Survival Of The Shiniest: Iridescence And Defense In Nature
(Inside Science) -- The rainbow-hued shimmer of fish scales, bird feathers, and insect bodies that change color and brightness depending on viewing angle can be mesmerizing, but biologists have long debated the purpose of the striking displays and why they are so widespread in nature.
Different organisms have evolved various mechanisms to produce iridescence, but most of them rely on light-manipulating structures. Peacocks, for example, have feathers that contain very small protein structures that break up incoming light waves and then recombine and reflect them as vibrant colors.
It's been suggested that iridescence, also called "interference coloration," plays a role in sex or species recognition. Another hypothesis is that the structures that create iridescence help repel water or reduce friction or help in regulating body temperature, and that the pretty visuals are only a side effect.
Now, new research suggests there is another possibility: that for some organisms, iridescence evolved as an anti-predator defense to dazzle and confuse predators with sudden shifts in color and brightness in a bid to gain a few precious moments for escape.
In a new study, published in this week's issue of the journal Biology Letters, Thomas Pike, a behavioral and sensory ecologist at the University of Lincoln in the U.K., designed an experiment to test this hypothesis.
Pike trained seven Japanese quail to catch virtual prey -- some of which changed colors to mimic iridescence -- on a touch-sensitive screen. Pike said he came up with the idea for his experiment after reading a study in which scientists found that moving prey with distinctive patterns such as stripes are particularly successful in avoiding capture.
"Iridescent prey are also often mobile, have distinctive coloration and are surprisingly abundant, given their distinctiveness," Pike said. "No one had adequately explained why iridescent coloration has evolved, and so I reasoned that avoiding predation may be one potential explanation."
Pike decided to use Japanese quail in his experiments because they naturally feed on insects and are relatively easy to train for the task he had in mind.
Pike initially trained the birds to peck at small, solid-colored moving dots on a screen. If they successfully "caught" the dots, they were rewarded with access to a feeder containing dried mealworms -- a favorite food.
Pike also tested the birds with dots that shifted colors, changing from metallic green to blue, depending on the angles they were moving. Pike said he modeled the color-changing dots on greenbottle flies, which have iridescent bodies that undergo similar color shifts.
The results showed that the birds required on average about four pecks to catch an "iridescent" dot, compared to about 2.5 pecks to catch the solid-colored dot. Furthermore, when the birds missed, they did so by a larger margin than when trying to catch the color-changing dots.
Martin Stevens, a sensory and evolutionary ecologist at the University of Exeter in the U.K., said the idea that iridescence makes striking a target more difficult is "very exciting, and opens up a whole new avenue of research questions."
"As far as I’m aware the idea is quite novel and has not been so clearly suggested before," said Stevens, who was not involved in the study.
Stevens noted that some Victorian naturalists in the 1800s suggested that the bright iridescent markings on some butterflies might dazzle, confuse, or even camouflage them in forest habitats, potentially while in flight.
"So this is a really nice modern test of a similar idea," he added.
Stevens also praised Pike's experimental design, calling it innovative and clever.
"Not many studies have managed to get birds to play computer 'games' like this before as they're normally done with humans acting as a proxy for predators," Stevens said.
Tim Caro, a biologist at the University of California, Davis, who has studied anti-predator defenses in birds and mammals, also praised the novelty of Pike's hypothesis, but he said was skeptical that evading predators was a primary driver in the evolution of iridescence.
"The evidence on dazzle coloration making it more difficult to target prey items pertains mostly to human subjects using computer screens, and it is mixed in some studies," said Caro said, who also did not participate in the study.
He added that "it would be good to have some natural history observations to support [Pike's hypothesis]."
Pike thinks it would be difficult to test the underlying reasons why iridescence allows prey to avoid predation in a natural setting. However, he notes that his experiment does make a prediction that he thinks can be tested in the wild.
"All other things being equal, iridescent prey should have better survival in the wild than non-iridescent prey," Pike said.
Ker Than is a freelance writer living in the Bay Area. He tweets at @kerthan.