Patterns of variation in spring emergence by black rat snakes (Elaphe obsoleta obsoleta)

Substantial individual variation in timing of emergence from hibernation has been reported among reptiles, although few studies have addressed the causes of that variation. Here we report patterns of spring emergence by black rat snakes (Elaphe obsoleta obsoleta) observed at 13 communal hibernacula in eastern Ontario from 1992-1997. Because our study area is at the northern extreme of the species' range, we expected synchronous emergence given the very short active season (about 5 months). Despite these apparent time constraints, however, the emergence period lasted an average of 40 days. Although we found significant variation in timing of emergence among years and among hibernacula, we found no evidence of a simple latitudinal gradient in mean emergence dates. Within hibernacula, significantly more snakes emerged on days with high maximum air temperatures, but maximum air temperature explained only a small amount of the variation among years in emergence dates. Larger individuals emerged earlier than smaller individuals, as predicted if early spring predation risk is higher for smaller snakes. Females tended to emerge slightly earlier than males. Condition was not a significant factor determining timing of emergence in our population. Individuals demonstrated significant repeatability between years in their time of emergence relative to other snakes in the same hibernaculum.