A multifunctional warning signal behaves as an agonistic status signal in a poison frog

Aposematic species use conspicuous "warning" signals to communicate unprofitability to potential predators. Although warning signals are classic examples of communication systems that evolved by natural selection, they can also function in the context of sexual communication and are therefore particularly useful for investigating conspicuous trait evolution under multifarious selection. To test whether aposematic signals also serve to mediate intrasexual disputes, we observed males from a highly territorial poison frog species (Oophaga pumilio) in their native territories and in experimental dyadic contests to assess the influences of body characteristics such as warning signal brightness and body size on the outcomes of territorial interactions. We report here that although neither male size (snout–vent length) nor mass significantly predicted male aggressiveness (latency to call) in dyadic contests, a male’s dorsal brightness was a significant predictor of willingness to initiate aggressive interactions, with brighter males exhibiting a shorter latency to call than duller males. Furthermore, brightness asymmetries between males predicted the outcomes of contests such that asymmetries were smaller in escalated aggression trials (where both males called), and brighter males were more likely to be the sole aggressor in trials with large asymmetries. These tests, combined with previous work, provide evidence that warning coloration has been co-opted as an agonistic indicator trait in this aposematic amphibian and reveal the potential evolutionary lability of conspicuous traits that arise through natural selection.