Age and sex-based differences in the use of prey sensory cues in wolf spiders (Araneae: Lycosidae)

Journal of Insect Behavior, Vol. 12, No. 6, 1999

Age and Sex-Based Differences in the Use of PreySensory Cues in Wolf Spiders (Araneae: Lycosidae)

Matthew H. Persons1,2 and George W. Uetz1

Accepted March 15, 1999: revised June 1, 1999

Differences in foraging patterns mediated by sensory cues were examinedbetween adult and juvenile male and female wolf spiders (Schizocosa rovneri;Lycosidae). Patch residence time for thirty-one spiders were tested amongjuveniles and adults in artificial foraging patches. Patches varied in sensoryinformation provided by live prey (crickets) as follows: visual stimuli alone;vibratory stimuli alone; visual and vibratory stimuli together; and control (nostimuli). Spiders moved between patches for one hour, but could not feed.Adult Schizocosa rovneri use primarily visual information to determine patchresidence time, but juveniles use vibratory cues as well. Significant age andsex-based differences in the use of sensory cues suggest that observed divergentforaging strategies are partly due to the use of different perceptual cues in preydetection.

INTRODUCTION

Much of traditional foraging theory emphasizes the net rate of energy intake toexplain how animals should exploit patch and prey types while moving mostefficiently between patches of food (Charnov, 1976; Pyke et al., 1977; Pyke,1984; Stephens and Krebs, 1986). Despite these assumptions, empirical studieshave often found that the proximate mechanism for leaving a patch may notbe directly related to prey consumption or energy gain. Sensory information isoften overlooked relative to post-ingestive feedback mechanisms as a predictor

KEY WORDS: age; sex; foraging; wolf spiders; Schizocosa rovneri; patch

1Department of Biological Sciences, ML 0006 University of Cincinnati, Cincinnati, Ohio 45221-0006.

2 To whom correspondence should be addressed at Department of Biology, Susquehanna University,Selinsgrove, Pennsylvania 17870.

723

0892-7553/99/1100-723$16.00/0 1999 Plenum Publishing Corporation

for determining foraging patterns (Bye et al., 1992; Rice, 1983; Bell, 1991).Sensory cues, even in the absence of prey capture, are known to be an importantsource of information in spider foraging. Crab spiders (Morse, 1993), funnel-weaving spiders (Riechert, 1985), orb-weavers (Pasquet et al., 1994), and wolfspiders (Persons and Uetz, 1996a, 1996b) have all been shown to use sensorycues in the absence of prey capture to either select foraging sites or determinehow long to remain at a site. In contrast, the use of prior feeding experience inselecting patches or choosing to remain in a patch may be relatively unimportantfor some spiders (Persons and Uetz, 1996a; Olive, 1982). Some wolf spidersdo not use prior patch experience to modify residence time (Persons and Uetz,1996a). Further, prior feeding experience (Persons and Uetz, 1997a) and hungerlevel (Persons, 1999) may be unimportant in mediating residence time decisionsamong wolf spiders as well (Persons and Uetz, 1997a, Persons, 1999).

Sex and Age Differences in Foraging

Classical foraging studies have found that foraging efficiency is largelyinfluenced by time and energy. Animals may adopt one of two extreme forag-ing strategies; maximizing the amount of energy or minimizing the amount oftime (Schoener, 1971). Although various aspects of foraging strategies of spidershave been examined (Morse and Fritz, 1982; Ford, 1978; Cady, 1984; Holmbergand Turnbull, 1982), few have tested differences in foraging as a function of sex(Givens, 1978; Singer and Riechert, 1994). In general, when sex-based foragingdecisions are found, it is often assumed to be attributable to differences in maleand female energy needs and time budgets (Charnov, 1982). Among spiders,Givens (1978) demonstrated that adult salticids exhibit sexually-dimorphic for-aging strategies, with males minimizing time and females maximizing energy.However, there has been little examination of the proximate causes of differ-ent foraging strategies for each sex nor have there been studies that examinesex-based differences in foraging strategies among juvenile spiders. Empiricalstudies on differences between preultimate juvenile male and female spiders arelacking, primarily due to the difficulty in sexing immature spiders. There are alsorelatively few studies that compare behavior of adult and immature wanderingspiders (Wise and Wagner, 1992; Aspey, 1975; Kreiter and Wise, 1996).

