Sparkly exoskeletons may help camouflage beetles from predators

Long thought to help insects stand out, iridescence may instead let some blend in

Asian jewel beetles

Asian jewel beetles (Sternocera aequisignata) stand out with their iridescent exoskeleton. New experimental evidence suggests this iridescence can actually help camouflage these beetles from both birds and humans in the wild.

K. Kjernsmo et al/Current Biology 2020, University of Bristol

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.  

beetle wing cases
Wing cases of the Asian jewel beetle tilted different ways show the diversity of colors iridescence can produce. Most animal colors are produced by pigments, but iridescence is structural. Microscopic layers interfere with how a surface reflects light, and can generate different colors depending on the angle of view.K. Kjernsmo

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.

Jonathan Lambert is a former staff writer for biological sciences, covering everything from the origin of species to microbial ecology. He has a master’s degree in evolutionary biology from Cornell University.

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