Iridescence as Camouflage

Iridescence is a striking and taxonomically widespread form of animal coloration [1], but that its intense and varying hues could function as concealment [2], rather than signalling, seems completely counterintuitive. Here we show that the colour changeability of biological iridescence, produced by multilayer cuticle reflectors in jewel beetle (Sternocera aequisignata) wing cases, provides effective protection against predation by birds. Importantly, we also show that the most likely mechanism to explain this increase in survival is camouflage, and not some other protective function such as aposematism. In two field experiments using wild birds and humans, we measured both the ‘survival’ and direct detectability of iridescent and non-iridescent beetle models, and demonstrated that the iridescent treatment fared best in both experiments. We also show that an increased level of specular reflection (gloss) of the leaf background leads to an increase in the survival of all targets and, for detectability by humans, enhances the camouflage effect of iridescence. The latter suggests that some prey, particularly iridescent ones, can increase their chance of survival against visually hunting predators even further by choosing glossier backgrounds. Our study is the first to present direct empirical evidence that biological iridescence can work as a form of camouflage, providing an adaptive explanation for its taxonomically widespread occurrence.