Bumpus in the snake den: Effects of sex, size, and body condition on mortality of red-sided garter snakes

Huge breeding aggregations of red-sided garter snakes (Thamnophis sirtalis parietalis) at overwintering dens in Manitoba provide a unique opportunity to identify sources of mortality and to clarify factors that influence a sn ake's vulnerability to these factors. Comparisons of sexes, body sizes, and body condition of more than 1000 dead snakes versus live animals sampled a t the same time reveal significant biases. Three primary sources of mortali ty were identified. Predation by crows, Corvus brachyrhynchos (590 snakes f rilled), was focussed mostly on small snakes of both sexes. Crows generally removed the snake's liver and left the carcass, but very small snakes were sometimes brought back to the nest. Suffocation beneath massive piles of o ther snakes within the den (301 dead animals) involved mostly small males a nd (to a lesser extent) large females; snakes in poor body condition were p articularly vulnerable. Many emaciated snakes (n = 142, mostly females) als o died without overt injuries, probably due to depleted energy reserves. Th ese biases in vulnerability are readily interpretable from information on b ehavioral ecology of the snakes. For example, sex biases in mortality refle ct differences in postemergence behavior and locomotor capacity, the greate r attractiveness of larger females to males, and the high energy costs of r eproduction for females.