Movement patterns and orientation mechanisms in garter snakes




Lawson, Peggy Margaret Ann

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Movements of animals presumably reflect their changing needs and the changing availability of necessary resources. In cold climates, snakes often make long seasonal migrations between hibernacula and summer habitats, Finding suitably deep hibernacula with minimal delay could be critical. I hypothesized that such animals should have highly developed navigational ability. By contrast, snakes living in mild climates can hibernate in shallow sites and probably do not migrate; if so, they might be expected to show poorly developed orientation mechanisms!. The objectives of this study were to determine movement patterns and navigational ability of garter snakes (Thamnophis) living in a mild climate and compare them with a congeneric population known to be migratory. From 1986 - 1988 I examined, using mark-recapture, movement behaviour of two populations of garter snakes at Spectacle Lake Provincial Park (SLPP) on Vancouver Island, British Columbia, focusing on several components typically associated with migratory behaviour - distances travelled, population directionality, fidelity to seasonal sites, distinctness of seasonal habitats, and route directness. Thamnophis sirtalis, the common garter snake, is the most widely distributed North American snake species and high latitude populations are migratory. Thamnophis ordinoides, the northwestern garter snake, is restricted to the Pacific northwest and migratory behaviour has never been reported. Both species displayed combinations of traits clearly suggesting nonmigratory behaviour. These included short-distance (< 500 m), random movements, a lack of den fidelity, and variation in the maintenance of specific home ranges between successive years. Home ranges overlapped between individuals, averaged less than 0.3 ha measured over a single active season, and were not clearly distinct from denning areas. Although some directionality of movement was evident, it was likely related to foraging strategy and unlike the typical unidirectional movements undertaken by migrating snakes, Sexual and reproductive differences in any of these traits generally were not significant. The navigational abilities of a migratory population of T. sirtalis from Wood Buffalo National Park (WBNP) in northern Alberta were examined as were those of the nonmigratory populations of snakes from SLPP. Displacement studies were carried out during the active seasons of 1986 - 1988 to determine the level of orientational abilities present in each population and to examine potential orientation cues. Snakes were displaced from their home range and tested in an arena under a variety of conditions, The results demonstrated that T. sirtalis from both SLPP and WBNP possessed advanced navigational abilities. Advanced skills may be absent in T. ordinoides. Thamnophis sirtalis at both study sites demonstrated time-compensated solar orientation as determined by 6 hr phase-delayed tests. Pheromone trails produced by recently copulated females (but not unmated females) also provided an orientation guide for displaced WBNP males, but results from SLPP were less conclusive. Thamnophis ordinoides did not respond in a discernible way to either cue. Navigational skills thus vary relatively little between migrating and nonmigrating populations of the same species but may be poorly developed in completely nonmigratory species.



Common garter snake, Thamnophis ordinoides, Locomotion, Migration, Garter snakes, British Columbia, Spectacle Lake Provincial Park, Garter snakes, Alberta, Wood Buffalo National Park