Eileen A. Hebets

Complex signal function: developing a framework of testable hypotheses

The basic building blocks of communication are signals, assembled in various sequences and combinations, and used in virtually all inter- and intra-specific interactions. While signal evolution has long been a focus of study, there has been a recent resurgence of interest and research in the complexity of animal displays. Much past research on signal evolution has focused on sensory specialists, or on single signals in isolation, but many animal displays involve complex signaling, or the combination of more than one signal or related component, often serially and overlapping, frequently across multiple sensory modalities. Here, we build a framework of functional hypotheses of complex signal evolution based on content-driven (ultimate) and efficacy-driven (proximate) selection pressures (sensu Guilford and Dawkins 1991). We point out key predictions for various hypotheses and discuss different approaches to uncovering complex signal function. We also differentiate a category of hypotheses based on inter-signal interactions. Throughout our review, we hope to make three points: (1) a complex signal is a functional unit upon which selection can act, (2) both content and efficacy-driven selection pressures must be considered when studying the evolution of complex signaling, and (3) individual signals or components do not necessarily contribute to complex signal function independently, but may interact in a functional way.

Female mate choice based upon male motor performance

Our goal in this essay is to review the hypothesis that females choose mates by the evaluation of male motor performance. We define motor performance as vigour, the ability to perform energetically expensive acts repeatedly, or as skill, the ability to perform difficult motor tasks well. Motor performance reflects most aspects of whole-organism performance that relate to survival, and thus should indicate, more reliably than ornaments do, individual male genetic quality and/or developmental history. Male sexual displays in many animal taxa contain elements of vigour and/or skill, and accumulating evidence suggests that females choose mates in nature based upon their evaluations of male motor performance. We note that male ornaments in many species are accompanied by conspicuous motor display, and we propose that ornaments often arise secondarily as a way to enhance the apparent skill or vigour of male motor performance. More and better methods to measure male vigour and skill are needed, as well as additional studies on the abilities of females to make discriminations of this type.

Subadult experience influences adult mate choice in an arthropod: Exposed female wolf spiders prefer males of a familiar phenotype

Current sexual selection theory proposes several potential mechanisms driving the evolution of female mating preferences, few of which involve social interactions. Although vertebrate examples of socially influenced mating preferences do exist, the invertebrate examples are virtually nonexistent. Here I demonstrate that the mating preferences of female wolf spiders can be acquired through exposure as subadults to unrelated, sexually active adult males. I first conducted exposure trials during which subadult females of the wolf spider Schizocosa uetzi were allowed to interact with mature males of an experimentally manipulated phenotype (either black or brown forelegs). After maturation, these previously exposed females were paired with a male of either a familiar or unfamiliar manipulated phenotype for mate-choice trials. Subadult females that were exposed to directed courtship by mature males of a particular morphological phenotype were subsequently more likely to mate with a male of a familiar phenotype as adults. Furthermore, females that were exposed as subadults were more likely, as adults, to cannibalize a courting male with an unfamiliar phenotype. Unexposed females did not distinguish between phenotypes in either mate choice or cannibalism frequency. These results suggest a previously uncharacterized mechanism influencing the origin of female mating preferences and ultimately the evolution of male traits: subadult experience. This study also stresses the potential importance of learning and memory on adult mate choice in an arthropod.

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.

Diet influences mate choice selectivity in adult female wolf spiders

Most studies of female choice have assumed that mating preferences are shared within a population or species. However, variation both within and among females exists in natural populations, and foraging history is among the many ways in which females may vary. Here, we used diet manipulations in an effort to understand how foraging history influences female mate choice. Immature Schizocosa wolf spiders collected from a mixed population of brush-legged and non-ornamented males were reared in the laboratory on two diets that varied in both quality and quantity (low/high diet). For low- and high-diet individuals, we recorded data on rates of development, adult size and adult mate choice. Consistent with previous work, we found that high-diet spiders matured more quickly and were significantly larger as adults than low-diet spiders. Males also matured earlier than females. Body condition varied with diet treatment and sex. High-diet individuals and females were both characterized by better body condition indexes. In addition, high-diet brush-legged males had larger brushes than low-diet brush-legged males. Upon maturation, females were paired simultaneously with a low- and a high-diet male of the same form (brush-legged or non-ornamented) in a mate choice trial. While no obvious differences were observed in courtship and/or mating effort between males, female mate choice varied with the females diet treatment. High-diet females mated more frequently with high-diet males than with low-diet males, whereas low-diet females showed no selectivity.

An introduction to multimodal communication

Though it has long been known that animal communication is complex, recent years have seen growing interest in understanding the extent to which animals give multicomponent signals in multiple modalities, and how the different types of information extracted by receivers are interpreted and integrated in animal decision-making. This interest has culminated in the production of the present special issue on multimodal communication, which features both theoretical and empirical studies from leading researchers in the field. Reviews, comparative analyses, and species-specific empirical studies include manuscripts on taxa as diverse as spiders, primates, birds, lizards, frogs, and humans. The present manuscript serves as both an introduction to this special issue, as well as an introduction to multimodal communication more generally. We discuss the history of the study of complexity in animal communication, issues relating to defining and classifying multimodal signals, and particular issues to consider with multimodal (as opposed to multicomponent unimodal) communication. We go on to discuss the current state of the field, and outline the contributions contained within the issue. We finish by discussing future avenues for research, in particular emphasizing that ‘multimodal’ is more than just ‘bimodal’, and that more integrative frameworks are needed that incorporate more elements of efficacy, such as receiver sensory ecology and the environment.

