex iehneumoneus
have been subjected to different selective pressures so that the control mechanisms may be
different). Evans (1966b) and Alcock & Ryan
(1973) have suggested that in another species of
sphecid wasp that digs tunnels of highly variable
lengths (Microbembex nigrifrons), variability in
soil moisture may be partially responsible for the
variability in tunnel length. Literature observations, then, seem to support the notion that soil
moisture may be an important additional factor
influencing tunnel length.
When one discovers in an experiment that an
animal is adjusting the depth to which it digs its
nest on the basis of surface light levels, one cannot help but ask why. What is the selective advantage for such behaviour ? I see at least three
possible explanations. First, it is, of course, possible that I have not measured the correct variables. Shading the nest entrance may simultaneously increase the surface soil moisture or
humidity. No experiments were conducted to
test the effect of these variables independent of
light levels. Therefore, it remains to be seen
whether the wasps are really responding to
shading or to changes in some other variable
closely correlated with shading. Second, it is also
possible that the animal might be using an easily
measured variable such as surface light level to
predict some difficult-to-measure but biologically
important variable such as soil moisture (which
is possible if the two are closely correlated).
Third, surface light levels may, in fact, be a
better predictor of some biologically important
condition deep in the soil where the larva is
developing than actual measurements of the
condition itself. For example, the integrated
temperature over the day may be an important
factor in the success of the developing larva.
Temperature levels in the soil vary enormously
444
ANIMAL
BEHAVIOUR,
over the course of the day, being cooler than the
surface early in the morning and warmer than
the surface late in the afternoon as a wave of heat
f r o m the hot afternoon sun flows deep into the
soil (many complexities are added to this by
other weather conditions, personal observations).
I f the female were to measure the temperature
deep in the soil she might have a worse indication
o f the integrated temperature than if she simply
knew whether she was digging in the sun or in
the shade and adjusted her behaviour accordingly.
Conclusion
The results collected so far indicate that a wasp
measures the length of the tunnel she is digging
and compares it with a preset value. She continues digging if the main tunnel is too short, and
she moves up the tunnel if it is too long. The
preset length may be altered up or down by about
one body length depending on surface conditions.
Surface light levels may act to reset the set-point
for tunnel length, or soil moisture encountered
during digging may act as a secondary mechanism controlling tunnel length. However, such
a proposed controlling mechanism appears to be
contrary to the observation that the lengths of
the tunnels dug by one wasp in one localized
area are highly variable. I do not think this is a
contradiction; for even within one area, local
conditions, such as the presence of a plant,
strongly influence local soil moisture and light
levels. In Exeter, where there was less variability
in the environment, i.e. equal exposure to sun
and more or less equal presence of plants, the
tunnel lengths were also less variable. I suggest,
therefore, that variability in tunnel length may
be largely a reflection of variability in local surface conditions.
The model proposed for the mechanism controlling tunnel length (Fig. 14) suggests that the
wasp is using more than one kind of information.
The animal is apparently not completely dependent upon one external stimulus source or one
sensory system. Such a redundant system could
be crucial for an animal that digs in such a
variety of soils over such a wide geographical
range. Because wasps choose to dig only in certain areas, only at specific times of the day
(Brockmann 1979), and only to particular
depths, and because of redundant mechanisms
controlling tunnel length, the end result of the
digging, i.e. the environment in which the larval
wasp develops, may be held within specific
limitations.
28,
2
Acknowledgments
I would like to express my particular gratitude to
Jack P. Hailman, whose thoughtful comments
and critical mind played a very important role in
the design of these experiments and in the writing
of this manuscript. I would also like to thank
W. P. Porter, A. O. W. Stretton, S. D. Carlson,
R. D. Shenefelt, and H. E. Evans for reading the
manuscript and for providing many helpful suggestions. The University of Michigan in Dearborn, Michigan, Carleton College in Northfield,
Minnesota, and Richard and Betty Brinckerhoff
of Exeter, New Hampshire, kindly provided
study sites. Orin Gelderloos, Gary Wagenbach,
Timothy Manning, Calvin DeWitt, Thomas
Brockmann, and Jean O'Meara provided assistance for this study. The University of
Wisconsin and Maxwell and Helen Brockmann
provided support for the research. Cheryl
Hughes prepared the figures and drawings from
slides and sketches by the author. John Dallman
and Ken Olsen assisted with the experimental
apparatus. This manuscript is a modification of
one submitted in partial fulfilment of the requirements for the Ph.D. from the University of
Wisconsin.
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