Reproductive behaviors of vertebrates are often underpinned by temporal patterns of
hormone secretion. We investigated interspecific patterns of circulating testosterone in male
birds to test the hypothesis that testosterone plays a crucial role in sexual selection as
determined by degree of polygyny and extra-pair paternity. We predicted that the evolution
of increased levels of polygyny and extra-pair paternity would have resulted in the evolution
of increased levels of testosterone to allow males more efficiently to compete for mates. This …