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 signaling seems completely counterintuitive. Here, we show that the color 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. [Display omitted] •Iridescence in prey can serve a counterintuitive function: concealment•The effects of this protective function are further enhanced by glossy backgrounds•Iridescence, even for signaling purposes, may be less costly than previously thought•This newly discovered function may explain the widespread occurrence of iridescence Kjernsmo et al. show that biological iridescence provides a survival benefit to prey by presenting iridescent and non-iridescent beetle models to birds and humans in a natural setting. They also show that the underlying mechanism for this is camouflage and that increased levels of specular reflection of the background further enhance this effect.