Behavioral Ecology Vol. 9 No. 4: 360-366
Presampling sensory information and prey
density assessment by wolf spiders
(Araneae, Lycosidae)
Matthew H. Persons and George W. Uetz
Department of Biological Sciences, University of Cincinnati, Cincinnati, OH 45221-0006, USA
Decisions regarding foraging patch residence time and the assessment of patch quality may be mediated by various sources of
information. This study examined the use of sensory cues by hunting spiders to assess prey density in the absence of prey
capture. Adult female wolf spiders [Schixocosa ocmeata (Hentz); Lycosidae] had food withheld for 4 days and then were exposed
to artificial foraging patches containing four densities of crickets (0, 3,10, 20) with different sensory stimuli (visual and vibratory
information, visual only, and vibratory only). The spiders were not allowed to feed during trials, and patch residence time was
recorded. The spiders varied patch residence time based on sensory cues alone and spent more time in patches with higher
prey density. With visual information only, spiders could apparently distinguish among prey densities almost as well as with visual
and vibratory cues combined, but residence time did not differ among prey densities when only vibratory information was
presented. Measurements of vibration levels produced by cricket activity under experimental sensory treatments conform to test
results, suggesting that visual detection of crickets is important in patch assessment used in determining patch residence time.
Key words: patch quality, prey density, residence time, Schixocosa ocreata, sensory information, wolf spiders. [Behav Ecol 9:360-
366 (1998)]
H
ow animals assess foraging patch quality and determine
how long to remain in a patch before leaving has been
of considerable interest in foraging theory (Charnov, 1976;
Cook and Hubbard, 1977; Iwasa et al., 1981; Oaten, 1977;
Pyke, 1984). Many species of sit-and-wait predators remain
longer in patches where encounters with prey are more frequent (Janetos, 1982; Turnbull, 1964; Zhang and Sanderson,
1993), but the proximate mechanisms mediating the decision
to leave or stay needs further investigation because the type
and accuracy of information used to make foraging decisions
may affect predictions of patch residence time and patch
choice (Morse, 1989).
For predators, each stage of the predation process constitutes a b«tter estimator of prey and patch quality. Endler
(1991) outlined six stages of predation: encounter, detection,
identification, approach, subjugation, and consumption. Various forms of information are gathered at different stages of
this process, including search time, prey density, subjugation
and handling costs, prey palatabiHty, and energy value. The
majority of empirical and theoretical foraging studies have
largely emphasized the foraging decisions of animals as a result of cumulative information gathered through the last
stages of predation (subjugation and consumption). However,
animals may make foraging decisions with less information,
particularly if the time costs associated with increased accuracy
outweigh the value of the information.
Sources of information used in assessing foraging patches
have been largely divided into two types: presampling and
sampling information. Although most sources of sampling information include some measure of prey capture rate per unit
time (Stephens and Krebs, 1986), individuals may also estimate patch quality before exploitation. Presampling inforAddress correspondence to M. H. Persons, who is now at the Department of Zoology, Miami Univenitjr, Oxford, OH 45056, USA. EmaiL personmhftmuohio.edu.
Received 14 February 1997; first revision 19 August 1997; second
revision 9 December 1997; accepted 22 December 1997.
O 1998 International Society for Behavioral Ecology
mation includes sensory cues, memory of patch location and
quality from previous patch sampling,, or information on the
relative distribution of resources within patch subtypes (Bayesian foraging) (Valone, 1991). Foragers that use sensory cues
or memory in predicting patch quality in spatially or temporally variable environments are known as prescient foragers
(Valone arid Brown, 1989).
Several mechanisms for the assessment of patch quality via
sampling have been suggested, and most fall under one of
several categories suggested by Formanowicz and Bradley
(1987): (1) prey capture rates or intercapture interval (Charnov, 1976); (2) gut fullness or other postingestion feedback
mechanisms (DeBenedictis et al., 1978; Johnson et aL, 1975;
Nakamura, 1968; Provenzaand Cincotta, 1993; Schuler, 1990);
and (3) encounter rate with prey (Hughes, 1979; Pulliam,
1974). The type of sampling information used to make foraging decisions may significantly affect predictions of patch
residence time or other foraging decisions.
