Garter Snakes

The Common Garter Snake and The Game of Love (Thamnophis sirtalis)

By Lynne Bittner

A couple of weeks ago while planting my zinnias, I was startled by a garter snake. I involuntarily jerked backwards while letting out a high pitched screech. It slithered calmly away, and I went back to planting my zinnias laughing at myself, grateful that my husband Richie who was mowing the lawn at the time didn’t hear me. It got me thinking about the resident garter snake that lives in a rock wall that I built for one of my flower gardens, and wanting to learn a little bit more about him, I researched the subject and discovered a curious thing. It appears that there is quite a controversy as to why certain male garter snakes use their ability to emit female pheromones.

As males are the first to emerge from their winter’s hibernaculum, (their wintering den - perhaps an old woodchuck burrow or underground cave or cranny which house sometimes hundreds or even thousands of garter snakes.) they wait in large numbers for the females who emerge slowly and singly from their den. Quickly, they surround her creating an incredible spectacle - a mating ball, most often comprised of a large single female at its center, and covered by literally dozens or even hundreds of males. Major competition for her suitors to be sure!

Little is known how the female decides upon choosing an appropriate mate, but one school of thought is that the males that mimic females by emitting female pheromones confuse their rivals, luring them away thereby giving the mimic a better chance. But this theory raises questions in my mind. Wouldn’t the female be repelled by the female pheromones given off by the mimics? Also, where could the mimic lead his amorous rivals to when they are all there together in that writhing tangle anyway? I don’t think that garter snakes are that easily fooled.

In the garter snake world, it is all about pheromones - not only are they used for sexual identification, but they are also used to communicate with each other. Their acute sense of smell enables them to track each other by the scent trails others have left behind through pheromones given off by their skin. As winter approaches, they follow the scent trails (which can sometimes be great distances) to locate a den. They depend on each other for winter survival, forming tight coils during hibernation to prevent body heat loss.

I am in agreement with another theory that concludes that male-female mimics in the garter snake world might have evolved through natural selection for the purposes of thermoregulation and predator defense rather than for sexual selection. When the snakes first emerge from a winter’s hibernation, they are chilled and move slowly. It is at this point when they are the most vulnerable to the predation of crows, hawks, skunks, raccoons and king snakes. (Also while in this state, they are not at their best to attract emerging females who require a warm suitor.) When an emerging male emits those female pheromones, it attracts the males who transfer their body heat in the aggregation created as they surround the she-male.

There have been scientific studies on this phenomenon - proving that a snakes' body temperature is significantly raised during the courting process in the mating ball. They even used dead snakes as courtship targets with thermistor leads implanted to measure deep body temperature to prove their point.

It has also been found that this she-male state is transitory - limited only to the first day or two that the males emerge from hibernation, in fact warmed up she-males regained their he-male status within a 3 hour time period as opposed to the those that were not heated up and remained that way for over 5 hours.

This gives me a lot to think about as I go about my gardening. The hidden dramas of the natural world continue to amaze and inspire me!