George W. Uetz

Estimating fitness : a comparison of body condition indices

Behavioral ecologists might often benefit by the ability to directly measure an animals body condition as an estimate of foraging success, and ultimately fitness. Here we compare the reliability and effectiveness of three indices of body condition that have been heavily used in the morphometrics literature. We examined the ratio index (body mass/body size), the slope-adjusted ratio index (based on regression slopes generated from a reference population), and the residual index (the residuals of a regression of body mass on body size). We present the results of tests performed in the field and laboratory on two ecologically and evolutionarily divergent spider species : the vagrant wolf spider Pardosa milvina (Araneae, Lycosidae), and the colonial orb-weaver Metepeira incrassata (Araneae, Araneidae). The ratio index correlated with body size, which weakened the strength of conclusions that could be drawn. The slope-adjusted ratio index requires an independent and large data set with which to generate the expected values, and was likewise sensitive to body size. The residual index, with appropriate transformations to achieve homoscedasticity, was the most reliable index because it did not vary with body size, and we recommend its general use in behavioral studies that require a condition estimate.

The influence of variation in litter habitats on spider communities

SummarySpider communities were sampled over an artificial gradient of litter depth (created by raking) and compared to those of two other forests exhibiting natural variation in litter depth. More species of spiders were present in areas of greater depth and/or complexity in all sites. Relative abundance of Lycosidae decreased, while relative abundance of Clubionidae, Thomisidae and Gnaphosidae increased over gradients of increasing depth and complexity. Similarity of species composition between areas within a forest was related to site differences in litter depth and structure.As litter depth increased, there were significant changes in prey species richness, litter complexity, and microclimate. Partial correlation analysis of grouped data from early-, mid-, and late season suggests that influential factors change with season. In the early season, prey abundance and temperature variation account for most of the variation in spider species richness. In mid-season, litter complexity and moisture fluctuations appear to influence richness, with complexity relatively more important. In late season, complexity and temperature range were the primary factors, with temperature relatively more influential. The relative importance of these factors in influencing community structure of spiders is discussed.

Multisensory cues and multimodal communication in spiders: Insights from video/audio playback studies

Spiders perceive the world using multiple sensory modes, including vibration, vision, and chemical senses, for prey detection and communication. These sensory modes are used in many communication contexts, either individually or in multimodal signaling. Selection for effective signaler-receiver communication and species discrimination is especially strong for these predatory and potentially cannibalistic arthropods, resulting in the evolution of considerable diversity in signaling behaviors. In this paper, we review sensory mechanisms involved in spider signaling and present an overview of recent work done on wolf spiders (Lycosidae) that use multimodal communication (simultaneous visual and vibratory signals) in sexual signals during courtship. The relative importance of visual and vibratory signaling modes, and the use of multiple modes varies among closely related species in the genus Schizocosa, providing a model system for investigating multisensory guidance of complex behavior. Here we examine previous and current research on responses of female spiders to components of male courtship behavior, using several experimental techniques including cue isolation (single sensory modes), video/audio digitization and playback, and cue-conflict (mixed conspecific/heterospecific components) to tease apart elements of multimodal signaling.

Female choice and pre-existing bias: visual cues during courtship in twoSchizocosawolf spiders (Araneae: Lycosidae)

Male wolf spiders, Schizocosa ocreata, possess tufts of bristles on the forelegs, which are used in visual courtship displays, but males of a closely related species, S. rovneri, lack tufted forelegs. In studies with live males, female S. ocreata showed receptivity more often to males with larger tufts than to males with smaller or shaved tufts. Male body size and/or shaving of tufts may aVect male courtship behaviour, however, which could influence female receptivity. Manipulated video images of the same courting male were used to control for male behaviour and to test the hypothesis that these tufts influence female choice. Video images of S. ocreata, and images of S. rovneri males (control: without tufts; experimental: with tufts added) were shown to females of both species. Female S. ocreata showed lowered receptivity to a video of a male with tufts removed and equal receptivity to a video of a male with enhanced tufts compared to a control video of a conspecific male with typical tufts. Responses of female S. ocreata to heterospecific video male S. rovneri were significantly lower than to conspecifics. In contrast, female S. rovneri are more receptive to video images of conspecific males with tufts added and heterospecific male S. ocreata than to control images. Phylogenetic analyses suggest that tufts are apomorphic, and that S. rovneri is more basal within the clade than S. ocreata. Since female S. rovneri respond receptively to both conspecific and heterospecific males with tufts and/or active leg waving displays, these results may provide evidence of pre-existing bias and/or sexual selection for a novel trait. ? 1996 The Association for the Study of Animal Behaviour

Sexual selection, male morphology, and the efficacy of courtship signalling in two wolf spiders (Araneae: Lycosidae)

Abstract Males of the brush-legged wolf spider, Schizocosa ocreata (Araneae: Lycosidae), possess a conspicuous male secondary sexual character: dark pigmentation and tufts of bristles on the tibiae of their forelegs. We tested several hypotheses relating to the role of this conspicuous trait in sexual selection. Triad mating experiments suggest that the tufts do not play an obvious role in the operation of sexual selection by either male competition or female choice, as there were no significant differences in the mating success of intact and experimentally shaved males. However, females mated more often with males that initiated courtship first, suggesting that capture of a female’s attention by male signalling may play a critical role. In behavioral experiments that paired a single male with a female in arenas that allowed both visual and vibratory signal transmission during courtship, female receptivity did not vary significantly with the presence or absence of tufts. However, experiments that isolated the visual component of communication (by eliminating vibratory communication) revealed a significant effect of the presence of tufts: females showed receptivity less often to males with tufts removed. Female response to visual signals was much greater in S. ocreata than in its sibling congener, Schizocosa rovneri, which lacks male tufts. We hypothesize that the tufts serve to increase the efficacy of visual displays of S. ocreata, as vibratory communication is constrained by the complex leaf litter habitat of some populations. Such environmental constraints may make visual signalling over distance a critical factor for effective courtship communication, which may in turn strongly influence male fitness.

