Irridescence – a Matter of Life and Death

Its sometimes difficult to notice, until you see the flow of brief, glowing patterns under the light – irridescence is more prevalent throughout the animal kingdom than you may realize, one of those traits that has long held a special place in evolution. Creatures as varied as the dragonfly, to the pond dwelling catfish and even boa constrictors have skin with shimmering colors, flickers of blue, neon and pink, if you look hard enough. Its something that we probably take for granted, as the world’s apex predator, but to the animals who wear it, on their scales or in their feathers, it may play a key part to their survival – providing the prime opportunity for momentarily confusing and eluding any potential predators that might make them into a meal.

Iridescence is far from the only type of conspicuous coloration meant to deliberately confuse predators, observed the author of the new study, Thomas Pike, who is a behavioral and sensory ecologist from the University of Lincoln in the United Kingdom.

Iridescence is the product of a process known as structural coloration, in which microstructures interfere with the wavelengths of light reflecting against them. It’s the same effect that gasoline has if it is accidentally spilled into a puddle of water, turning the reflection of light into a ripple of pink and blue. Several years ago, scientists learned that the rainbow squid is equipped with several sets of cells for changing color, the most unique being iridophores, which are charged with electrical stimulation from the squid’s nervous system. Perhaps the most intriguing thing about this defense in squid is that while these creatures are color blind, their bodies produce a signal of different colors. They’re not even the only mollusk that does it either – the paua shell has one of the most distinct patterns of iridescence on the inside. Most predators could hardly tell if the squid are coming or going, allowing for a swift escape.

Besides the obvious advantages of mimicry learned by many insects and amphibians, a pattern of contrasting stripes may be advantageous in helping animals escape potential predators. This is likely in part because stripes on animals such as the zebra, which has little other defense, can make it difficult for any lurking predators to correctly judge the animal’s speed and movement, said Pike.

“It struck me that the conspicuous colors displayed by iridescent animals may also allow them to evade predators using a similar mechanism,” he said in a statement to LiveScience.

In order to test his hypothesis, Pike first trained seven female Japanese quails to hunt down and peck at a virtual black bug, which was really a 0.8-inch-diameter circle moving along a touch screen. So they would keep their focus on the target, Pike gave them a reward of dried mealworms each time the birds pecked within 0.4 inches of the target.

After the quails had become proficient hunters, he decided to mix up the game a little, still offering rewards, but along with the regular black circles, he threw in some iridescent circles. Within each of Pike’s trials, every quail met 20 circles. Half of the circles that appeared were black and the other half only gave off iridescent colors. The order in which they were presented was completely random. On average, the birds pecked less than three times when looking to snare one of the black ‘bugs.’ However, a subsequent average of the data showed that trapping the iridescent bugs required an average of one extra peck, Pike learned, something that could give a real insect just the right amount of time necessary to escape.

Something else Pike found was important as well. When it came to hunting iridescent bugs, the quails also happened to have less-accurate aim, which could indicate that they were confused. In each trial, they pecked approximately 0.6 inches away from the center of the iridescent circles — which is an average of about 0.08 inches further away than where they pecked at the black circles.

These results indicate that any alterations of color or brightness, something known to biologists as “interference coloration,” could give the would be predators a hard time when it comes to tracking down prey, Pike said. Beetles and butterflies, which are also the best known families of mimic artists have iridescence in their shells and wings, but so do birds such as the peacock, which also works iridescence into its mating display, and also fish, reptiles, cephalopods, particularly squid which are also excellent shape shifters, and there is at least one species of mammal that also does it, the golden mole, according to Pike.

All the trait needs is enough survivors to pass it along to the next generation. Some argue that many sexual selection traits may actually be detrimental to the animal’s survival. While some traits are attractive to would be mates, such as the brightly colored scales on a collared lizard, they can also make things easier for predators – sticking out in the Southwestern deserts where they are found as prime choices for a hawk flying overhead.

“My findings suggest that significant anti-predator benefits may indeed be gained by having interference coloration, and may explain why interference colors are so widespread,” Pike said. “More generally, this raises the intriguing possibility that changing appearance may be important in predator avoidance, and that interference colors are a special case of this phenomenon.”

The new study has been published this month in the journal Biology Letters.