Other empirical and theoretical foraging studies have examined the use of presampling sources of information including public information (Iivoreil and Giraldeau, 1997; Templeton and Giraldeau, 1995), prior patch experience (Valone,
1992), or the interaction of sampling and presampling information (Bayesian foraging) (Iwasa et aL, 1981; Mangel and
Clark, 1983; McNamara and Houston, 1980, 1982, 1987; Oaten, 1977), but few studies have examined the use of sensory
information specifically (Cordon and Bell, 1991; Marden,
1984; Morse, 1989). The majority of empirical treatments of
information use, residence time, and patch assessment have
used avian models (Krebs et aL, 1974, 1978; Nishimura,
1991a,b; Nishimura and Abe, 1988; Pyke, 1978b; Templeton
and Giraldeau, 1995; Valone, 1991; Wolf and Hainsworth,
198*t ZasfeandEsuls, 1976) or Hymenoptera (Heinrich, 1979,
1983; Hodges, 1981; Pyke. 1978a, 1980), both of wnieh have
been found to use prior patch experience under some circumstances.
Wolf spiders (Schixocosa ocrtata) have not been demonstrated to base residence times in food patches on prior feeding
experience (Persons and Uetz, 1997b) or prior sensory infor-
Persons and Ueti • Prey density assessment by wolf spiders
mation (Persons and Uetz, 1996a), and have been found to
move randomly between patches that differ in the presence
of prey (Persons and Uetz, 1996a). Studies show that wolf spider patch residence decisions may be based on the intensity
of current sensory stimuli without using prey capture, feeding
information, or prior patch experience. Although sensory
cues alone may be of central importance in determining residence time, almost no study has separated this mechanism
explicitly from energy gain. If an animal uses sensory assessment rather than energy gain to make residence time decisions, it should vary its patch time with different numbers of
food items in a patch, even if no feeding occurs. In previous
studies, we demonstrated that wolf spiders vary their patch
residence time based on different types of sensory information in the absence of feeding (Persons and Uetz, 1996a,b).
Sensory cues from prey result in longer foraging patch residence time for 5. ocrtata than when prey are captured in a
patch (Persons and Uetz, 1997b). Studies have also found prefeeding cues to be important determinants of choosing foraging sites among orb weaving spiders (Pasquet et al., 1994)
funnel-web spiders (Riechert, 1985), and crab spiders (Morse,
1993), indicating that sensory cues may serve as a better predictor of foraging behavior than feeding among other spiders
as well. Further, patch residence time in spiders may be determined more by prey perception than the more commonly
assumed feeding rates or prior patch experience that have
been studied in birds. We examined the influence of two sensory channels, visual and vibratory, in the absence of feeding,
on patch residence time of S. ocreata under different prey
densities.
METHODS
Study species
Schizocosa ocreata (Araneae, Lycosidae), like most wolf spiders,
is a mobile adt-and-wait forager with frequent changes in foraging site (Cady, 1984; Ford, 1978) and does not build webs
to snare prey. These ground-dwelling arthropods are found
in complex leaf litter of deciduous forests in the eastern United States. Their foraging behavior has been studied largely
within the context of distribution and population dynamics
(Cady, 1984; Wise, 1993; Wise and Wagner, 1992). Some wolf
spiders locate and attack prey using both substratum-coupled
vibrations and visual cues (lizotte and Rovner, 1988; Persons
and Uetz, 1996a), but the threshold level of different types of
sensory stimuli necessary to elicit an increase in residence
time is unknown. Under natural conditions, these spiders frequently are presented with only visual or vibratory cues from
prey. Prey vibrations may be transmitted only the distance of
a single leaf (Scheffer et. a]., 1996), but spiders are able to
perceive prey visually at greater distances (T. Valerius and J.
