In this thesis, I investigate the relationship between two variables for which persistent directional sexual selection is an evolutionary driver: condition dependence and sexual dimorphism. This joint dependence on sexual selection predicts that among traits within a given species, greater dimorphism should be associated with stronger condition dependence. Very few studies have tested this prediction, and those that have focus on species with highly exaggerated and strongly dimorphic traits between the sexes. Here, I quantified variation in a suite of morphological traits in a dipteran species – the antler fly, Protopiophila litigata – in which sexual dimorphism is less extensive. I manipulated condition via different larval diets and then quantified the effects on adult body size and shape in both sexes. Across traits, I found that the extent of sexual dimorphism was positively associated with the strength of condition dependence in males but not in females. These results suggest a shared developmental basis to condition dependence and sexual dimorphism in body shape, and suggest that this has arisen via sexual selection in males despite the absence of extremely dimorphic shared traits.

One of the major research challenges is the ability to test selective forces in a wild population. A recent discovery of a new dipteran species, Protopiophila litigata, can enable researches to test selection in the wild. Most research has focused on mating behaviour, male mating success and senescence. In this study a small sample of wild mating and non-mating flies were collected, cuticular hydrocarbons were extracted and morphometric traits were obtained to assess the strength of sexual selection. There was significant linear sexual selection on cuticular hydrocarbons and, mid tibia length, hind tibia length and wing length. Overall, further establishes P. litigata as a model species for studying selection in the wild. Earlier studies have demonstrated strong sexual selection on male cuticular hydrocarbons in Drosophila serrata. Recently wing interference patterns have been documented to be under sexual selection in Drosophila melanogaster. A sample of cuticular hydrocarbons and wing interference pattern values were analyzed to understand the combined effects on male mating success. Cuticular hydrocarbons were under sexual selection, however wing interference patterns were not. Overall, this study confirms selection on cuticular hydrocarbons, but highlights the difficulty in accurately capturing and measuring wing interference patterns.

Aspects of white-tailed deer (Odocoileus virginianus) reproductive ecology remain understudied. The accuracy of the fetal age estimation equation in current use is unknown. Knowledge is also limited for female choice of secondary sexual traits like antlers and body size. To address previous fetal estimation equations, I developed a model that included litter characteristics using 110 fetuses with known ages of 54 to 175 days. To address female choice, I manipulated antler size and paired large and small males while controlling allometrically related traits. I then allowed estrus females to choose between pairs of segregated males with either large and small antlers or large and small bodies. My predictive fetal aging model generated more accurate fetal ages under a range of sample timing and composition variation. Using various behavioral indications of choice, I demonstrated that females prefer males with larger antlers and lack a preference for body size or age.

Sexually selected male weaponry are among the most diverse and elaborate traits seen in the animal kingdom and often aid males in securing resources and mates, providing a fitness advantage to those that bear them. Yet, while intra-sexual competition between males to secure mates is common, weaponry is a rare trait that is observed in few taxa. Building upon previous theory, Emlen predicted that exaggerated animal weaponry evolves when, (i) there is intense competition between males for access to females, (ii) the biology or behaviour of a species generates high variance in male reproductive success, and (iii) competitions between males promote extreme weapon size evolution. While there are many species bearing weaponry that appear to fit these contexts, it has yet to be tested if these conditions are sufficient to initiate the evolution of exaggerated weaponry. For my research, I created three environmental treatments that emulated the three conditions proposed by Emlen. Using these treatments, I conducted experiments using Drosophila melanogaster to examine the effects of spatial structure on natural and sexual selection with a focus on the initial stages of the evolution of exaggerated trait weaponry that arise from this process. From these experiments, I found that the strength of sexual and other components of natural selection vary with environmental complexity and mutational effect. I also found that these environmental treatments generate weak differences in condition dependence and allometry of fly morphological traits that could be indicative of early weapon evolution. With these experiments, I provide the initial framework to test whether the conditions proposed by Emlen are both necessary and sufficient to lead to the initial evolution of exaggerated animal weaponry and demonstrate that these conditions may indeed create the circumstances that allows this evolution to occur. / Thesis / Master of Science (MSc) / Sexual selection drives the evolution of extreme animal weapons that are used in competition between individuals for access to mates, however while competition is common, these traits are rare. Theory predicts that animal weapons will evolve when (i) there is intense competition between males for mates, (ii) there is high variance in male reproductive success, and (iii) competitions between males mostly occur as duels. For my research, I created three environments that emulated these conditions to differing extents and used the fruitfly as a model to see how these conditions influence the initial stages of animal weapon evolution. I found that these environments are variable in their effects on sexual selection and create morphological differences that could indicate early weapon evolution. With this research, I provide the initial framework to test whether these conditions can create the circumstances for the initial evolution of exaggerated animal weaponry to occur.

Darwin first proposed sexual selection as a process to explain the evolution of extravagant morphological traits in males. Despite being potentially detrimental to individual survival, such traits evolve because they increase a male's reproductive success, and provide a net benefit to their fitness. Mate competition is the source of sexual selection, and healthier, more vigorous males are likely to be superior competitors. Because most genes are likely to impact an individual’s health/vigour, sexual selection should act across much of the genome to favour the same alleles as natural selection, thereby promoting adaptation. On the other side of the coin, adaptation to an environment should enhance male sexual fitness, since it is likely to increase the overall health/vigour of individuals within a population, though tests of this prediction are rare and results are mixed. Taking advantage of a long-term evolution experiment involving replicate populations of Drosophila melanogaster, I performed a reciprocal transplant in which the sexual fitness of males was compared when raised in an environment to which they are well adapted and in one to which they are not. I improved on past tests via a comprehensive measure of male sexual fitness that included pre- and post-copulatory reproductive success in a competitive assay under conditions that closely mirrored those to which the populations have been evolving. I found that sexual fitness was higher in locally-adapted males from these experimental populations, a result that was consistent across environments that also manipulated the context in which mate competition occurred.

