In many species, members of one sex (usually males) compete for access to mating opportunities. When this competition manifests through inter-sexual interactions, female preferences commonly drive the evolution of elaborate male traits. Many different models have been proposed to explain the evolution of these traits.
Anurans are a taxonomic group that is particularly dominated by inter-sexual selection. Male frogs spend a significant amount of time and energy producing advertisement calls, and these calls are used by females to select from prospective mates. This widespread importance of a single type of sexual advertisement across an entire taxonomic group is unique within the animal kingdom. As such, anurans are an ideal model taxon for studies of inter-sexual selection, and the main aim of this thesis was to demonstrate this usefulness of anurans for investigating various models of inter-sexual selection.
First, the possible functions of male ornaments in anurans were investigated in two separate studies. First, calls from the Australian frog Litoria chloris were used to investigate whether calls in this species are used as signals of quality, or if the main function of calls is for individual recognition. A number of general properties of the advertisement calls of L. chloris were assessed to determine which of the two possible functions of calls appeared to be more important. It was found that some aspects of the calls were consistent with quality signalling, although most were not. In addition, many features of the calls indicated that they would be useful for individual recognition.
The function of frog calls was also investigated, using a meta-analysis to quantify the relationships between male signals (also known as ornaments), male quality, and female preferences. The strengths of these relationships were found to be consistent with inter-sexual indicator models, which assume that male ornaments act as signals of quality. This study provides the first evidence of male ornaments acting as signals of quality across an entire taxonomic group.
Finally, a comparative study was carried out to determine whether sexual selection has played a major role in the adaptive radiation of anurans. Models that evoke sexual selection as a driver of speciation propose that diverging female preferences within a population can result in reproductively isolated groups, thus facilitating sympatric speciation. These models, therefore, predict that closely related species should have divergent sexual signals. The calls from a large number of frog species were analysed, and the data collected were used to assess the phylogenetic patterns that were present within a variety of different call features. These patterns indicated that many of the call features that were analysed did not indicate that closely related species had divergent signals. This finding indicates that speciation in anurans is unlikely to have been influenced by inter-sexual selection.
The field of inter-sexual selection is vast, and has many questions that remain unresolved. However, this thesis has demonstrated how anurans can be used to investigate some of the fundamental ideas involved in inter-sexual selection. In addition, these results open up further avenues of research, which could resolve many more questions to do with inter-sexual selection.
