Iridescence sparkles across many branches of the tree of life, from dazzling ruby-throated hummingbirds to bright, metallic beetles. While ostentatious coloration can woo mates, scientists had assumed it also attracted predators. But new evidence suggests an unexpected benefit of iridescence — camouflage.
Asian jewel beetles (Sternocera aequisignata) boast brilliantly iridescent exoskeletons, and the fact that both males and females share this trait suggests its importance outside of mating. To see if iridescence affected whether beetles were detected by hungry birds, behavior ecologist Karin Kjernsmo at the University of Bristol in England and colleagues pinned mealworm-stuffed iridescent beetle wing cases to forest leaves along with non-iridescent ones artificially colored blue, green, purple, rainbow or black. All 886 targets — iridescent and matte — represented the spectrum of colors in the iridescent shell, allowing researchers to disentangle the effects of individual colors from the ever-changing sparkle of iridescence.
After two days, the iridescent “beetles” were less likely to have been attacked by birds than all the other colors, except black, researchers report January 23 in Current Biology. Birds “killed” 85 percent of purple and blue targets, but less than 60 percent of iridescent targets, Kjernsmo says. “It may not sound like much, but just imagine what a difference this would make over evolutionary time.”
It’s unclear if birds had trouble seeing iridescence or were avoiding it, for example if they associated it with poisonous prey. But Kjernsmo suggests the rapidly changing colors could disrupt normal image-forming processes.
Humans proved worse than birds in detecting iridescent beetles. In a second experiment, 36 people walked a forest path while trying to spot both iridescent and dull beetle cases affixed to leaves in plain sight. Humans on average identified nearly 80 percent of matte blue and purple cases, but only 17 percent of iridescent cases — suggesting to the researchers that iridescence can function as camouflage.