Millions of garter snakes seem to live in the Manitoba Interlake Region. At least that is what it seemed to be when I walked through the Narcisse Snake Dens, a wildlife management area north of Winnipeg, Manitoba. Lucky for me, I met up with Rocky Parker, a PhD student from Oregon State University on the trail. His field studies have brought him here. I was very interested to hear from him how the study of these snakes brings together a wide variety of fields from ecology and physiology to genomics.

As we peered into the pits, we could see large balls of garter snakes slowly moving about. Rocky explained that females will exhibit receptive behavior (most notably by gaping their cloacae) depending on a number of factors. Most importantly, it is the
vigor of courtship she's receiving, the length of time she's been out of hibernation, and her condition that determine her receptivity. However, female receptivity is one of the biggest unknowns of our system. We know that estrogen plays a role, but even that is cloudy at best. We DO know that mating makes a female UNreceptive, and that the presence of sperm in her oviduct may contribute to a physiological change in her that contributes to her lack of receptivity. Apparently there can be up to 100 hopefuls, but only one male actually inserts his sperm. The female's sex pheromone on her skin doesn't change, but the copulatory plug the male leaves inside the female DOES appear to tell other males that she has mated. Though it is EXTREMELY rare, some females can mate with two males at once. After large females mate, they will linger at the den and mate at least a second time with another male prior to dispersing to the feeding ponds.

This is not the end of the sperm competition though as the female garter snake has a very long oviduct and will mate with several males over the duration of the mating season. It is speculated that there can be some further sperm selection even at this stage. Research into the use of stored sperm from multiple matings, and post-copulatory sexual selection is still ongoing at the Mason Lab at Oregon State University. Rocky chuckled as he spoke about garter snakes giving birth after living for years of captivity. Early scientists believed they had discovered a form of parthenogenesis when in fact they were seeing examples of extremely long sperm storage prior to fertilization.




Rocky also told me that these garter snakes prefer mating with snakes from their very own “pit”. Various studies bringing snakes together from further afield show that the pheromones are specific to the small populations. I asked Rocky how this fit with our current species concept and he suggested that genomic studies are certainly in the future with regard to resolving this question. I will certainly be watching with interest.