Determinants of the northern range limits of amphibians and reptiles in Canada

Reptiles and amphibians are overwhelmingly tropical and subtropical taxa. However, there are some Nearctic species that range into Canada, a few of which reach relatively high latitudes characterized by severe winters that cause them to spend at least half of their lives in hibernation. We posit two general hypotheses about what limits northern ranges, which we term the long-winter and the short-summer hypotheses. The former suggests that winters are too long (and severe) for survival of at least one life stage. The latter posits that summers are too short for successful reproduction, defined as young surviving their first winter. Turtles clearly favour the short-summer hypothesis, because all turtles are oviparous with long incubation periods that approach the onset of cold weather at their northern limits, and because adults can survive in simulated hibernation longer than they would have to in nature. The similarity of the northern ranges of turtles and egg-laying snakes suggests that the latter are also limited in their northern extent by incubation time restraints. For other groups, results of studies to date are more equivocal, largely because there are few studies of overwintering neonates. Nevertheless, we suggest that the short-summer hypothesis may apply to all taxa.