Jewel Beetles’ Iridescent Shells Deter Hungry Birds—By Freaking Them Out

It may not be the best strategy for insects to avoid birds. Biologists have shown that iridescence can camouflage green jewel beetles among sun-dappled leaves. Birds appear to have an innate wariness of the color changes themselves and iridescence may be another way to protect them.

It is the first time that iridescence has been shown to deter predatory animals. The hallmark of iridescence is changeability and it is important for this protective function.

To find out how birds react to different colors of beetles, Kjernsmo and her colleagues set out a variety of shells, including one that was glossy green, another that was matt green, and a third that was color-changing. They baited the shells with mealworms and then gave the day-old domestic chick a buffet to eat.

The chick scarfed down the mealworms under the green shells, but hesitated at the glossy shells. The avoidance of the color shifting nature of iridescence was documented for the first time in a paper from a lab.

Johanna Mappes was not involved in the new paper but she praised the way the team controlled for each shell finish.

According to the new findings, iridescence is an evolutionary two-for-one deal, as it helps the jewel beetles hide but can also scare off predatory animals. It's possible that this might explain why so many insects are iridescent.

This hypothesis could explain why jewel beetles evolved to use iridescence instead of a more typical warning color. These vivid colors are used by some poisonous insects to warn birds to back off. If they get noticed, these insects have a backup. Extra attention from a traditional warning color may not be worth it for this species of jewel beetle.

It's not clear what it is about that scares birds. There are insects that have chemical defenses. An analysis of the insect family tree to see how often iridescence coincides with poison could be used to confirm this idea. Mappes theorizes that iridescence might confuse a predator, since he might not be able to classify a beetle as safe or dangerous.

Though more work is needed to fully understand iridescence as a warning, she is proud of how this study demonstrates nature's complexity. She says it is important to realize that colors can serve multiple purposes at the same time and that they don't necessarily evolve for one particular purpose.