How Beetle Iridescence Can Hide Them and Scare Predators

Some beetles have a beautiful shiny rainbow of brilliance radiating from their shells. While the effect can be brilliant, it can also keep predators away, even after the insects have been detected, a new study finds.

Iridescence is a quality where colors seem to shift and change, depending on the light and angles. It’s a shimmering effect that is present in some animals including birds, reptiles, and insects.

Researchers were curious how this distinctive metallic coloration worked so well at warding off predators.

“Interestingly, we recently discovered that even something as striking as glossy jewel beetle iridescence could work as a highly efficient form of camouflage, but obviously, this is likely to be very illuminant and background dependent,” lead author Karin Kjernsmo of the University of Bristol’s School of Biological Sciences, tells Treehugger.

“A glossy green iridescent beetle might be perfectly well-camouflaged against a glossy green leaf in a dappled forest environment, but if it moves onto something brown, like tree bark or dead leaf, it might suddenly stand out like a sore thumb. This is why we wanted to investigate whether iridescence and gloss could also provide a protective effect even when the disguise had been revealed and camouflage potentially broken against a mismatching background.” 

Iridescence Helps Survival

For their study, researchers analyzed the function of the reflective coloring but had to separate the impact of the flashy color changing from just having a lot of colors at the same time. They also looked at the different effects of iridescence versus gloss.

In early research, they looked at how iridescence provides camouflage for some species. They created prey from both real iridescent beetle shells (elytra) and from non-iridescent beetle models, which acted as controls.

They hid tempting mealworms under the targets and placed hundreds of them on leaves in a nature preserve. They then watched to see which of these beetle models endured versus hungry wild birds.

“We found that the iridescent beetles survived the best, which was very exciting,” Kjernsmo says. “To disentangle whether it was most likely that the iridescent beetles survived due to camouflage, or due to warning coloration, we then re-did the study, but with humans as surrogate predators instead.”

To the researchers’ surprise, they found that humans were “surprisingly bad” at spotting the iridescent target beetles. Humans only were able to spot about 17% of the iridescent ones, whereas they found about 75% of non-iridescent beetles.

Kjernsmo says that was a striking result that definitely speaks in favor of the camouflage hypothesis.

Does Background Matter?

This time around, they investigated whether iridescence and glass could help protect against bird predators, even when the beetles were up close and against a background that didn’t match them.

For this study, they presented both iridescent and non-iridescent, as well as glossy and matte versions of the beetles, to birds in a controlled laboratory environment. They used newly hatched chicks as predators so they could be sure the birds had neither good nor bad experiences with the prey of certain gloss or iridescence attributes. Then they observed the birds’ responses toward the beetles.

They found that birds were much less likely to be willing to prey on the iridescent beetles and that gloss also has an independent impact.

“Interestingly, we found that even when presented up close and against a brown background, iridescence and gloss significantly reduced the attack-willingness of the birds, providing yet another adaptive explanation for the widespread existence of iridescence and gloss in many animals,” Kjernsmo says.

“I believe that this is an important finding because it also shows that iridescent animals don't necessarily need to have secondary defenses such as sharp spines or toxins in order to have a protective function. Indeed, all prey items were equally edible in our experiment, which is why iridescence and gloss could be considered as a ‘deceptive’ form of warning coloration in this study.”

The findings also suggest that when iridescence is used to attract potential mates, it may not also be as dangerous as previously thought, in terms of also attracting predators.

Their findings were published in the journal Animal Behaviour.

Coat of Many Colors

Based on these findings, researchers believe that it’s highly likely that iridescence could serve two protective functions. It could work both as camouflage and as a warning coloration for potential predators.

“This is an incredibly intriguing thought because it means that iridescence would allow animals to move around in search for food and mates at a much lower risk of being eaten by a predator compared to other protective coloration strategies,” Kjernsmo says.

“Iridescence truly is nature's enigmatic coat of many colors, I find it endlessly fascinating!”