Environmental and Biodiversity Correlates of Sexual Signal Variation

Explaining the diversity of sexual signals is a fundamental problem in evolutionary biology. Generally, two major factors have been hypothesized to explain signal diversity: the habitat in which signals are produced and the presence of other species with similar sexual signals. These two explanations are not mutually exclusive, but research in any given system often focuses on only one explanation. Relatively few studies have examined how the contribution of both habitat features and other signaling species impacts the evolution of sexual signals. In this thesis, we addressed this issue using spadefoot toads (Spea bombifrons). The males of this species produce two auditorily distinct, geographically distributed mating call types: a slow call in northern regions and a fast call in southern regions. We contrasted these calls using five call characteristics—call rate, call duration, pulse rate, dominant frequency, and call effort—to quantify the extent of difference between the two call types. We also examined the extent to which each call characteristic explains the variation of either call type. We then analyzed whether four environmental variables—dew point temperature, temperature, net primary productivity, and precipitation—and species interactions predicted the mean principal component scores for the different calls. We found that both environmental variables and species interactions explained variation in the call types. This study provides important insights into the factors that might contribute to the diversification of sexual signals.