Small-scale Geographic Variation in Antipredator Tactics of Garter Snakes

Garter snakes (Thamnophis sirtalis parietalis) in Manitoba court and mate at communal overwintering dens and then disperse to feeding ranges in summer. The restriction of mating activities to den areas may reduce gene flow between populations from adjacent dens, in turn allowing divergence in traits such as antipredator behavior. We quantified responses of adult male snakes to the approach of human observers at four dens, prior to dispersal in late spring. Larger snakes bit us more frequently than did smaller individuals, at all four dens. Although some of the dens were ,20 km apart (and hence, individuals from each would likely encounter each other during summer), we found strong geographic differences in antipredator tactics (approach distance and propensity to bite). These differences may reflect genetic isolation of den populations, due in turn to den-based mating and philopatry. However, to rule out alternative explanations such as learned responses to different predation exposure, studies are needed on naı̈ve snakes.