Tuesday, March 4, 2014

Eastern Garter Snakes: Mating Balls & Sex in the Trees

Last week, Duke Forest staff photographed Eastern Garter Snakes (Thamnophis sirtalis sirtalis) intertwined on the forest floor. They had a discovered a "mating ball" of small males vying for the chance to fertilize a mature female.

Eastern Garter Snakes mating in the Duke Forest, February 20, 2014; Photo courtesy of Sara Childs.

Other garter snake species are better known for their mating balls, including the Red-Sided Garter Snake (Thamnophis sirtalis parietalis). In Manitoba, as many as 25 male Red-Sided Garter Snakes will compete for the chance to fertlize one female, with hundreds of snakes congregating in spring outside their hibernacula or winter dens. The females release a pheromone that drives male Garter Snakes wild (video link here).

Both Eastern Garter Snakes and Red-Sided Garter Snakes have been documented to mate in the trees, a full yard off the ground. Researchers believe that Garter Snakes are driven to arboreal mating by their thermoregulatory needs -- it's easier to make sweet snake love while warm. Typically, Garter Snakes are only seen mating in trees after a rain. The rain cools the ground, but the trees and shrubs remain warmer at air temperature. On sunny days, the ground tends to be warmer than trees, and Garter Snakes are more likely to be seen mating there.

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