Each year, as the warmth of summer turns into the cool of autumn and the cold of winter, snakes disappear from the Canadian landscape. For several months of the year, winter weather at high latitudes is much too cold for snakes to be active in the open and they must seek subterranean shelter to hibernate. This long period underground is one during which these animals are subject not only to the possibility of mortality, but also to lost opportunity for activities such as foraging and the acquisition of resources for reproduction. Winter is thus a major constraint on the life histories of temperate-zone snakes. Short, cool summers further restrict their foraging and reproductive opportunities. In apparent response to this challenge, most high-latitude snake species are viviparous, counter to what is seen in warmer climes. Viviparity allows gravid females to “manipulate” the developmental temperature of their progeny, via behavioral thermoregulation, until those offspring are independent, an option not open to oviparous species. However, viviparity also has costs, not least of which is a pronounced reduction in feeding during pregnancy, which means that the post-partum female has only a short time before winter to make up the reserves (“capital”) that she spent on reproduction and will need for future reproduction; therefore, reproduction in consecutive years is not always possible. Evidently, the demographic costs of viviparity are outweighed by its advantages, but what remains unexplained is how some oviparous species manage to persist at high latitudes. Demographic advantages of oviparous over viviparous species, due to shorter “pregnancy” of the former, are not apparent from limited temperate-zone studies. More likely, cool-climate oviparous species also reproduce successfully by taking advantage of the thermal heterogeneity of the environment, especially by thermoregulating precisely while gravid and/or by careful selection of nest sites.