Although most strepsirrhines do not exhibit apparent physical signs of sexually selected traits, researchers have suggested that olfactory communication is sexually selected. The goal of this thesis is to (1) review sexual selection theory with an emphasis on sensory communication, and (2) test whether olfactory communication is sexually selected in strepsirrhines. I examined the relationships between primate mating systems and several measures of olfactory communication in 22 species: scent marking rates, the number of scent marking methods, and the volume of the main and accessory olfactory bulbs. I also evaluated qualitative data on olfactory communication in three lemur species to determine whether they meet the criteria of a sexually selected trait. Polygynandrous and monogamous species did not significantly differ from each other in scent marking rates, scent glands, or volume of the main and accessory olfactory bulbs. Three species of strepsirrhine met all criteria of having sexually selected olfactory traits, suggesting that polygynandrous lemurs are subject to sexual selection on several levels of olfactory communication. / text

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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.

My Ph.D. research has examined the evolution of mating preferences and their role in speciation. I have addressed these topics empirically, using sympatric species pairs of limnetic and benthic threespine sticklebacks, and theoretically, with multilocus population genetics. Sticklebacks are small fish that occur in lakes, streams and estuaries throughout British Columbia. Most lakes contain one type of stickleback, however, several lakes support two differentially adapted sympatric species: a large benthic form (benthic), and a smaller zooplanktivorous form (limnetic). Chapter 2 examines the role of species interactions in shaping male mating preferences. I determined that the mate preferences of the small species have shifted from preferring large females (the ancestral state) to preferring small females due either to selection against hybrids (reinforcement), or to egg predation by the larger benthic species. Chapter 3 explores the idea that sexual imprinting may facilitate assortative mating between benthic and limnetic sticklebacks. Sexual imprinting occurs when individuals imprint on the phenotype of their parents, and subsequently prefer mates that resemble their parents. The results suggested that sexual imprinting does not contribute to assortative mating between the sympatric species pairs, implying that genetics are more important than early learning for the formation of mate preferences. Chapter 4 focuses on differences in male breeding colour between benthics and limnetics. The results reveal that limnetic males have more intense red and blue coloration than benthic and solitary males. These differences in colour could be due to reinforcement, to differences in visual sensitivity of females, or to territorial interactions between males. Chapter 5 examines the evolution of female mating preferences under different scenarios of sex-linkage, when the male display trait is sexually antagonistic. Theoretical analysis suggests that sexually antagonistic traits on the X chromosome (males XY, females XX), females will evolve to prefer mates carrying alleles beneficial to their daughters. In contrast, with a Z-linked trait (males ZZ, females ZW), females more often evolve preferences for mates carrying alleles beneficial to their sons (e.g., flashy displays). This provides an explanation for why males in ZW species have more elaborate sexual displays than males in XY species.