Foraging strategies of spiders.

Abundant and ecologically important in many ecosystems as predators of insects (including agricultural pests), spiders have recently gained attention as a result of a number of significant studies. In addition, because of their small size, short lifespan, and the strong influence of genetic control on their behavior, spiders are regarded with keen interest as model organisms in behavioral ecology.

Leg ornamentation and the efficacy of courtship display in four species of wolf spider (Araneae: Lycosidae)

Abstract This study used both correlative and experimental video playback methods to test the hypothesis that the secondary sexual traits of male wolf spiders act to increase the efficacy of visual courtship displays. Direct observations of courtship of several lycosid genera and a review of the literature revealed a significant association between ornamentation and visual courtship displays. This suggests that the ornamentation may be playing the role of amplifier for a visual display. To test this hypothesis, male courtship behaviors of four Schizocosa species were experimentally manipulated using video-imaging techniques. Females of species with non-visually displaying, non-ornamented males (Schizocosa duplex and S. uetzi) did not increase in frequency of receptivity when tufts were added to conspecific males. In a species with a visual display and foreleg pigmentation (S. stridulans), the addition of foreleg tufts increased female receptivity. In a tufted species (S. crassipes), females tended to decrease their receptivity when male ornamentation was completely removed. In visually displaying species, ornamentation acts to increase female receptivity, supporting its role as an amplifier of a visual display.

Trade-offs in foraging success and predation risk with spatial position in colonial spiders

SummaryColonial web-building spiders respond to trade-offs between selective forces relative to spatial position within colonies and thus provide support for the selfish herd theory. The size distribution of spiders within colonies of Metepeira incrassata, a colonial orb-weaver (Araneae: Araneidae) from tropical Mexico is nonrandom; larger (mature) spiders and females guarding eggsacs are more prevalent in the center, whereas more small (immature) spiders are found on the periphery. Experimental field studies with spiders of selected size classes show that larger spiders actively and aggressively seek protected positions in the center of the colony webbing, even though prey availability and capture rates are significantly higher on the periphery. Attacks by predatory wasps, other spiders, and hummingbirds are more frequent on the periphery than in the core of the colony. Reproductive females on the periphery are at greater risk because they are captured more often than smaller spiders, and if their egg sacs consequently remain unguarded, chances of cocoon parasitism are increased. As a result, spiders in the core of the colony have greater reproductive success, producing more egg sacs with greater hatching frequency. Colonial spiders thus appear to be making a trade-off between foraging and protection from predation and show a spatial organization predicted by the selfish herd theory. The influence of such trade-offs on individual fitness and the structure of colonies is discussed.

Morph-independent mate selection in a dimorphic jumping spider: demonstration of movement bias in female choice using video-controlled courtship behaviour

Abstract The importance of female choice in maintaining an extreme male dimorphism seen in the jumping spider, Maevia inclemens, was investigated. It was demonstrated that female M. inclemens apparently do not choose between the two strikingly different male morphs based on phenotypic characteristics alone, but instead select mates on the basis of initial male movement. Because of the difficulties associated with manipulating live males a video playback and computer digitization/animation technique was developed. Using this technique, male courtship behaviour was experimentally controlled and the behaviour of two different males was synchronized before presentation to females. Results confirmed that female mate preference is dependent on the male that the female is first oriented towards, independent of male morphology. This suggests that females may not be preferentially mating with one male type. Recent studies have shown that females may select mates that exploit a pre-existing sensory mechanism, and it is known that jumping spiders always orient to movement in their peripheral field of vision. Given the sensory mechanisms of jumping spiders and many other species, initial male movement may be an important factor in the mate selection process.

Video image recognition by the jumping spider, Maevia inclemens (Araneae: Salticidae)

Abstract Jumping spiders of the family Salticidae have well developed eyes, which mediate their highly stereotyped predatory and commnicative behaviour. Experimental studies presented here provide behavioural evidence that salticids are capable of perceiving televised (video) images. Juvenile and sexually mature individuals of the dimorphic jumping spider, Maevia inclemens (Walckenaer), were tested in V-maze choice test and behavioural chambers where they were allowed to view images of videotaped prey and spiders presented on Sony Watchman micro-television unuits. Results from this study suggest that jumping spiders interpret video images as real, because: (1) spiders did not discriminate between live prey and its simulateously presented video image; and (2) they behave appropriately when presented with televised images of prey insects (e.g. stalk and attack televised prey), conspecifics (e.g. courtship and sexual receptivity behaviour directed toward televised conspecifics) and heterospecific jumping spider species (e.g. retreat from predator stimulus). These observations support a previous estimate, of low critical flicker fusion frequency values in this group, and enable the development of a powerful new method for the study of animal behaviour.