Seismic signals are crucial for male mating success in a visual specialist jumping spider (Araneae: Salticidae)

The diversity of courtship displays throughout the animal kingdom is immense and displays can range from seemingly simple, to incredibly complex. Signallers often possess elaborate morphological adaptations for signals directed at a specific sensory modality in receivers. In some cases, these signals are so compelling to human observers, the possibility that important signals exist in other sensory modalities is ignored, potentially resulting in an incomplete characterization of the communication system. Jumping spiders (Salticidae) have remarkable visual capabilities. Yet one species, Habronattus dossenus, has recently been shown to have a complex repertoire of multicomponent seismic courtship signals in addition to and produced in concert with its multiple visual ornaments and movement displays. Here, we demonstrate the importance of these seismic signals in the courtship display of H. dossenus by comparing mating frequencies across experimentally manipulated treatments. Virgin females were paired with males from one of two experimental groups: nonmuted males or muted males. We found that females were significantly more likely to copulate with nonmuted males than with muted males. Furthermore, in all pairs that copulated, the latency to copulation was significantly shorter in nonmuted pairings than in muted pairings and precopulatory cannibalism rates were significantly lower. These results demonstrate that seismic signals are a critical component of male H. dossenus courtship displays. Additionally, we demonstrate that many other Habronattus species include a diversity of seismic signals in their courtship displays and we discuss potential selection pressures that may drive the evolution of multimodal displays even in species that already possess elaborate morphological adaptations for signals directed at one sensory modality.

Geographical variation in male courtship behaviour and sexual isolation in wolf spiders of the genusSchizocosa

We surveyed 12 populations of the wolf spider Schizocosa crassipes (Walckenaer) and S. nr. crassipes in Tennessee, Mississippi, Louisiana and Florida, in the U.S. to determine the extent of variation in male courtship behaviour when observed in standard laboratory conditions. We observed variation in both the frequency of occurrence and the sequence of the four principal male courtship behaviours and, when compared statistically, most of these behaviours differed between populations. Although there was no clinal pattern in the frequency of courtship behaviour, we observed geographically consistent patterns in the sequence in which male courtship behaviours are displayed. We conducted two subsequent studies to determine whether sexual isolation among selected populations existed. In the first study, we performed male-female reciprocal crosses of brush-legged spiders (S. nr. crassipes) between two populations from the Yazoo-Mississippi Delta physiographical region and one population from the Loess Bluff region of Mississippi and measured mating success, number of matings that produced egg sacs, and the number of egg sacs that hatched young. The results of that experiment suggest that sexual isolation occurs among populations in different physiographical regions but not between populations within regions of the state. In a second crossing study, we paired the brush-legged spiders from one of the Mississippi physiographical regions (Yazoo-Mississippi Delta) with Schizocosa ocreata (Hentz) from Ohio. The results of that cross suggest that the Mississippi Delta population is more similar to the northern S. ocreata populations than to S. crassipes. We discuss the results of the crossing experiments in terms of what was revealed about geographical patterns in male courtship behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Diversification under sexual selection: the relative roles of mate preference strength and the degree of divergence in mate preferences

The contribution of sexual selection to diversification remains poorly understood after decades of research. This may be in part because studies have focused predominantly on the strength of sexual selection, which offers an incomplete view of selection regimes. By contrast, students of natural selection focus on environmental differences that help compare selection regimes across populations. To ask how this disparity in focus may affect the conclusions of evolutionary research, we relate the amount of diversification in mating displays to quantitative descriptions of the strength and the amount of divergence in mate preferences across a diverse set of case studies of mate choice. We find that display diversification is better explained by preference divergence rather than preference strength; the effect of the latter is more subtle, and is best revealed as an interaction with the former. Our findings cast the action of sexual selection (and selection in general) in a novel light: the strength of selection influences the rate of evolution, and how divergent selection is determines how much diversification can occur. Adopting this view will enhance tests of the relative role of natural and sexual selection in processes such as speciation.

Age-related female mating decisions are condition dependent in wolf spiders

Female mating behaviors are known to be sensitive to a variety of individual factors both external and internal to a female; however, mating decisions are likely due to a suite of interacting factors. By independently manipulating female and male diet in the wolf spider Rabidosa rabida and testing females across age groups, we demonstrate that, in addition to its independent effect, female nutritional condition interacts with female age to influence female mating behavior. Overall, high-quantity diet (HD) females were more likely to mate than low-quantity diet (LD) females. Within the LD females, older individuals were more likely to mate than younger individuals, while within HD females, mating probabilities were equal across females of different age classes. With respect to mate choice, only female age influenced the likelihood of mating based on male diet. Young females were choosier as they were more likely to mate with HD males than LD males; in contrast, older females were equally likely to copulate with males of each diet treatment. In addition, the likelihood of pre-sexual cannibalism was dependent on both female and male diet. High-quantity diet females were more likely to cannibalize than LD females, and attacks were directed towards LD males most often. We discuss our results in terms of costs versus benefits of female mate choice.