Thermal Preference and Habitat Use of Eastern Garter Snakes (Thamnophis sirtalis sirtalis) in a Southern Ontario Peatland
Garter snakes are the most widespread reptile in North America. Despite occupying vastly different biogeoclimatic zones across their range, evidence suggests that the thermal preference (Tsel) of garter snakes has not significantly diverged among populations or different Thamnophis species. The reason for garter snake’s wide thermal tolerance could lie in their flexible thermoregulatory behaviours and habitat use. I aimed to investigate this relationship by first identifying the Tsel of a common garter snake species (Thamnophis sirtalis sirtalis) via a thermal gradient in the lab. I used this Tsel parameter (27.82 – 32.15 ºC) as a benchmark for calculating the thermal quality of open, mixed, and forested habitats all used by the species. I measured the thermal profiles of these habitats by installing a series of operative temperature models that mimicked the thermal properties of living garter snakes and recorded environmental temperatures as living snakes experience them. Lastly, I used coverboards to survey the habitat usage of T. s. sirtalis. Of the three habitats, I found that the open habitat offered the highest thermal quality throughout the snake’s active season. In contrast, I recorded the greatest number of snakes using the mixed habitat which had considerably lower thermal quality. Although the open habitat offered the greatest thermal quality on average, environmental temperatures regularly exceeded the upper limits of the animals’ thermal tolerance, greatly restricting the activity window for Eastern garter snakes. Therefore, the open habitat may be less thermally attractive to T. s. sirtalis. My data show that not all habitat types used by Eastern garter snakes share the same quality and the relationship is not as simple as warm, open habitats are better. Rather, these animals may preferentially seek out areas that offer a mix of both open and closed-canopy spaces to suit their thermoregulatory needs. In conclusion, T. s. sirtalis may select more thermally stable habitats over those that present them with thermal extremes and greater constraints to their surface activity.