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

Jewel Beetles' Iridescent Shells Deter Hungry Birds--By Freaking Them Out thumbnail

Flashy iridescent shells may not be the best evolutionary strategy for insects trying to avoid birds. But in recent years, biologists have shown that iridescence–lustrous shifts in color, depending on the angle of view–can actually camouflage green jewel beetles among sun-dappled leaves. A new study in Animal behavior has shown that iridescence can also protect these insects even when they are not visible. It appears that birds have an innate fear of color changes .

This is the first time that iridescence has been proven to deter predators. It is not just glossiness or bright colors. Karin Kjernsmo (a researcher at the University of Bristol, England, and the study’s principal author) says that it is actually the “changeability”, the very hallmark of Iridescence, that is important to this protective function.

To test how birds reacted to iridescent beetles’ varying colors, Kjernsmo and her colleagues set out real Sternocera aequisignata jewel beetle shells, along with three types of artificial shells: one a glossy green, one a matte green and one color-shifting but matte. Kjernsmo and her colleagues baited the shells using mealworms and then gave this buffet to day-old domestic chickens (this was to make sure that any reactions were not learned predation techniques).

The chicks ate the mealworms in the matte green shells, but they hesitated when they saw the glossy shells or the two types of color-changing shells. A 2017 paper from another lab had shown that birds shy away from glossiness, yet the specific avoidance of the color-shifting nature of iridescence had never been documented before.

Johanna Mappes, a University of Helsinki biologist who worked on the 2017 study but was not involved with the new paper, praises the way Kjernsmo’s team controlled for each type of shell finish, “especially creating matte iridescence signals–it’s really genius.”

The new findings suggest that iridescence may be an evolutionary two-for-1 deal. It helps jewel beetles hide, but can also scare away predators that still spot them. Kjernsmo speculates that this might explain why so many insects are iridescent: “allows them to protect in many different contexts.” Kjernsmo speculates that this might help explain why so many insects are iridescent: it “allows them to be protected in many different contexts.”

This hypothesis could also explain why jewel beetles developed iridescence instead of a more common warning color like bright red or orange. These colors stand out more than other colors, and some poisonous insects like ladybugs or monarch butterflies use them to warn birds to back off. In case they are noticed, their poison acts as a backup. This species of jewel beetle does not have any chemical defenses so extra attention might not be worth it. It is better to blend in than stand out.

It is not yet clear what it is about iridescence which seems to frighten birds. These jewel beetles could be mimicking other iridescent insect with chemical defenses. Kjernsmo suggests that this idea could be confirmed by looking at the insect family tree to determine how often iridescence is associated with poison. Mappes suggests that iridescence could also confuse predators. If a beetle’s colors shift, a predator might not know whether it is safe or dangerous.

Kjernsmo is proud of the way this study shows nature’s complexity and nuance. It can be tempting to find one answer and stop looking, she says, but “it’s really important to realize that colors can actually serve multiple purposes at the same time–that they don’t necessarily evolve for one particular purpose.”

ABOUT THE AUTHOR(S)

    Kate Golembiewski is a science writer based in Chicago. Her work has appeared in the New York Times, the

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