Previous studies of foraging patterns in the wolf spider, Schizocosa ocre-ata, have found significant sex and age effects on foraging patterns, but hungerwas not a significant determinant of these patterns (Persons, 1999). Since sen-sory information has been found to be a major factor determining residencetime in wolf spiders compared to feeding experience or hunger, we hypothesizedthat observed foraging differences between sexes and age classes of some wolfspiders may be mediated by differential use of sensory information. If males,females, and juveniles use different sensory channels in prey detection, this may

724 Persons and Uetz

explain vanation in foraging patterns between groups. In this study we: 1) com-pared adult and juvenile sex differences in foraging strategies among the wolfspider Schizocosa rovneri, and 2) determined if variation in the use of sensoryinformation alone is sufficient to explain any observed differences between sexesor age classes of spiders.

Schizocosa rovneri is a common species of wolf spider inhabiting com-pressed deciduous leaf litter of eastern floodplains (Uetz and Dondale, 1979).Like most wolf spiders, they do not build webs but are sit-and-wait predatorswith frequent changes in feeding sites (Ford, 1978). Wolf spiders use multi-ple sensory channels to locate prey including substratum or air-borne vibrations(Lizotte and Rovner, 1988), chemical cues (Persons and Uetz, 1996b), and visualinformation (Persons and Uetz, 1996a). However, the extent that each of thesesensory modes is used in prey detection and how that, in turn, influences thedecision to stay or leave a particular foraging patch remains unknown.

METHODS

Collecting and Maintenance

The methods and experimental protocols are similar to those described inPersons and Uetz (1996a). 125 immature S. rovneri wolf spiders were fieldcaught September 25-30th, 1992 at Burnet Woods Park, Hamilton Co., Cincin-nati, Ohio. Each spider was maintained in an opaque container, fed a diet ofthree one-week old cricket nymphs every four days to standardize hunger levelsprior to testing and provided a constant source of water. Spiders were kept atroom temperature (23-25°C) in an environment with stable humidity (ca. 60%)and a 13L: 11D photoregimen. All spiders were labeled, and tested three or fourmolts prior to maturation. Spiders were tested again no earlier than one weekafter maturation.

Experimental Apparatus and Protocol

Foraging patterns of wolf spiders were studied in an experimental appara-tus that controlled for visual and vibratory cues used in prey detection and hasbeen used in other foraging studies, but is reiterated here (see Persons and Uetz,1996a) (Fig. 1). The test apparatus was composed of four chambers made ofwhite foam-core board. Each of the four sensory chambers consisted of a 20 cmdiameter arena with transparent acetate walls. The source of the stimuli usedfor this experiment was 12 live, one-week-old crickets introduced behind eachacetate wall (except in the control chamber). Sensory stimuli presented to thespider differed for each chamber: 1) visual information alone; 2) vibratory alone;3) both visual and vibratory information; and 4) control (no crickets present). Thevibration only treatment consisted of an area with white paper over the acetate

Age and Sex Differences in Prey Cues 725


Fig. 1. The foraging patch apparatus used for spider testing. Spiders are placed inthe central area prior to each experiment (spider shown in center) and allowed tomove between four patches for 1 hour after a 15 minute acclimation period. 12crickets serve as stimuli behind acetate screens. Shown clockwise from upper-left,the sensory treatments are: 1) control—no crickets behind an acetate screen; 2)visual stimuli only—the crickets and spider are mounted on separate substrata; thecrickets are visible behind the screen; 3) vibratory stimuli only—a paper barrier isbetween the spider and crickets, and 4) visual and vibratory stimuli together.

that prevents transmission of visual information. The visual only treatment had thecricket enclosure mounted on a separate foam-core block from that of the spider.The visual and vibratory treament lacked either of these modifications. Each cham-ber was isolated from each other by foam blocks between chambers to eliminateany incidental vibratory and visual stimuli between chambers and in the lab. Thebox was elevated on a 70 cm high table and sponge foam was placed between thebox and table to further dampen ambient vibrations. Vibration level differencesbetween chambers were verified with a Bruel and Kjaer accelerometer (Type 4366)high sensitivity vibration pickup, loaded to a Bruel and Kjaer sound level meter(Type 2203) prior to testing (Persons and Uetz, 1996a).

