The causes, magnitude and phenotypic determinants of mortality are central features of population demography. Although critical to effective conservation planning, these attributes remain poorly understood for most wild vertebrates. Exceptionally dense aggregations of garter snakes (Thamnophis sirtalis parietalis) in and around communal overwintering dens in central Manitoba provide an exceptional opportunity to enumerate these factors. Over the course of long-term field studies we obtained data on mortality of snakes in autumn as they migrated back from their summer ranges, and in winter during and after their entry into the underground dens. Some mortality sources were affected by a snake's phenotype: for example, snakes killed by motor vehicles were smaller, on average, than surviving snakes. However, two major episodes of mortality at the dens themselves (freezing during midwinter due to atypically light snow cover; and drowning due to flooding after heavy rains late in autumn) were essentially random with respect to phenotype. Mark-recapture estimates suggest that the winterkill event eliminated >60,000 snakes, comprising almost the entire populations of the three largest dens. Although such events are cryptic because the bodies of frozen animals remain underground, they are of much greater magnitude than overt mortality sources for active snakes (roadkills, predation, etc.). Thus, relatively minor habitat manipulations around the dens may contribute more to conservation of these populations than would extensive efforts to protect active snakes during their spring and autumn migrations.
- Body size
- Natural selection