She-male garter snakes: some like it hot.

Male garter snakes limping out of hibernation in northern Manitoba can mimic females and drive dozens of other guys to wriggle over them. The force behind this deluded orgy may not be sex, though.

Until now, scientists presumed that female mimicry gives its perpetrators an edge in mating, explains Rick Shine of the University of Sydney in Australia. But there's no evidence for any mating advantages for the "she-male" garter snakes, nor do scientists know how these awakening snakes attract other males.

The fakery needs a new explanation, argue Shine and his colleagues in the Nov. 15 NATURE. They propose that these snakes creeping out of 8 months of chilled inactivity find that the ball of suitors provides body heat and protection from crows and other birds.

"If you're weak and slow and cold, what you want is a whole bunch of warm snakes on top of you," says Shine.

The animals observed in the new study belong to a subspecies of the garter snake found across much of North America. In Manitoba, garter snakes converge on the few spots suitable for hibernation without freezing. In spring, males emerge and wait for the sporadic rousing of females. "You can have 25,000 to 30,000 snakes in a den the size of an average living room," says Robert T Mason of Oregon State University in Corvallis, a coauthor of the new study. When a female slides by, up to 100 males knot around her. She permits just one to mate.

Mason and a colleague first described a she-male mating ball in 1985, but scientists are still searching for its benefits.

To check heat transfer, the researchers monitored female snakes that birds had killed. Males courted the corpses, often heating them more than 3 [degrees] C. In a temperature test with live females that started at 4 [degrees] C, those courted in a mating ball warmed to 20 [degrees] C faster than did those separated from any suitors.

The researchers also explored recovery from hibernation. They caught newly emerged males that attracted male attention. The she-males that researchers warmed to 28 [degrees] C turned into regular guys within 3 hours, but those at 10 [degrees] C still inspired courtship after 5 hours. This season, Mason hopes to check just-emerging male snakes for pheromones.

When the snakes emerge, birds gather and kill hundreds, say the investigators. Garter snakes have no venom and can only flee to defend themselves. In sprint tests, however, cold snakes move slowly. A courtship tangle could protect the insiders, the researchers propose.

"Female mimicry is pretty common," says Stephen M. Shuster of Northern Arizona University in Flagstaff. Some animals show a clear mating benefit from the deception. In a pill bug relative that Shuster studies, males with antlers on their rears defend cavities where females gather. Occasionally, a male with no antlers and the domed body shape typical of females flirts with the defender, enters the cavity unchallenged, and sires up to 60 percent of the females' offspring.

Barry R. Sinervo of the University of California, Santa Cruz sees mating advantages to female mimicry among side-blotched lizards. For she-male snakes, though, he calls the heat-and-safety payoff "a plausible idea" and predicts that researchers will consider it for other species. He says, "It really takes just one example, then people start looking more closely."