Habitat use and abundance of the common garter snake, Thamnophis sirtalis, at the northern limit of its range in Manitoba
Wiens, Jonathan P.
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The common garter snake (Thamnophis sirtalis) is a wide-ranging species that reaches the northern limit of its range in central Manitoba. Although Thamnophis sirtalis (and especially the subspecies parietalis) has been the subject of intense biological research, there are large gaps of knowledge regarding the ecology of this species in extreme northern latitudes. A recently discovered northern hibernaculum in the boreal forest region near Jenpeg, Manitoba, provided an ideal opportunity for comparing the ecology and biology of this species with geographically distant populations. The climate at Jenpeg is sub-arctic continental and consists of cold winters, and cool short summers. The area lies on the Canadian Shield. and is characterized by surface outcrops of granitic bedrock. Garter snakes were found colonizing artificial habitat features for winter habitat including dikes and rock quarries. Research on the ecology of this species was conducted from May 2005 to May 2007. Snakes emerged in late April and early May. Activity at the den lasted approximately two weeks before snakes dispersed for the summer. Mark-recapture results from 2005 indicate that this population is small (~79 individuals +_ 10.6) and exhibits a wide variation in adult sizes (350-1340 mm total length). Radio telemetry and funnel trapping have shown that summer habitat use is concentrated around wetlands. with wood frogs (Lithobates sylvatica) constituting the most common food source (56%). Many snakes dispersed over distances exceeding two kilometres, despite the apparent abundance of prey species near the den site. Analysis of colour patterns revealed substantial variation in the skin folds between the lateral scales. The majority of snakes expressed no red colouration on the lateral scales (45%) while some individuals expressed rare examples of bright red colouration (erythristism) (6%) and dark colouration (melanism) (1%). Traditional local knowledge gathered from aboriginal hunters, fishers and trappers outlined the long-term presence of garter snakes in the region, and provided local distribution data for the species. Aboriginal peoples stated that the overall population density of snakes in the region was low, and provided additional support for the findings of large variation in body sizes and colour. It is hypothesized that relative reproductive isolation and a prolonged overwintering period are factors contributing to the uniqueness of this northern population. Information from this study will benefit our understanding of garter snake biology and provide valuable information to assist the conservation efforts of wildlife and landscape managers in the region.
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