An experimental trial consisted of a single spider introduced into the centerof the apparatus, where it was allowed to move freely for one hour after a 5 minuteacclimation period. Each trial was videotaped from above and duration and numberof chamber visits were determined by analyzing the videotape recording.

Statistical Methods

Statistical methods are similar to those used in other studies (Persons andUetz; 1996a, 1996b, 1997a, 1997b). A complete four-way, partially nested,mixed model ANOVA was used to analyze differences between treatments.Multiple visits by each spider were used as replicates with individual spidertreated as a random effect nested within sex. All other variables were fixed,fully crossed effects. Additional ANOVAs were done to examine main effectsin more detail and interactions between factors since three and four-way inter-actions are difficult to interpret. Due to the nesting of individual within sex,individual effects were difficult to evaluate. To account for this, four two-waymixed-model ANOVAs were used to analyze the variation in duration of patchvisits (Zar, 1984). The dependent variable of patch residence time was testedusing individual (random) and sensory chambers (fixed) as categorical variables.The four ANOVAs were done separately for adult males, adult females, imma-ture females, and immature males respectively. Four three-way mixed modelANOVAs were also done to determine differences between immatures and adultsas a function of age and sex. All data for all ANOVAs were natural log (ln)transformed to conform to ANOVA assumptions of normality.

Spiders that did not visit all four chambers at least twice as both juvenilesand adults were omitted from analysis. Data from thirty-one spiders were ana-lyzed based on this criterion (N = 16 females and 15 males). Since previousstudies have demonstrated that spiders chose patches at random and do not varypatch residence time based on previous visits to the patch, all sequential visitsinto a sensory treatment by a single spider were regarded as independent repli-cates of that individual (Persons and Uetz; 1996a, 1997a, 1997b).

RESULTS

Spiders had longer patch residence times when sensory stimuli werepresent, indicating that in the absence of feeding information, spiders vary patchresidence time based on sensory cues alone (Table I, II). Spiders of differentages and sexes responded differently to sensory information (Fig. 2). In gen-eral, spiders had longer patch residence time when visual cues alone or visualand vibratory information were presented together than in the presence of onlyvibratory cues. Vibratory cues were an important determinant of residence timefor immature spiders, but not for adults.


Table I. Four- Way Mixed Model ANOVA Comparing Effects of Sensory Stimuli,Individual, Age, and Sex on Residence Time for Spidersa

Source

Individual (Sex)AgeSexSensory StimuliAge*StimuliSex*StimuliInd*StimuliInd*Sex*AgeInd*Age*StimuliInd*Sex*StimuliSex*Age*StimuliInd*Sex*Age*Stimuli

df

211333323346

F

54.31376.4423.0451.50

3.943.612.76

73.234.680.816.640.64

P

*0.0001*0.0001*0.0001*0.0001*0.0081*0.0128*0.0406*0.0001*0.00290.4857

*0.00010.7020

aAsterisk = significance at A = 0.05.

ANOVA results indicate that male and female spiders have significantly dif-ferent residence times, both as immatures and as adults (Table III). There werealso significant age effects for both sexes (Table IV). Immature males spent thelongest time on average in each chamber for all sensory treatments. This trend


Table II. Summary of Mixed Model Two- Way ANOVAs Comparing Effects ofSensory Stimuli and Individual on Residence Time for Immature and Adult Male and

Female Spiders

Source

Adult FemalesIndividualSensory StimuliInd*Stimuli

Adult MalesIndividualSensory StimuliInd*Stimuli

Immature FemalesIndividualSensory StimuliInd'Stimuli

Immature MalesIndividualSensory StimuliInd*Stimuli

df

153

45

143

42

153

45

143

42

Fa

18.3010.822.32

59.0212.40

2.11

24.976.072.29

3.603.693.53

P

*0.0001*0.0001*0.0001

*0.0001*0.0001*0.0001

*0.0001*0.005 < p < 0.002

*0.0001

*0.0001*0.05 <p<0.01

*0.0001

aF values are calculated for a mixed model two-way ANOVA. Asterisk = significance ata = 0.05.


