Female choice is an important selective force shaping the evolution of communication and speciation in animals. However, predation risk can impose severe costs on longer searches and choosiness, thereby limiting the expression of female preferences for specific male traits. The work detailed in this dissertation explores how mate choice and sexual selection can be influenced by predation risk in túngara frogs. I begin by examining the effects of multiple simulated cues of predation risk on female search behavior and mate choice, taking a departure from the standard presence/absence paradigm used in similar studies to explore responses to quantitative variation in perceived predation risk. I demonstrate that light, longer travel times, and acoustic cues of predators are all sufficient to sway females away from otherwise more attractive conspecific males. Next, I explore the role of predation risk in altering female permissiveness, or the range of signals females will respond to. Using an artificial series of calls intermediate between heterospecific and conspecific, I demonstrate that predation risk dramatically increases the range of signals females will respond to, including a small number of females choosing pure heterospecific calls. Next I attempt to bridge a logical gap with our understanding of search costs, testing questions about how female search paths change with increasing distance. I demonstrate that females use more direct paths and move faster under higher light conditions, potentially reducing sampling but also reducing encounter rates with predators. Lastly, I examine factors that influence how individual females vary in their response to perceived risk, particularly hormonal state and experience. I demonstrate that naïve, captive-bred females respond to acoustic cues produced by natural predators in a manner similar to wild females and that, while hormonal state is obviously important in determining female receptivity, it has little effect directly on how females respond to predators. Together, these studies demonstrate that predation risk not only changes how females respond to conspecific males, but also increases female permissiveness and constrains search behavior. Predation risk can strongly influence and potentially even negate the expression of female preferences, having profound consequences for communication and the evolution of reproductive isolation between populations. / text

In this study I present a new mechanism for the evolution of male display as a consequence of female choice. I use a population genetic model to show that if female preferences for better males are based on a cue that is an integral part of male adaptation, a display may evolve if it amplifies the variance in this cue, and hence increases female resolution power with respect to male quality. This evolutionary mechanism is used as a core of a theory that explains the evolution of male display and adaptive female choice (i.e. female preferences that evolve because of their association with high quality genes). I argue that because an amplifying display (termed "amplifier") decreases mating success of males of poor quality, modifiers are likely to evolve that decrease the expression of the amplifier when associated with the poor quality males. As a result, the amplifier's expression becomes an indicator of male quality, and provides sufficient conditions for the evolution of a new type of female choice that is based on the amplifier's expression. This process may lead, in turn, to further changes in both female choice and male display, emphasizing either the amplifying effect of displays or their indicating effect. I show that the direction of these changes may depend on the cost that the amplifier confers on male viability, and on the degree of polygyny of the mating system in concern. I also outline explicit predictions for empirical tests.

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The face holds a central role in both human and nonhuman primate social interactions, through the communication of feelings and intentions via facial expressions and by acting as a means of recognising individuals. Humans, however, also employ their faces in mate attraction and assessment, an area that has received little attention in nonhuman primates. Many researchers have proposed that human aesthetic judgments of facial attractiveness have a biological basis, and these preferences have evolved via sexual selection processes during human evolution. The use of the face in attractiveness assessments need not be limited to humans. Rather, there is good reason to suggest that this may also apply to other nonhuman primates, based on homologies in the way in which primates use their faces, and on evidence that the face is a site of sexual selection for many primate species. It was the aim of this thesis to explore whether facial traits may also play a role in judgements of attractiveness in a nonhuman primate, the rhesus macaque( Macaca mulatta), in an effort to understand whether humans are unique in utilising the face as a mechanism of mate assessment. Three factors that are reported to influence facial attractiveness in humans are facial symmetry, sexual dimorphism, and averageness. To assess whether they also play a role in nonhuman primates, a series of experiments were conducted where digital images of adult male and female rhesus macaque faces were altered for these features. Opposite-sexed images were then displayed to adult males and females in a captive setting. Eye gaze measures were utilised to assess visual preference for, and the relative importance of, these traits. These experiments yielded mixed results. Increasing facial symmetry of opposite-sexed conspecifics positively influenced the dependent gaze measures employed here. Manipulating degree of facial sexual dimorphism had little influence on the visual gaze of either sex. Facial averageness positively influenced visual preferences for opposite-sexed conspecifics among both sexes, although increasing degree of averageness did not. The last topic to be explored was facial colouration. Rhesus macaques like, various other species of anthropoid primates, possess facial displays of red secondary sexual colouration. As above, animals viewed digitally altered pale and red versions of opposite-sexed conspecifics. Although females displayed preferences for red male faces, males displayed no clear preferences based on female facial colour. This raises the possibility that male and female facial colour may serve different roles in intraspecific signaling. While it cannot be concluded that visual preferences are indeed indicative of real-life preferences, the results do indicate that animals are not indifferent to variations in conspecific facial features. The present findings have important implications regarding the evolution of facial attractiveness, as they provide the first experimental evidence suggesting that facial features may serve as a mechanism for mate selection across primate taxa and that both human and nonhuman primates may employ similar criteria to appraise facial attractiveness.

