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Some Snakes Find Safety In "Cross-Dressing"

Hillary Mayell
for National Geographic News
November 15, 2001
 
Scientists who study "cross-dressing" garter snakes have come up with a new answer to the question: "Why do they do it?"

Nearly 20 years ago, researchers discovered that some male garter snakes mimic female behavior when they emerge from hibernation. This behavior is not unique in the wild; in several species the males are known to pose as females.











The common assumption has been that the "she-males" are seeking a reproductive advantage or attempting to avoid aggression from larger males.

Not so, at least as far as garter snakes are concerned, reports a team of U.S. and Australian scientists in the November 15 issue of the journal Nature. The researchers have concluded that the male snakes pose as females simply to get warm and to reduce their exposure to predators.

The idea that the she-male trait evolved as a response to environmental factors (natural selection) instead of as part of a reproductive strategy (sexual selection) is a new one.

Rebounding from Hibernation

Manitoba, Canada, is famous for having the largest concentration of snakes in the world. The garter snakes (Thamnophis sirtalis parietalis) leave their underground dens by the thousands in early spring, after eight months of hibernation.

The males emerge first—cold, weak, slow, and highly vulnerable to attacks from predators such as crows, magpies, and other birds.

"The temperature underground is about 4 degrees Celsius (39 degrees Fahrenheit)," said Bob Mason, a reproductive biologist at Oregon State University and a co-author of the study. "They're wet and cold, have to vent out a lot of built-up waste, and are rotten sprinters.

"They're pretty lethargic, and when they first come out, the average male is really a rotten courter," he added.

Even so, the male garter snake's mind is on just one thing. "They move around a lot, waiting for females, constantly looking, looking, looking," said Mason.

When a female finally emerges from the den, she is immediately engulfed in a giant mating ball, swamped by the attention of as many as 100 males competing for the opportunity to mate. Once she is fully claimed, the unsuccessful suitors slither off, looking for other females.

In this mix are she-males. When they emerge from the den, they too are swamped with male interest.

Until now the female-mimicking behavior was generally seen as a ruse by which a she-male would increase his own mating opportunities by duping other males—essentially tricking them into expending a great deal of energy battling for non-existent opportunities to mate.

Once the other males gave up the fight and slithered off, exhausted, the she-male would revert to being fully male, and in prime condition for courting.

But the explanation troubled some scientists. "There were always nagging issues," said Mason.

Rethinking the Theory

Mason and Rick Shine, a scientist at the University of Sydney, Australia, and lead author of the new study, had worked together earlier on studies of she-males and decided to jointly seek an answer to the she-male garter snake's behavior.

"This is a perfect example of collaborative science," said Mason. "Rick is an evolutionary biologist, while I'm a reproductive biologist. We bring different skills and approach the problem from slightly different angles."

The research showed that the she-males did not have greater success in reproduction than other males. What, then, was the purpose of their behavior?

To improve the she-males' chances of survival, the researchers say.

"It occurred to me that the one true, obvious, and indisputable effect of female mimicry was to attract a crowd of amorous suitors," said Shine.

High numbers of male garter snakes die soon after emerging from hibernation because they are attacked by crows. She-males at the center of a mating ball, however, are less exposed to predators.

Moreover, as the courting males basically wrestle one another in an effort to claim a female snake in their midst, they transfer heat. The she-males use this heat to raise their body temperature, thereby accelerating their recovery from hibernation and improving their ability to avoid capture.

"This speaks directly to the paradox between sexual selection versus natural selection," said Mason. "We almost always think of these traits as providing some kind of reproductive advantage, but sometimes the simplest explanation is the best."

The she-male garter snakes, he said, "are just cold, and they gain a survival advantage" from their female-mimicking behavior.

For other species in which males mimic females, it's not known whether this behavior evolved as the result of sexual selection or natural selection.

"I do think [this study] is a cautionary tale," said Shine, "in that sometimes simple explanations are ignored in our search for more complex and elegant hypotheses."
 

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