Understanding the evolution of social systems, such as cooperative breeding, has been of major interest to biologists. Comparative work has identified several selective factors favoring sociality that receive significant support in global datasets. In this dissertation, I tested the importance of two of these hypothesized drivers independently, and performed the first comparative analyses of multiple potential drivers of cooperative breeding. First, I investigated the relationship between cooperative breeding and brood parasitism at a global and regional scale. I found a strong correlation between the two that may be due to brood parasites being attracted to cooperative breeders rather than by parasites driving the evolution of sociality, as previously supposed. Second, I tested the relationship between promiscuity and social system, using relative testis size as measured from museum specimens as a proxy for mating system. This greatly increased the sample size (by an order of magnitude) over genetic measures of promiscuity and eliminated a strong bias towards data on species from the northern hemisphere. While there were some discrepancies among analyses, I found that cooperative breeding and relative testis size exhibited the negative association expected under indirect benefits models of the evolution of cooperative breeding, despite my less biased and much larger sample of species. Finally, I combined five factors previously suggested to be important in the evolution of cooperative breeding into a single comparative analysis to determine the relative importance and interactions among them. Correlations between cooperative breeding and these predictors were highly dependent on the phylogenetic tree and choice of analysis, but favored a positive association with brood parasitism overall with little support for interactions. I make recommendations for ways forward toward better understand the evolution of cooperative breeding in a comparative framework.
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