Sexual selection can be defined as the difference in reproductive success among individuals that is related to their ability to acquire mates and fertilizations. Mathematical models have shown that combined with natural selection, sexual selection can be an important driver for speciation. There have also been a few case studies showing that sexual selection alone can lead to speciation in nature. Over the past 30 years, the barn swallow has become a classic model animal for sexual selection. The barn swallow has at least two sexually selected traits in different subspecies: the length of tail streamers in European barn swallows, H. r. rustica and ventral plumage colour in north American barn swallows, H. r. erythrogaster. Meanwhile molecular research on barn swallows shows that these barn swallow subspecies are recently derived and thus the barn swallow becomes an ideal model animal to test the theory that sexual selection drives speciation. The Barn Swallow species complex is comprised of six closely related subspecies distributed throughout the Holarctic. Whereas experimental studies in Europe, the Middle East and North America have been conducted, little is known about populations distributed across Asia. During my PhD study I collected barn swallow samples from more than 20 locations across China and compared the difference in morphology of Chinese barn swallow populations. The results show that in the northeastern part of China the barn swallow is intermediate in phenotype between subspecies in North America (H. r. erythrogaster) and subspecies in Europe (H. r. rustica), and is characterized by rusty ventral plumage colour and medium length tail streamers. Using morphological and colour traits, northeastern Chinese swallow populations can be separated from other Chinese populations, and this pattern of phenotypic variation may form under both natural and sexual selection. I also conducted an observational and experimental study on one of these populations, aiming to determine the extent to which variation in plumage color and tail streamers is underlain by sexual selection. The observational study reveals that for male barn swallows in my study population, clutch initiation date, the number of both social and genetic offspring and the body condition of nestlings can be predicted by the colouration of their ventral plumage, while the experiment shows that the reproductive success of male barn swallows tended to increase with experimental enhancement on their plumage colouration. My research supports that the ventral plumage colour is the sexually selected trait in northeastern Chinese barn swallows, and further experiments are needed to clarify the effect of male ventral plumage colour manipulation on their breeding success with the limitation of small sample size in my study.

Condition-dependent sexual selection has the potential to align natural and sexual selection and accelerate adaptation. When the expression of a sexually selected trait is constrained by the condition of the bearer, it offers a reliable signal of quality on which females can base mate choice decisions. Individuals with highly expressed sexually selected traits are therefore expected to possess advantageous genes given the prevailing environmental conditions. Such genes can then spread by their naturally selected benefits as well as their sexually selected advantages, thereby accelerating adaptation. I investigate the effect of condition-dependent traits on mating and signalling behaviour to explore the potential for alignment between natural and sexual selection in the wild. By studying a wild population of the field cricket, Gryllus campestris, I explore condition-dependent sexual selection in a natural context. This avoids some limitations of laboratory studies, which may overestimate effects in the absence of natural and environmental variation. I employ a direct experimental test of the effect of condition on sexually selected traits and mating success, finding that while male acoustic signals are condition-dependent, modest increases in calling effort do not result in increased mating success. I investigate the effect of body size on mating success and find mating success to be independent of body size. I explore the possibility of condition- and context-mediated flexibility in mate-searching tactics, finding that while population density influences tactic choice, individual condition is unlikely to predict which tactic a male will adopt. Finally, I consider the role of female condition in sexual selection and find that mating latency in females is not condition-dependent, but that mating history has an important effect on female choosiness.

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Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

The evolution of mate choice remains one of the most controversial topics within evolutionary biology. In particular, the coevolutionary dynamics between ornaments and mating preferences has been extensively studied, but few generalizations have emerged. From a theoretical standpoint, the nature of the genetic covariance built up by the process of mate choice has received considerable attention, though the models still make biologically unrealistic assumptions. Empirically, the difficulty of estimating parameters in the models has hindered our ability to understand what processes are occurring in nature. Thus, it is the goal of this dissertation to contribute to the field both theoretically and empirically. I begin with a review of the evolution of mate choice and demonstrate how the lack of cross-talk between theoretical and empirical pursuits into studying mate choice has constrained our ability to extract basic principles. The review is followed by a new model of intersexual selection that relaxes some of the critical assumptions inherent in sexual selection theory. There are two empirical studies whose goal is to measure mating preference functions and genetic correlations in a way that can be related back to theory. Finally, I conclude by setting the stage for future endeavors into exploring the evolution of mate choice. The results presented herein demonstrate four things: (i) a lack of communication between theoretical and empirical studies of mate choice; (ii) genetic drift plays a much larger role in preference evolution than previously demonstrated; (iii) genetic correlations other than those explicitly modeled are likely to be important in preference evolution; and (iv) variation in mating preferences can eliminate intersexual selection altogether. From these four findings it can be concluded that a tighter link between theory and empiricism is needed, with a particular emphasis on the importance of measuring individual-level preference functions. Models will benefit from integrating the specific phenotypes measured by empiricists. Experimentation will be more useful to theory if particular attention is paid to the exact phenotypes that are measured. Overall, this dissertation is a stepping stone for a more cohesive and accurate understanding of mate choice evolution.