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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.

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.

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

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Sexually selected traits are among the most costly, complex, and conspicuous elements of the phenotype. In polygynous reproductive systems, they evolve under strong selection by females. Why females favor those traits, however, is an on-going debate. Here, I use túngara frogs as a model system to study the evolution of communication under sexual selection. The wealth of available information on the behavior, neurophysiology, and reproductive biology of túngara frogs make them an ideal system to understand the patterns of signal evolution and explore the processes that have shaped them. In chapter 1 and 2, I review the taxonomy of túngara frogs (Engystomops) from western Ecuador. I describe three new species including their external morphology and advertisement calls. In chapter 3, I explore the phylogenetic relationships of túngara frogs, testing the support for alternative relationships previously reported for this group. The new phylogeny provides the framework for the comparative analysis carried out in chapters 4 and 5. In chapter 4, I present new female preference and male advertisement call data to test the sensory exploitation hypothesis of sexual selection. Using ancestral character reconstruction, I found that female preferences for complex calls did not originated before the appearance of complex calls, as predicted by sensory exploitation. Instead, my results suggest that the origin of complex calls and their preference originated at similar times. Finally, in chapter 5, I analyze the macroevolutionary patterns of call variation in male túngara frogs. A generalized least squares model demonstrates that advertisement calls have a strong phylogenetic signal. Although most species in Engystomops have distinctive calls, they share a common acoustic structure with two components that evolve at different rates. I did not found evidence of greater call differentiation among sympatric species relative to allopatric species.

The function of the complex song of the European Starling (Sturnus vulgaris) was examined. Song playback experiments showed that both male and female starlings were attracted by song, and complex song inhibited males from entering nestboxes. Wild starlings (at least two years old when first recorded) showed extensive changes in the composition of their song phrase repertoires and most also increased the size of their repertoires, which will result in a correlation between age and repertoire size in this species. Females prefer males that have more complex song, and this preference remained significant when preferences for certain nest sites were controlled. Males with larger repertoires did not spend more time incubating or make more feeding visits to nestlings than did males with smaller repertoires. The evolution of complex song in the European starling is consistent with an age-indicator model of sexual selection, in which aspects of male quality correlated with age are advertised by the complexity of male song.

The occurrence of three reproductive tactics in a population of American redstarts was estimated: extra-pair fertilization, egg-dumping, and male polyterritoriality. Using DNA fingerprinting we show that none of 108 nestlings samples were the result of egg dumping. In contrast, 40% of nestlings were the result of extra-pair fertilizations. Males sired a greater proportion of their broods as they aged. In addition, promiscuous females were never fertilized by males that were younger than their social mates. Whether the poor reproductive performance of younger males was caused by female preference for older males or by intra-sexual competition was not determined. In any case, females would have benefited by mating with older males, if the traits that allowed the survival of the father were inherited by the females' offspring. Only three of 80 males attracted two females through polyterritoriality. One male fledged two broods: he sired both nestlings from his first brood, but none of the four nestlings from his second brood. We concluded that male redstarts can improve their reproductive success by trying to obtain extra-pair copulations, but less so by establishing a second territory.

In the first part of this thesis, a general one dimensional theory of strategically determined phenotypes is developed and applied to biological signalling games. Abstract modular modelling techniques are utilised to solve hitherto analytically intractable problems including error-prone signalling, and how to incorporate genetic features into optimization models. Links are drawn between previous biological models, such as the War of Attrition and Strategic Handicap mod- els. Mistakes in previous biological models are recognised and, where possible, rectified. A number of novel insights into biological phenomena arising from the models are presented, including analyses of: when free signals are possible; honest signalling of future paternal investment; dimorphic signals; the effects of the mechanisms of female discrimination in sexual selection on male signalling strategies; and the effects of relatedness on the magnitude and stability of equi- librium signalling strategies. It is argued that Zahavi's proposed demarcation between signal selection and natural selection is unjustifiable from a theoretical perspective. The second part of the thesis concerns the epistatic handicap process of sex- ual selection. Unlike the conditional and revealing handicap mechanisms, the epistatic or 'Zahavian' handicap mechanism of sexual selection has hitherto found scant support in the theoretical literature, as it appeared to function only under the most extreme conditions. A continuous game theory model, a quantitative genetics model, and a three locus major gene model are presented which show that the epistatic handicap mechanism can function, independent of the Fisher process of sexual selection, under reasonable assumptions. More- over, the game theory model illuminates the connection between the strategic and epistatic handicap mechanisms. The quantitative genetics and major gene models, together with a fourth model, are also used to show that a general argu- ment concerning indirect genetic correlations, which has appeared in a number of papers on sexual selection, is specious. Finally, a general theorem on games that satisfy the single-crossing condition (also known as the sorting, Spence-Mirrless, or constant sign condition) which underlies many of the results presented in the first part of the thesis is proven in appendix C. Applying a limit result to this general theorem provides a new proof of, and extensions to, Nash's existence result for equilibria to strategic- form games without having to resort to Kakutani's fixed point theorem.