Think about the last time you stood squinting in front of a full-length mirror, trying to decide whether the colors in your outfit went together. Now imagine you're a reptile, and you wouldn't even understand a mirror if you saw one, but somehow you need to find a rock that matches your skin color. Otherwise you might get eaten by a bird today. Oh, and the skin color you need to match is on your back. Certain lizards in Greece manage to pull this off every day, though how they do it is a mystery. University of Cambridge graduate student Kate Marshall and her colleagues went to four Greek islands—Folegandros, Santorini, Syros and Skopelos—to study a little reptile called the Aegean wall lizard. Podarcis erhardii is common in the Aegean islands, where it likes to bask on sunny rocks during the day. Unfortunately, this exposes the lizards to predatory birds flying above. Aegean wall lizards are multicolored, wearing a blend of browns, grays and greens. Males may have flashier colors on their undersides, such as bright orange or blue, to show off to females. The more neutral colors on their backs help camouflage them to the eyes of hunting birds. But these lizards can't change color rapidly like a chameleon does. So for maximum camouflage, they should stick to rocks that best match their own body colors. To see whether the lizards really do this, the researchers went around the islands and photographed 263 lizards. They also photographed the surfaces the lizards were sitting on. "We wanted to know whether the lizards were choosing rocks to sit on that made them more camouflaged to the eye of a hunting bird, such as raptors and crows," Marshall says. But these birds don't see the same way a human does. For one thing, they have four types of color-detecting cone cells in their eyes, unlike the three we have. Marshall and her colleagues used data from other researchers on how sensitive these birds' cone cells are to different colors. For each photo, they figured out how closely the color and brightness of the lizard's back matched the color and brightness of the rock it sat on—to a bird's eye. The researchers then measure how well each lizard would match the basking rocks of other lizards on the same island. In both color and brightness, they found that lizards matched their own rocks significantly better than others. In other words, the lizards had chosen perches that matched their own colors. Females seemed to be a little better camouflaged than males, with their showoff color patches. On average, the authors write, female lizards were "probably indistinguishable against their chosen backgrounds" in a bird's eyes. The camouflage effect was weakest on the island of Folegandros, which has fewer predators than the others. It may not be as important here to stay hidden from birds. But how does a lizard know that it should seek out a grayer or browner rock to rest on? How can it judge which perching spots are a close match to the skin of its back? "This is a puzzle that we're still not really sure about," Marshall says. There may be a genetic component: lizards programmed to pick the best rocks are more likely to survive. Or the lizards might learn through experience which rocks keep them safest, or which rocks other lizards with similar colors tend to get plucked from. Alternately, there may be a totally different reason each lizard prefers a certain kind of rock—maybe it provides the best warmth for that lizard's body, say—and that trait has evolved together with the lizard's color. For example, some lizards may have evolved to be both grayish and to like the feel of a grayish rock. "It could be none of the above, or it could be a combination [of factors]," Marshall says. "It's an intriguing question." She hopes she'll find an answer to this puzzle soon. If someone can figure out how to train the lizards to do wardrobe consultations, that would be helpful too.
Images: Kate Marshall.
Marshall, K., Philpot, K., & Stevens, M. (2016). Microhabitat choice in island lizards enhances camouflage against avian predators Scientific Reports, 6 DOI: 10.1038/srep19815