Fig. 2. Mean (± S.E.) residence time/spider (seconds) by sensory treatment (male N = 15,female N = 16). Treatments include 1) con = control (no crickets); 2) vib = vibratory stim-uli only; 3) vis = visual stimuli only; and 4) both = visual and vibratory together. Let-ters above treatments indicate significant differences between treatments based on Tukeypost-hoc comparisons of means (alpha = 0.05). Identical letters indicate groups that are notdifferent.

was reversed later in life with adult male spiders having the lowest mean resi-dence time for all patch types. Adult females had significantly longer residencetimes than adult males, but much shorter than immatures of either sex (Fig. 2).Tukey post-hoc tests failed to show a significant increase in patch residencewith vibratory stimuli over the control for both adult females and adult males(Fig. 2), but vibratory cues were significantly above the control for both maleand female immature spiders (Fig. 2). All spiders showed significantly differentpatch residence times based on the type of sensory stimuli available. There weresignificant interactions of age and sensory stimuli, sex and sensory stimuli, and athree way interaction of age, sex, and stimuli for spiders (Table I). This indicates

ontogenetic and sex-specific changes in foraging responses to sensory stimuli.Since there was a significant effect of sex on residence time in immature spiders(Table III), sex-specific foraging strategies exist for juveniles as well as adults.

All ANOVAs revealed significant individual effects as well as significantinteractions between individuals and sensory treatments (Table I, II). When Tukeypost-hoc comparison of means tests were done for adult male, adult female, imma-ture male, and immature female spiders, the results indicate that all four groups hadslightly different uses of sensory information (Fig. 2). This demonstrates that notall individuals were using the same sensory cues while foraging.

DISCUSSION

Previous studies have demonstrated that wolf spiders perceive prey throughvisual and vibratory stimuli (Barth, 1982, 1985; Land, 1985; Lizotte and Rovner,1988), but these studies have largely focused on the physiological mechanisms,and less on the motivational, cognitive, or ecological correlates that mediatethe use of various types of sensory information. Use of sensory informationby Schizocosa rovneri to make foraging decisions, as shown here, is consistentwith previous studies of a closely related species, S. ocreata (Persons and Uetz;1996a, 1996b, 1997a, 1997b). Results indicate that visual cues are the primarysensory channel used in determining residence time for S. rovneri. Although S.rovneri showed a visual bias in determining residence time as adults, immaturespiders rely on vibratory information as well. Therefore, some foraging differ-

Table III. Summary of Three-Way Mixed Model ANOVAs Comparing Adult andImmature Spiders and Indicating the Effects of Sensory Stimuli, Individual, and Sex on

Residence Time

Source

Immature SpidersIndividual (Sex)Sensory StimuliSexInd*StimuliSex*StimuliInd*Sex*Stimuli

Adult SpidersIndividual (Sex)Sensory StimuliSexInd*StimuliSex*StimuliInd*Sex*Stimuli

df

231333

231333

Fa

57.7015.444.133.760.640.60

61.1536.1960.15

3.381.460.51

P

*0.0001

*0.0001*0.()427*0.01080.59020.6174

*0.0001*0.0001*0.0001*0.01770.22250.6764

aF values have been calculated for a three-way mixed model ANOVA. Asterisk =significance at a = 0.05.


ences between age classes of spiders are due to differences in the use of sensoryinformation.

Almost all the literature on foraging residence time decisions has invokedsome measure of energy intake as a decision-making criterion (Stephens andKrebs, 1986). This is not surprising since feeding on prey affords information onpalatability, digestibility, and handling time as well as having direct fitness conse-quences on the forager. However, perceptual cues may represent the most parsi-monious proximate mechanism for acquiring information used in patch residencedecisions, and for this reason, may be widespread in many animals. Further, sen-sory information is much less time-intensive than sampling information, since itrequires only the immediate perception of prey in the area. This study, like others(Persons and Uetz, 1996a, 1996b, 1997a, 1997b), demonstrates the importance ofprey perception without energy gain as criteria for basing foraging decisions. How-ever, the utility of various forms of perceptual information for foraging decisions isclearly dependent on the age and sex of the forager, and this may be due to differentenergy needs and time constraints across these categories.

