Summary
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.
Methodology
This study was conducted at the Queen's University Biological Station in eastern Ontario as part of a general radiotelemetry study of black rat snakes (Weatherhead and Charland, 1985 ; Weatherhead and Hoysak 1989). Snakes were caught opportunistically during the summers of 1982 and 1983 and implanted with transmitters having an expected longevity of approximately 18 months. (For details of the transmitter design and implantation technique, see Weatherhead and Anderka (1984).) The data reported here came from two groups of snakes. The spring 1983 emergence data came from three female and three males snakes implanted with transmitters the previous summer, all of which occupied the same hibernaculum that winter. The data from autumn 1983 and winter 1983--1984 came from three males and one female snake in which transmitters were implanted during the summer of 1983. One of the males was part of the spring 1983 group, having had his transmitter replaced during the summer. Other snakes tracked in 1982 had their transmitters removed in 1983, with the exception of one female that did not emerge from hibernation. In the fall and winter group, two males occupied one hibernaculum and the female and remaining male occupied another. The male snakes weighed between 375 and 650 g and the females weighed between 410 and 578 g.
The transmitters were designed so that the pulse rate decreased with temperature. Each transmitter was calibrated in a water bath before being implanted in a snake. This allowed direct conversion of transmitter pulse rates to temperatures. Accuracy of the transmitters at the time of calibration was ± 0.25°C. After transmitters were removed, recalibration showed that their performance varied less than 0.3 % from the original calibration. Each time a snake was located, either above ground or in a hibernaculum, a series of 10 pulses was timed to determine the pulse rate. A second series was always timed to ensure no mistake had been made on the first. Temperatures then were read from the calibration curve for the appropriate transmitter.
In April 1983 the hibernaculum was visited approximately once a week until the end of the month, by which time all but one snake had emerged and dispersed. Regular checks at the hibernaculum through the early summer confirmed that that snake had not emerged, but allowed continued monitoring of the temperature in the hibernaculum, at least at the location of that transmitter. In the autumn the snakes were tracked almost daily through September and early October. Once all the snakes were permanently below ground, the hibernacula were visited approximately once a month through March 1984.