Sexual selection’s role in the persistence of polymorphism in an aposematic signal

The persistence of polymorphisms in aposematic species remains one of the most interesting paradoxes in evolutionary biology because aposematism theory suggests that polymorphisms should be unstable over time. We offer an explanation for the persistence of aposematic polymorphisms that considers not only the role of natural selection but also the role of sexual selection. While predation and mate choice generally act to erode signal variation, intraspecific competition may facilitate polymorphisms by reducing mate competition for males bearing the rarer warning signal. We tested this hypothesis in a population of the strawberry poison frog (Oophaga pumilio), where red (common) and yellow (rare) morphs coexist using (1) a 10-year capture-mark-recapture experiment to study natural selection, (2) a territorial intrusion experiment, and (3) previously published mate choice experiments to study sexual selection. We found that rare yellow males suffer less aggression from male conspecifics, suggesting negative frequency dependent selection. Moreover, the more common and choosier red females have lower apparent survival than their less choosy yellow counterparts, suggesting that survival may be better explained by costs of exercising mate choice rather than by predation. Our work highlights the importance of considering multiple sources of selection in explaining the paradoxical persistence of aposematic polymorphisms.