We now know why natural selection may favor iridescence in some insects

(ORDO NEWS) — Myriads of animals possess brilliant iridescent hues of iridescent coloration that make them stunningly beautiful, but why natural selection so widely favors such structural colors remains a mystery.

The change of metallic sheen has evolved independently many times in the animal kingdom – from dazzling hummingbirds to iridescent skinks and gem-like flies. In some animals where the iridescence is more prominent in one sex than in the other as in peacocks sexual selection has clearly played its part.

But in many insects both sexes are equally iridescent, and some animals are iridescent only in their larval form, like shrew beetles (Trirhabda bacharidis). A new study has provided experimental evidence that color changes in these animals may also serve a protective function.

“One of the challenges in studying the function of such highly reflective structural coloration has been to separate the effect of color variation, a hallmark of the iris, from the effect of simply having multiple colors at the same time,” explains evolutionary biologist Karin Kjernsmo from the University of Bristol. “And also to separate the glitter effect from the iridescence effect.”

To achieve this, Kjærnsmo and her colleagues set up an experiment in which they tested young chickens with a variety of “prey” in the form of real and artificial wings of the ground beetle (Sternocera aequisignata) with a delicious mealworm snack inside.

Chicks that had never encountered such prey before were presented with wings that were either matte with a static gradient, or glossy with a static gradient, or matte with shifting shades, or glossy with shifting shades.

The birds did not hesitate to attack the “prey”, which displayed several colors at once. But they thought twice before attacking the “prey”, whose color changed in a rainbow way. Gloss also caused fluctuations, but not as strong as color changes. Because they had no previous experience with rainbow colors, this behavior was instinctive.

“Here, for the first time, we effectively tested each of these two effects separately and showed that both the iris and glitter can protect prey even after detection, providing another adaptive explanation for the evolution and wide distribution of the iris,” says Kjærnsmo.

In 2020, the same team presented evidence that the iris can be an effective form of camouflage, although this may seem counterintuitive given how strikingly attractive some of the brilliant structural colors can be.

“This idea is actually very old, but it’s never been shown before. I think it’s just been neglected or forgotten,” Kjernsmo told Discover magazine in 2020.

Of course, these results need to be demonstrated in vivo to confirm them and to make sure that there is nothing in the laboratory that could affect the results.

“Our results are important because they demonstrate that even when prey is presented up close and against an inappropriate background, iridescent coloration can confer a survival advantage by inducing hesitation or even, as is sometimes observed, a reaction of aversion in attacking birds,” write Kjærnsmo and his team. in your article.

They explain that this disgust may be due to a form of aposematism, when animals use colors to advertise their poisonousness.

A 2017 study on the rainbow-colored alpine leaf beetle (Oreina cacaliae), which has known chemical defenses, found that the sheen enhances its warning signal, supporting the idea of ​​a poisonous warning.

“Future studies could establish the frequency with which the combination of iridescent coloration and secondary protection occurs,” the researchers of the 2022 study suggest.

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