Sexual selection in animals

Abstract

This thesis covers a range of topics falling under the purview of sexual selection.

In the first two chapters I focus on how polyandry influences female fitness. In Chapter 1, I collaborated on a comprehensive meta-analytical review of the literature to determine if there is broad taxonomic support for the hypothesis that females gain indirect genetic benefits from mating multiply. The results suggest that polyandry has a small but beneficial effect on clutch production, fertility, and egg hatching success. Furthermore, polyandry had a detectable effect for the composite measure of offspring performance.

In Chapter 2, I investigated how polyandry and male harassment influence female fitness in mosquitofish. Polyandry had no effect on female fitness components and no indirect effect on offspring performance. Male harassment, however, had pervasive effects on females. Male harassment had transgenerational consequences: fry from harassed females were smaller at birth and were more likely to be male.

In Chapter 3, I performed a field study investigating how male fiddler crabs attempt to increase their reproductive success by controlling burrows, a resource that is essential to female reproduction and survival. I used a series of observational studies to show that males have more burrows around them than expected by chance. These burrows are used to attract additional female neighbours and are defended from rival males.

In Chapter 4, I investigated the phenotypic link between secondary sexual characters (SSCs) and sperm quality. I performed a meta-analysis of the available data to determine if there is support for this hypothesis. There was very little correlation between SSCs and sperm quality (r~0.07), regardless of how the data was parsed. The only statistically significant effect size was for sperm viability. Intriguingly, this is the aspect of sperm quality, which is most likely to impact females' direct fitness.

In the final two chapters I investigated mate choice/preference patterns in two different species. In Chapter 5, I investigated male mate choice patterns in mosquitofish and asked how competition from other males influences the focal male mating preferences. Males preferred larger females. This preference was, however, modified by the presence, but not the size, of competitor near an otherwise preferred female.

In Chapter 6, I investigated female mating preferences in humans. I generated 343 three-dimensional male figures varying in height, shoulder to hip ratio, and penis size to test female preferences (rating of sexual attractiveness). Multivariate selection analyses show that there is significant linear selection on each of these traits. However, quadratic selection was also significant indicating that there was a limit to attractiveness as the size of these traits increased as it began to asymptote at higher trait values. Finally, there were significant interactions between the traits (i.e. correlational selection) that affected a male's net attractiveness.