Renneker, personal communication). Spiders may also be located on the opposite side of the same leaf that a potential
prey item occupies, where it can detect vibrations from its
movement but cannot see the prey.
Spider collection and twi
We caught 28 immature female S. ocrtata in April 1993 at the
r.inHnnari Nature Center, Qermont County, Ohio, USA. Each
spider was housed in its own opaque container, provided water ad libitum, and fed three 1-week-old cricket nymphs every
4 days to standardize hunger level for testing. We kept all
spiders under identical controlled conditions at room temperature (23-25°C) in an environment with stable humidity
and a 12:12 h lightdark photoperiod. The spiders were allowed to mature before being subjected to experiments.
361
Experimental protocol
We conducted three experiments. The first experiment tested
if spiders modified their patch residence time in the presence
of increasing density of prey without prey consumption or
capture but with both visual and vibratory information. The
second and third experiments tested if spiders altered their
patch residence time when visual or vibratory information
alone was present to determine if sensory channels were used
differentially or if the spider's accuracy to perceive differences
in density varied. Thus, each spider was presented with four
density treatments (0, 3, 10, 20 crickets) and diree sensory
treatments (visual and vibratory information, visual information alone, and vibratory alone) in a fully crossed (non-nested) design. We analyzed each experiment separately.
The experimental apparatus controlled for visual and vibratory cues used in prey detection (Figure 1). The test apparatus
for the first experiment (visual and vibratory information)
consisted of four containers made of white foam-core board.
Each container housed two round chambers 20 cm in diameter with transparent 0.08-mm acetate walls. One chamber
served as a neutral chamber into which the spider was introduced, and the other chamber contained cricket stimuli (sensory chamber). In the sensory chamber, behind each acetate
wall, crickets were introduced at four prey densities: 0, 3, 10,
and 20. We randomly assigned each spider to a container and
tested it under all four density levels. For the second experiment, only visual information was presented to the spider.
This treatment had the cricket enclosure mounted on a separate foam-core block from that of the spider so that vibrations could not be transmitted to the substratum that the spider occupied. The third experiment presented only vibratory
information, as the acetate wall separating the spider and
crickets was covered with an opaque paper barrier.
An experimental trial consisted of a single spider introduced into the neutral (no cricket stimuli) chamber under a
clear plastic vial. Before testing, spiders were maintained on
three crickets every 4 days. All spiders tested were used after
having prey witheld for 4 days and were therefore under similar hunger levels. After a 5-min acclimation period, the vial
was removed and the spider was allowed to enter and exit the
single sensory chamber freely for 30 min. We videotaped each
trial from above and determined duration and number of
chamber visits by analyzing the videotape. Clean white paper
was placed on die floor of each chamber of the apparatus
before each trial. The chambers were also swabbed with dean,
dry cotton between trials to remove any silk draglines from
previous spiders that might affect subsequent foraging behavior. We tested each spider under each density treatment and
all three types of sensory stimuli.
Statistical methods and analysis
Many spiders fail to move into a sensory chamber at all during
an entire trial. Others do not visit all density treatment sensory chambers at least twice, which is required for an ANOVA
analysis of individual variation and interactions with prey density effects (because there is no replication within the individual factors). Therefore, for statistical reasons, we decided a
priori to analyze only residence times of spiders that visited
each density treatment and sensory treatment at least four
times. This produced a more balanced data set for ease of
interpreting statistical results (Shaw and Mitchell-Olds, 1993).
The last visit into a sensory chamber was omitted from analysis
if the spider was in the chamber when the trial time expired
because including it would underestimate the true patch residence time. Only IS of the 28 spiders tested matched all
analysis criteria, and although it may have biased the data set
Behavioral Ecology Vol. 9 No. 4
362
Figure 1
The artificial foraging environment used for spider testing.