Social dominance can influence the allocation of resources in animal groups and has important consequences for individual fitness. In my thesis, I examined the structure, formation, maintenance and consequences of dominance, in the cooperatively breeding pukeko (Porphyrio melanotus melantous: Aves). I first describe a quantitative analysis of pukeko dominance networks (Chapter 2), which included one of the first applications of exponential random graph models in behavioural ecology. This study demonstrated that pukeko form highly ordered dominance hierarchies, and that dominance relationships were influenced by both the attributes of individual birds, as well as self-organizational processes such as winner and loser effects. Additionally, I demonstrated that hatching order has an important influence on the formation of dominance relationships, with earlier hatched chicks achieving higher dominance ranks as adults (Chapter 3). To maintain dominance relationships, pukeko use their red frontal shield as a ‘status signal’, with larger frontal shields indicating more dominant individuals. I showed that sexual dimorphism in frontal shield size is dramatically different in two pukeko populations, probably due to differences in the intensity of intrasexual competition (Chapter 4). Furthermore, by manipulating apparent frontal shield size, I demonstrated that shield phenotype both influences, and is influenced by, social interactions (Chapter 5). This bi-directional relationship between signals and social interactions challenges conventional signalling theory, and has important implications for how honesty is maintained in this signalling system. Finally, I expanded my findings on pukeko colour traits by exploring interspecific patterns of bill colouration in over 1600 bird species (Chapter 6). This study revealed that colourful bills likely evolved as a signal used in competitive interactions, rather than as a sexual signal. Taken together, my research provides a significant advancement in our understanding of the complex nature of dominance in a wild bird, and provides both a methodological and theoretical basis for future studies on animal social behaviour. / Thesis / Doctor of Science (PhD)

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Elaborate sexual ornaments evolve because mate choice exerts strong sexual selection favouring individuals with high levels of ornament expression. Consequently, even at evolutionary equilibrium, life history theory predicts that ornamental traits should be under directional sexual selection that opposes contrasting selection to reduce the costs associated with their maintenance. Otherwise, the resources used to maintain ornaments should be used to improve other life history functions. Elaborate female ornaments have only evolved in a few species, despite females commonly experiencing strong sexual selection. One explanation for this rarity is that male preferences for female ornaments may be self-limiting: females with higher mating success become less attractive because of the lower paternity share they provide to mates with every additional sperm competitor. The unusual species in which female ornaments do occur can provide rare insight into how selection can favour the expression of expensive characters in females despite their costs. The main goal of my thesis was to determine how sexual selection acts on exaggerated sexual ornaments, and give new insight into how these ornaments may have evolved, in spite of the self-limiting nature of selection on male preferences. To determine the strength of sexual selection acting on female ornamentation in dance flies, we developed new microsatellite markers to assess polyandry rates by genotyping stored sperm in wild female dance flies. We first used polyandry rates to determine whether ornament expression was associated with higher mating success in female Rhamphomyia longicauda, a species that has evolved two distinct and exaggerated female ornaments. Contrary to our predictions, we found no evidence that females with larger ornaments enjoy higher mating success. We then compared polyandry rates in R. longicauda to those of two other species of dance fly, one (Empis aestiva) that has i independently evolved female ornaments on its legs, and another (E. tessellata) that does not possess any discernable female ornaments. We also estimated the opportunity for sexual selection, which we found to be similar and relatively low in all three species. Moreover, the standardized sexual selection gradients for ornaments were weak and non-significant in all three species. Females with more elaborate ornaments, in both within- and cross-species comparisons, therefore did not enjoy higher mating success. Overall, these results suggested that sexual selection operates rather differently in females compared to males, potentially explaining the general rarity of female ornaments. Our amplifications of stored sperm were able to reveal more than just mate numbers. We developed new methods to study patterns of sperm storage in wild female dance flies. We investigated how the skew in sperm genotypes from mixed sperm stores changed with varying levels of polyandry. Our data suggested that sperm stores were dominated by a single male in R. longicauda, and that the proportion of sperm contributed by this dominant male was largely independent of the number of rival males’ sperm present in the spermatheca. These results were consistent with the expectation of males using sperm ‘offence strategies’ in sperm competition and that the most successful male is likely to be the female’s last partner before oviposition. As a whole, my thesis contributed new molecular resources for an understudied and fascinating group of organisms. It exploited these new resources to provide the first estimates of lifetime mating success in several related species, and suggested that the general prediction that ornament expression should covary with sexual selection intensity does not seem to hold in this group. Instead, both the unusual prevalence of ii ornaments and the inconsistent evidence for sexual selection that sustains them in dance flies may owe their existence to the confluence of two important factors. First, the conditions under which sperm competition occurs: as last male precedence is likely, males are selected to prefer the most gravid females to secure a high fraction of her offspring’s paternity as they are unlikely to mate again before oviposition. Second, potent sexually antagonistic coevolution between hungry females and discerning males: females have evolved ornaments to disguise their stage of egg maturity to receive the benefits of nuptial gifts, while males face the challenge of distinguishing between gravidity and ornamentation in females.

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