Iridescence as Camouflage

Abstract

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. VIDEO ABSTRACT.

Keywords: Sternocera aequisignata; anti-predator adaptation; camouflage, gloss, specular reflection; iridescence; predation, protective coloration.

Figure 1

Prey Target Designs for All Six Treatments From left to right: iridescent on privet, static rainbow on bramble, green on beech, black on holly, purple on English ivy, and blue on bramble. Also illustrated in these images is the varying level of specular highlights (gloss) between the different backgrounds.

Figure 2

Avian Predation and Human Detection Data (A) Odds ratios (±95% confidence intervals [CIs]) from Cox mixed-model survival analysis comparing all treatments to the iridescent in the avian predation experiment. (B) Mean (±95% CI) probability of detecting targets for each treatment in the human detection experiment. In both experiments, the iridescent targets survived significantly better than all except the black treatment. Bla, black; Blu, blue; Gre, green; Irid, iridescent; Pur, purple; Stat, static rainbow. See also Figure S1A.

Figure 3

Results from the Human Detection Experiment Mean probability of detecting targets (A) and mean detection distance (B) as a function of gloss for each treatment. Lines are best fits from GLMMs. The iridescent targets became significantly more difficult to detect as substrate gloss increased, more so than other treatments. See also Figures S1C and S1D.