Spiders are placed in the "neutral" chamber before each experiment (spider* shown in
stimulus chambers) and allowed to move freely between
the two chambers after a 5-min
acclimation period. Using
cricket nymphs as stimuli, each
pair of chambers differs in the
density treatments presented
to the spider. From left, the
treatments are no crickets, 3,
10, and 20 crickets. Figure represents visual and vibratory
stimuli presented. Other sensory treatments include visual
stimuli only and vibratory stimuli only.
toward more mobile spiders, this procedure did allow more
reliable interpretation of the analysis of variance.
For each experiment, we used a fully crossed, mixed-model
two-way ANOVA to analyze the variation in duration of patch
visits. Each of the three experiments was analyzed separately.
Patch residence time was natural log transformed to conform
to ANOVA assumptions of normality. The primary parameter,
patch residence time, was tested using individual (random effects) and density (fixed effects) as categorical variables. The
F ratio for prey density effect was constructed with the interaction term mean squares in the denominator (Zar, 1984) for
the appropriate F ratio for a mixed model. We used a mixedmodel ANOVA in this analysis because it is more conservative
than a repeated-measures analysis and accurately accounts for
individual spiders as a random effect (see Bennington and
Thayne, 1994). All computations were performed with a mainframe version of SAS software (version 6.07). Differences
among individual spiders are likely to occur and may affect
inferences that may be drawn about patch assessment abilities
of individuals when treating them as groups. This design overcomes this problem (Martin and Kraemer, 1987). We used
repeated visits of individual spiders as replicates for the individual factor. Repeated visits by an individual spider have been
tested statistically for independence in previous studies of
patch residence using an identical or similar test apparatus
(Persons and Uetz, 1996a, 1997a,b). Spiders have shown no
evidence that prior experience biases their residence time
(Persons and Uetz, 1996a, 1997a,b), and lengths of sequential
visits to the same patch are not correlated (Persons and Uetz,
1996a, 1997a, b). Thus, sequential visits by spiders to a particular aeatmont wexe used as replicates for the individual spider factor as a measure of individual variation.
Despite the apparent independence of sequential visits into
a chamber by an individual spider, use of sequential visits may
be construed as pseudoreplication (Hurlbcrt, 1984). To avoid
this problem, we conducted two other analyses of the data.
Individual variation was collapsed to a mean value for each of
the 13 spiders and a one-way ANOVA was done for each of
the sensory experiments (vibratory only, visual only, visual and
vibratory together). A Tukey post-hoc comparison of means
analysis determined differences in density treatments within
each experiment. To examine more closely an individual spider's ability to perceive differences in patch quality, we conducted one-way ANOVAs for each individual spider to determine significant differences in residence time for each density
treatment in the presence of visual and vibratory cues from
prey.
Vibration testing
Vibration levels were expected to increase with cricket density.
Vibration levels for each sensory and density treatment were
measured in decibels using a Bruel and Kjaer accelerometer
(type 4366) high-sensitivity vibration pickup, loaded to a Bruel
and Kjaer sound level meter (type 2203). Dedbel level was
measured every 15 s for 30 measurements under each experimental treatment. We placed the accelerometer along the
edge of each chamber nearest the crickets because most spiders had a strong tendency to move along die edges of the
chambers. A one-way ANOVA was performed and a Tukey
post-hoc comparison of means test was used to distinguish
between all sensory and density treatments for the stimulus
and neutral chambers. We compared 24 chambers: 3 sensory
treatments with 4 density treatments each, and 2 chambers
(neutral and stimulus) for each density.
RESULTS
Patch residence times of spiders varied significantly with prey
density aad type of sensory information (Figure 2). When
both visual and vibratory information were presented to the
spider, significant density effects were found (.Fj,» = 18.34, p
=• .0001) as well as significant variation among individual spiders (F a u 7 - 5.57, p - .0001; Table 1). Using individual
spiders as die level of replication and a mean value of residence time for each density treatment, spiders spent signifi*
Persons and Uetz • Prey density assessment by wolf spiders
BOO
500
400
300
200
100
Figure Z
Patch residence times by sensory treatment and density (mean
SE) per adult female spider (n => 13).
candy more time in higher density patches (F^^ = 2.874, p =
.0001). A Tukey post-hoc analysis of means found significant
(a = 0.05) differences between 0 prey and 3 prey and between 10 and 20 crickets, but not between 3 and 10 prey (Figure 2).