Sex and Age Differences in Foraging

Time budgets of male and female spiders appeared to be different, a findingwhich is consistent with other results from other hunting spider studies (Givens,1978; Cady, 1984). Givens (1978) found significant differences in patterns of

Table IV. Effects of Sensory Stimuli, Individual, and Age on Residence Time for Maleand Female Spiders

Source

Male SpidersIndividualAgeSensory StimuliInd*AgeInd*StimuliAge*StimuliInd*Age*Stimuli

Female SpidersIndividualAgeSensory StimuliInd*AgeInd*StimuliAge*StimuliInd*Age*Stimuli

df

1413

1442

342

1513

1545

345

Fa

70.1360.5515.117.172.399.211.87

24.015.69

16.9517.362.670.171.87

P

*0.0001*0.0001*0.0001*0.0001*0.0001*0.0001

*0.02<p<0.01

*0.0001*0.05 <p< 0.02

*0.0001*0.0001*0.00010.5

*0.05 <p< 0.02

aF values have been calculated for a three-way mixed model ANOVA. Asterisk =significance at a = 0.05.


prey utilization between adult male and female jumping spiders. Males con-sumed less of each prey item than females. In this case, each prey item itselfcan be considered a patch with the spider choosing to remain in the patch fora varied period of time based on the energy value of the prey item (Cook andCockrell, 1978). However, this evidence of sex differences in spiders was basedon energy intake and time expenditure rather than sensory perception and spatialpatches of food. Edgar (1971) found that adult male Pardosa lugubris wolf spi-ders do not increase their weight upon maturation, but female prey consumptionincreases dramatically, resulting in a doubling in body weight in a short period oftime. This suggests that males and females have widely different energy needs.Adult males have much shorter residence times than immature spiders or adultfemales. This indicates that males may be attempting to cover a wide area toincrease the chances of encountering a female. The proportion of time malesspend searching for females versus food is unknown, but there were significantdifferences in residence time based on prey stimuli, indicating that some atten-tion to prey still occurs and prey presence is capable of modifying adult malelocomotor patterns.

Kreiter and Wise (1996) found an abrupt age-related shift in the movementpatterns of fishing spiders (Dolomedes tritori). These results were explained byenergy requirements of females for yolk production and possible differential pre-dation of juveniles and adults. It has also been suggested that such a shift in for-aging strategy from ambush to active searching is guided by increases in preydensity. In this study, the prey density is held constant suggesting that the shiftin foraging strategies between juveniles and adults may be innate and not neces-sarily dependent on external factors. However, previous studies with wolf spiderpatch residence demonstrate that increases in prey activity levels (Persons andUetz, 1997b) and prey density (Persons and Uetz, 1998) are sufficient to increaseresidence time, although not in a linear manner.

To our knowledge, only one study has examined possible differencesbetween juvenile male and female spiders and these were penultimate individualswhere primary sex characters were already evident (Singer and Riechert, 1994).Singer and Riechert (1994) found no differences in habitat choice in penulti-mate instar males and females which indicates an abrupt shift between male andfemale behavior upon maturation. Singer and Riechert (1994) suggested that sex-specific behavior may not occur prior to hormonal influences. The differencesbetween pre-penultimate immature male and female spiders seen in this studyclearly indicates that subtle differences do exist for some spiders and reflect dif-ferent foraging strategies throughout development. Males and females exhibitsexual size dimorphism and males may mature several weeks before females.These differences show divergent developmental schedules mediated by differ-ent energy requirements and hunger threshold levels. This may also explain dif-ferential use of sensory information between juvenile males and females.


A number of factors may contribute to differences in immature and adultuse of sensory information including the molting process and size differencesbetween immature and adult spiders. Differences in the development of opticneuropiles or changing numbers of vibratory organs may also contribute to age-related shifts in the use of different sensory channels (Barth, 1985). Studies withorb-weaving spiders have found large differences in the central nervous systemof immature and adult spiders as well as males and females (Babu, 1985). Otherchanges in motor neuron size, growth, and neuropile development suggest thatneurologically-mediated changes in behavior are likely (Babu, 1975).

