Evolutionary adaptation breeds gender-identification confusion

Rise of camouflage results in surprise for some male lizards

PORTLAND, Ore. — The recent evolution of camouflage among lizards in the powdery dunes of New Mexico’s White Sands National Monument can lead to some misunderstandings when some males choose to make love, not war.

MIX-UPS In the white dunes of New Mexico, eastern fence lizards have evolved a pale form (top) that predators find harder to see against the sand. But their recently evolved coloration has also caused them some confusion about the sex identity of darker males from a separate population, as shown in a recent study which put a male of each hue face to face. E.B. Rosenblum

Since the dunes developed a few thousand years ago, a light-colored form of the normally dark-shaded eastern fence lizard (Sceloporus undulates) has arisen that blends in with the landscape.

The change in hue can produce confusion over sex-recognition signals, Jeanne Robertson of the University of Idaho in Moscow reported June 27 at the Evolution 2010 meeting. At least it did when she and an associate arranged encounters between pale White Sands lizards and their darker cousins from the area surrounding the dunes.

Whenever she and University of Idaho colleague Erica Bree Rosenblum put a pale male lizard and a male from dark soil together, the dark lizard gets ready to fight, positioning himself sideways and showing off his bulk and his blue belly. He does some menacing push-ups and prepares for the head-butting and tail-whipping common in aggressive lizard encounters.

Meanwhile, the pale lizard starts wooing, making rapid shuddering movements and starting little romantic chases. So what might have been a territorial fight becomes “a very confused dark-soil male trying to engage in combat despite being courted,” Robertson said.

The confusion appears to arise from the blue belly splotches that both light and dark forms use as handy clues in social situations, Robertson said. The pale males now sport bigger patches than their darker counterparts, whose more modest blue blotches are more like those of sand dune-living females. Thus male light-colored lizards probably perceive their darker brethren as female — or at least as female enough to court.

This confusion looks as if it develops as an indirect effect of dodging predators through camouflage. That roundabout origin is “the most interesting part,” says evolutionary biologist Liam Revell of the National Evolutionary Synthesis Center in Durham, N.C.

Aside from causing amusing problems with courtship, there’s a deeper message here, says Jonathan Losos of Harvard University. Sex-identification miscues can make it less likely for members of different populations to mate if they happen to meet. “As a result, adaptation to different environments may have the incidental effect of leading to reproductive isolation, and hence the origin of distinct species,” he says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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