When visual information alone was present, the results were
similar to those found when visual and vibratory information
were present together. Significant density (F^ x = 7.37, p =
.002) and individual (Flit lia = 9.98, p = .0001) effects were
found. The ANOVA analysis of individual means across density
treatments also found a significant density effect (FSM =
10.49, p = .0001). A Tukey post-hoc comparison of means
showed significant differences between 20 prey and 3 or less
prey, but 3 prey and 10 were not significantly different from
each other, nor were 10 and 20 prey significantly different
from each other.
Vibratory information presented alone did not produce any
significant density effects ( f j j , = 0.308) on patch residence
time, but individual variation was still significant (Flt,iM »
13.07, p = .001). Individual means across density treatments
also were not significantly different with vibratory information
alone (Fxv ™ .12, p = .95).
Individual spiders with access to visual and vibratory cues
varied considerably in their relation of residence time to increasing prey density (Table 1), but the overall trend was for
increasing residence time with increasing prey density. Seven
of the 13 spiders increased residence time with cricket number; but 3 showed mean residence times for the control treatment that were not the lowest of all treatment groups. Only
6 of 13 spiders showed significant differences (a = 0.05) in
residence time among density treatments in the presence of
visual and vibratory cues from prey.
Although vibratory information was not significant in modifying residence time, vibration levels should be related directly to cricket movement, which has been shown to be a
strong stimulus in prey detection in wolf spiders (Rovner,
1991) and a factor in residence-time decisions in conjunction
with other stimuli Vibration levels conformed to expected
density and sensory treatments, with significant vibration differences between treatments (FBet6 = 112.30, p = .0000; Figure 3). A Tukey post-hoc comparison indicated that vibrations
in the neutral chambers were not significantly different from
each other or from background noise. The experiment using
both visual and vibratory stimuli together found significant
increases in vibration level with cricket density. Similar results
were found when only vibratory stimuli were presented. As
expected, in chambers with visual cues only, no detectable
difference in vibrations between density treatments nor between stimulus chambers and neutral chambers were found.
DISCUSSION
The results show that spiders modify their patch residence
time in response to changes in visual, but not vibratory, cues
with increasing prey density. These findings are consistent
with previous studies demonstrating that visual rather than
vibratory cues influence patch residence time when prey density is held constant (Persons and Uetz, 1996a). Even at high
prey density, vibratory cues alone apparently are not used to
Table 1
Mean (±SE) reridence times (•) of individual spiden for each prey density treatment in die presence
of visual and vibratory cues
Prey density (no. of crickets)
Spider
0
3
10
20
F
P
I
2
3
4
5
6
7
8
9
10
11
12
13
121 2t 26
37 2: 15
58 2i 17
51 2: 11
65 2: 3 0
88 2l: 12*
43 d: 2
67 2: 12
4 8 2 : 19*
77 21 11*
52 2:8*
40 2:8*
46 2: 5*
159 :t 43
192 2t 129
5 9 : t 20
6 4 : t 25
6 6 2 1 16
8 3 : t 12*
7 4 : !: 22
112 :t 18
4 6 : 1 8*
90 :1 31*
7 4 : 1 16*
65 :I 36*
59 :i 14*
379 :t 133
8 8 : t 30
176 :!: 28
179 :i 47
4 9 2 :21
139 :iff
9 4 : i 53
293 2i 135
271 :i 42"
2 1 3 :139*
139:t 73*
1%:tiSf
582: 9 *
370 :1 114
ill :158
212 :t 81
3 3 2 : £ 151
55 :'.8
187:t 49"
2 0 8 : i 67
527 :I: 407
1 6 9 :• 8 8 "
889 :H459*
226 :1 38"
36421 147"
177 2: 42"
2.61
0.80
3.15
2.63
0.23
3.48
2.69
1.34
4.74
7.69
3.46
3.72
7.22
.10
.52
.06
.10
£7
.05
.09
.31
.02
.04
.05
.04
.01
F ratios and p values based on one-way ANOVAs for individual spiders. Superscripted letters indicate
fignifjr?m differences between density treatments based on a Tukey post hoc comparison of means
test Identical letters indicate no significant difference between those treatments.
