More on Garter Snakes
Female Impersonators and Other Courtship Matters
Compiled by Melissa Kaplan
She-Male Snakes Need
Their Love To Keep Them Warm
NARCISSE, CanadaOne of nature's strangest spectacles happens every spring in southern Manitoba. Tens of thousands of red-sided garter snakes emerge from their underground hibernation dens and engage in mating balls.
The male red-sided garters emerge first and wait patiently for the females to follow. Each time a female appears, the males surround her. The males look like a mass of living spaghetti, said Robert Mason, a zoologist at Oregon State University. The ball of snakes will writhe and sometimes even roll over land, until one male finally mates with the female.
The weirdest part of the spectacle occurs when some male red-sided garters impersonate females and find themselves at the center of a mating ball. These she-male snakes fool the males by exuding a female pheromonea chemical released by one animal that affects another animal. The female pheromone fools male red-siders into thinking the she-male is a female.
Why the she-males act out this charade has long been a mystery. Now Mason and Rick Shine, a biologist from the University of Sydney in Australia, think they've found the answer.
All snakes are ectothermic, or cold-blooded. When a garter snake emerges from the cold ground, it is cold and lacking in energy until it's warmed by the sun. In such a state, the snake is vulnerable to attacks by predators. Previous studies have shown that she-male garters are slower and weaker than male garters and, hence, even more vulnerable.
The she-male's charade has two purposes, say the scientists. One: It warms the she-male garter like a blanket; and two: It surrounds him-her with a phalanx of bodyguards.
The annual red-garter mating balls are a big tourist attraction in Manitobaand a source of many tales. One unsuspecting couple built a house on top of an empty snake pit one summer, only to find their property swarmed by thousands of red-sided garters returning to their traditional hibernation den in the fall. The couple quickly relocated their new house.
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°C. In a temperature test with live females that started at 4°C, those courted in a mating ball warmed to 20°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°C turned into regular guys within 3 hours, but those at 10°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."
Bumpus in the snake den: effects of sex, size, and body condition on mortality of red-sided garter snakes. Shine R, LeMaster MP, Moore IT, Olsson MM, Mason RT. Evolution Int J Org Evolution 2001 Mar;55(3):598-604
Conflicts between courtship and thermoregulation: the thermal ecology of amorous male garter snakes (Thamnophis sirtalis parietalis, colubridae). Shine R, Harlow PS, Elphick MJ, Olsson MM, Mason RT.Physiol Biochem Zool 2000 Jul-Aug;73(4):508-16
The transvestite serpent: why do male garter snakes court (some) other males? Shine R, Harlow P, LeMaster MP, Moore IT, Mason RT. Anim Behav 2000 Feb;59(2):349-359
Plasma triglyceride and beta-hydroxybutyric acid levels in red-sided garter snakes (Thamnophis sirtalis parietalis) at emergence from hibernation. Whittier JM, Mason RT.Experientia 1996 Feb 15;52(2):145-8
Female mimicry in garter snakes. Mason RT, Crews D. Nature 1985 Jul 4-10;316(6023):59-60
Social dynamics of group courtship behavior in male red-sided garter snakes (Thamnophis sirtalis parietalis). Joy JE, Crews D. J Comp Psychol 1985 Jun;99(2):145-9
Ovarian development in red-sided garter snakes, Thamnophis sirtalis parietalis: relationship to mating. Whittier JM, Crews D. Gen Comp Endocrinol 1986 Jan;61(1):5-12
Hibernation in garter snakes (Thamnophis sirtalis parietalis): seasonal cycles of cold tolerance. Joy JE, Crews D. Comp Biochem Physiol A 1987;87(4):1097-101
More on garters from PubMed, keyword: thamnophis
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