Results showed that there was greater variation in residence time amongjuvenile spiders compared to adults. This may be attributable to probable greatervariation in juvenile age relative to adults. Since we field-collected juveniles, theexact age of each spider could not be determined. If the development of sen-sory systems is driving age-based differences in foraging patterns, then this mayaccount for the larger variance in residence time found among juvenile spiders.Because juvenile spiders are significantly smaller than adults, we cannot ruleout the possibility that variation in locomotor patterns could be explained by thesimple physics of juveniles having smaller legs. However, the most obvious for-aging pattern difference between adult and immature spiders was that immaturespiders spent longer periods of time immobile than adults. This suggests thatthe differences in the physical size of adults and juveniles is probably not theprinciple explanation for significant differences in residence time between thesegroups.

Adult males showed significant differences between all sensory channelsexcept for the control group and vibratory cues only, with males primarilyexhibiting a visual foraging bias. This differed from adult females, which stayedlonger in all patches than their male counterparts. Although different in mean res-idence time across patches, adult males and females had qualitatively the samesensory biases during foraging with females also exhibiting a visual foragingbias. Male and female juveniles showed the longest patch residence times. Vibra-tory information was also an important determinant of residence time amongjuveniles, but the importance of this sensory channel diminished among adults.

Changes in the visual and acoustic properties of wolf spider foraging habi-tat over the development of the spider may contribute to changes in sensorymodes used while foraging. Schizocosa rovneri is commonly found inhabitingcompressed leaf litter associated with floodplains in the Eastern Deciduous For-est (Uetz and Dondale, 1979; Uetz and Denterlein, 1979) which conducts soundvibration well relative to other types of leaf litter (Scheffer et al., 1996). Sucha substratum would also transmit visual information well, since floodplain leaflitter has a strong horizontal orientation relatively unimpeded by visual barriers.The depth and degree of compression of the leaf litter changes with the timeof year and periodic inundation, both of which may contribute to the efficiency


of visual and vibratory channels used for prey detection. Schizocosa ocreata, aclosely related wolf spider, showed similar sensory biases as adults (Persons andUetz, 1996a), but lives in complex loose leaf litter with different acoustic prop-erties than that found in floodplain environments (Scheffer et al., 1996). Thissuggests that variation in habitat structure as a contributing factor in the use ofsensory channels seems unlikely.

The sensory biases exhibited in these spiders are likely a product of theinteraction of developmental changes in the spider sensory systems, the chang-ing efficiency with which different sensory channels can be carried through theenvironmental medium in which they live, conspicuousness of prey through var-ious sensory channels, and possibly phylogenetic effects.

Despite statistically significant results, sampling error can not be dismissedas a possible causative factor in apparent differences between groups since therewas large inter- and intra-individual variation in patch tenure. There were alsosignificant differences in how individual spiders used various sensory channels.Large variation in cricket movement between chamber visits (Persons and Uetz,1998) and stochasticity inherent in the residence time decision rule (Persons andUetz, 1997b) are both important factors contributing to this variability.

In summary, the patch tenure differences of males and females in differ-ent stages of development, coupled with the large degree of variation in resi-dence time, indicates dynamic foraging strategies for these spiders. The diver-gent movement patterns between patches are likely to be mediated by decisionrules based not only on different energy needs, but on different use of sensoryinputs as well.

ACKNOWLEDGMENTS

This research was supported in part by funds from the National ScienceFoundation through grant IBN-9414239 (support for G.U.), the Department ofBiological Sciences (Weimann Fellowship), and the Arachnological ResearchFund of the University of Cincinnati. Portions of this research were submittedin partial fulfillment of the requirements for M.S. and Ph.D. degrees in Biologi-cal Sciences at the University of Cincinnati. We are grateful to B. C. Jayne fortechnical criticism and statistical advice. Additional thanks go to W. McClin-tock, A. DeLay, K. Cook, D. Kroeger, V. Casebolt, and A. McCrate for variousadvice and hours of help with spider collecting and husbandry.

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Age and sex-based differences in the use of prey sensory cues in wolf spiders (Araneae: Lycosidae)