Behavioral Ecology Vol. 9 No. 4
564
Vibratory Only
3
10
20
Visual and Vibratory
m
"3
"o
ao
46
45
44
43
42
41
40
39
38
37
36
20
Visual Only
46
45
44
43
42
41
40
39
38
37
36
a
10
a
a
20
Prey density
Figure 3
Vibration levels generated by crickets at various densities in each of
three different apparatuses providing different sensory wimuli
Means with different letters are significantly different at p < .05; n
=> 30 for each experimental treatment.
assess patch quality. This suggests that not all sensory channels
are used equally in determining residence time, resulting in
different patch times based on the source of information.
However, the difference in residence time due to combinations of Visual and vibratory cues rather than visual cues alone
suggests that various sensory channels may interact in complex ways in the assessment of patch quality. Other studies
have found that residence time may differ based on chemical
cues from prey as well (Persons and Uetz, 1996b). Given that
most predators use multiple sensory channels to detect prey,
and the importance of these channels may change with environmental heterogeneity, this is an important consideration
in understandings proximate mechanisms of patch foraging
decisions.
Environmental constraints on transmission of various types
of sensory information may partially explain these results.
Schixocosa ocreata inhabits complex, loose leaf litter where vibrations from prey attenuate rapidly, often being conducted
no farther than the distance of a single leaf (Scheffer et aL,
1996). This represents a smaller area within which to perceive
prey and could result in selection for using visual cues while
foraging. Using visual cues instead of vibrations would allow
for a larger perceptual "patch" in which to forage and consequently less frequent movement between patches and greater foraging efficiency (Bye et al., 1992; Rice, 1983).
Many studies have examined different types of presampling
or prescient foraging information (Nishimura, 1991a,b; Valone, 1992; Valone and Brown, 1989; Valone and Giraldeau,
1993) in assessing patch quality, but all of these use prior experience as a parameter or assume that patches with static
patch quality may have been estimated by the forager from
prior patch experience. Present sensory information may be
an even more important source of information than prior experience for many animals. Wolf spiders rely heavily on present sensory information rather than on any other type of presampling information. Schizocosa ocreata move randomly between patches that differ in sensory information or that differ
in the presence of prey (Persons and Uetz, 1996a). They also
do not base residence time on previous patch sensory experience (Persons and Uetz, 1996a), previous prey attack (Persons and Uetz, 1997a) or feeding (Persons and Uetz, 1997b),
suggesting that bayesian foraging is unlikely in wolf spiders
and that patch quality assessment and residence time decisions are based primarily on present sensory information rather than any other source.
Nishimura (1994) modeled decision makfng in a sit-andwait predator when the forager finds one of three possible
patch states: finding no prey, finding prey without attacking
(analogous to the conditions in this study), and rinding prey
and attacking (prey capture). Nishimura predicts that when a
forager moves between patches without learning or memory
and chooses patches at random, it bases its decision to stay
on the long-term probabilities of the patch containing no
prey, one prey, or two prey. Under diis condition, the experience of finding prey without attacking does not contribute
to exploitation efficiency, and the forager should treat all
patches the same. Wolf spiders uphold the assumptions of this
model. They experience no difference in energy gain among
prey density treatments, have been found to move randomly
between patches (Persons and Uetz, 1996a), and appear not
to use prior sensory experience to modify residence time, yet
they vary residence times using sensory cues of prey density.
This suggests thatforagingpatch decisions with some animals
need not employ memory, sampling information, or Bayesian
means of assessment to determine patch quality and that the
most parsimonious source of information, present sensory information, may be the one weighted most heavily.
