Temporal and thermal aspects of hibernation of black rat snakes (Elaphe obsoleta) in Ontario

Radio telemetry was used to determine the time of emergence from and entry into communal hibernacula by black rat snakes (Elaphe obsoleta obsoleta) and to gain insight into the proximate factors involved in these behaviours. Emergence in mid-April appears not to be triggered by marked reversal of thermal gradients in the hibernaculum as has been proposed, or by changes in day length. By default, endogenous rhythms seem most likely to be responsible. Entry in early October may be triggered in part by outside temperatures. Even with the limited sample size used in this study, considerable individual variation in emergence and entry patterns was observed. Through the winter the snakes steadily got colder, reaching their coldest point just before emergence. The 7-month hibernation period was substantially longer than has been observed for more southerly populations. Given the period of inactivity immediately before and following hibernation, the active period is less than 4 months, which perhaps explains why the population in this study is the northernmost in the species' distribution.