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 co-exist
using (1) a 10-year capture-mark-recapture experiment to study natural
selection, and (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.