Wolf spiders detect prey primarily through visual detection
of movement (Rovner, 1991, 1993). As such, the probability
of detecting prey in a patch may be directly related to the
probability of any prey moving while the spider remains in a
patch. Therefore, even in a simple environment, cricket movement patterns may be sufficient to generate uncertainty about
pateb quality, and prey encounter may be considered a Poisson process. If prey detection is based on prey movemest,
spiders do not have complete knowledge of patch quality. Using video images of prey, previous studies completed with S.
ocreata fixed die proportion of time a prey item spent moving
in a foraging patch and therefore removed this source of stochasticity, and found that S. ocreata wolf spiders use a fixed
365
Persons and Uetz • Prey density assessment by wolf spiders
probability of leaving a patch that is influenced by both visual
detection of cricket movement and stochasticity inherent in
the decision rule (i.e., not all variation in residence time is
accounted for by stochasticity in prey movement) (Persons
and Uetz, 1997a). It is important to note that in this study,
even 25 sequential visits into a patch with a standardized
amount of cricket movement (using a video-generated cricket) failed to modify the behavior of the spider (Persons and
Uetz, 1997b). This also emphasizes the lack of importance of
prior sensory experience in a patch. However, lunging at prey
without feeding does significantly modify wolf spider residence time (Persons and Uetz, 1997a). This may simply reflect
that crickets may not be detected on every patch visit Studies
on residence time in crab spiders found that they also exhibit
a fixed probability of leaving a patch even though this does
not result in optimal prey encounter (Kareiva et aL, 1989).
Kareiva et aL suggest that this is due to extreme stochasticity
in prey encounter. Morse (1993) later found that crab spider
decisions regarding patch choices may be modified by distance cues radier than by prey capture directly.
Chamov's marginal value theorem (Charnov, 1976) assumes a continuous and deterministic gain function that accelerates negatively over time in a patch, and the decision to
leave is partially contingent on the gain function for alternative patches. The energetic gain for patches in this study is
zero, and residence time is largely determined by the probability of detection of prey movement, which is likely to be a
linear function of residence time. Differences in vibration levels across density treatments reflect linear increases in cricket
movement and therefore increases in the probability of detecting prey for any given patch visit The spiders had a choice
with regard to the patch: remain in the presence of sensory
stimuli or leave for another patch void of sensory cues. The
two patches are mutually exclusive with respect to sensory
stimuli, but identical with respect to energy gain. To determine if sensory cues function in an analogous way to energy
gain with respect to the marginal value theorem would require alternative patches being offered with greater or lesser
degrees of sensory stimuli radier than an empty neutral patch.
The threshold of response for spider detection of prey vibrations may be higher than that for any foraging patch used
for this study. However, spiders in patches with vibratory information alone were occasionally observed lunging at the
acetate screen when a cricket was walking nearby. This strongly suggests diat the vibration levels were sufficient for prey
detection but were not used to determine residence time.
This laboratory study demonstrates diat spiders are at least
capable of responding to differences in prey density even if
prey capture success is minimal for a particular site. It also
demonstrates diat a postingestion feedback mechanism is not
necessary to explain foraging time at a particular site and that
sensory information should be considered more strongly in
proximate and theoretical studies of residence time decisions.
This research was supported in part by funds from the National Science Foundation through grant IBN-94142S9 (support for G.U.), the
Department of Biological Sciences, the Arachnological Research Fund
of the University of Cincinnati, and a University of Cincinnati Research Council Fellowship. Portions of this research were submitted
in partial fulfillment of the requirements for M5. and Ph.D. degrees
in Biological Sciences at the University of Cincinnati. We are grateful
to B. C Jayne for «t3p«rirai advice and technical criticism. We thank
D. Wise, A. Cady, J. Shann, and two anonymous reviewers for their
helpful comments on this manuscript. We acknowledge the people of
the Cincinnati Nature Center for their willingness to provide a collecting site for Schizocosa ocrtala. Additional thanks go to W. Mcdiniock, A. DeLay, K. Cook, D. Kroeger, and A. McCrate for their
advice and assistance with spider husbandry.
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