The ecology of garter snakes, Thamnophis sirtalis and T. elegans in Southeastern British Columbia
Date
1988
Authors
Farr, Daniel Richard
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Abstract
Explaining patterns of life history variation within a wide-ranging species requires comparative ecological data from populations in different parts of the species' range. Two species of garter snakes, Thamnophis sirtalis and T. elegans, have been studied in a variety of environments, and the sample of populations from which patterns can be discerned is greater than for most other snakes. I used markĀrecapture to document the body size, growth, reproduction, and other aspects of the ecology of each of these species at Creston, in southeastern British Columbia, during 1986 and 1987, and I examined variation between these populations and those in other regions.
Adult body size in both T. sirtalis and T. elegans at Creston is small compared to many other populations. Whether higher adult mortality, slower growth, smaller asymptotic size, or a combination of factors causes this difference is not known. In T. elegans, small female size accounts for the relatively small litter size (7-8 young). In T. sirtalis, however, female size is insufficient to explain differences in litter size between snakes at Creston (7-8 young) and other locations. Food availability and local adaptation are possible causes of these differences. Within each species at Creston, females reach larger maximum sizes than males, and females are heavier, with wider, longer heads than males of similar body length. Both sexes exhibit a decrease in relative head size with increasing body size.
Garter snakes at Creston do not overwinter communally, and the longĀ-distance migrations between winter and summer habitats observed in other populations apparently do not occur. The abundance of suitable overwintering sites adjacent to feeding areas may explain the relatively limited seasonal movements of snakes at this location. Variation in movements among individuals was extensive, ranging from less than 50 m to over 2000 m between captures within a 30-day period, with T. elegans probably moving longer distances than T. sirtalis. Small snakes and gravid females were probably more sedentary than other individuals. The diet of T. sirtalis consisted of primarily (80%) earthworms, amphibians and small mammals, and that of T. elegans almost exclusively (90%) slugs and small mammals. Although neither the diet nor the overall proportion of snakes with food was particularly unusual, dramatic shifts in the type and size of prey eaten occurred as snakes grew larger. Large snakes ate relatively larger prey, and tended to eat a wider range of prey types and sizes than small snakes. The largest T. elegans, however, excluded small prey (slugs) from their diet, and instead specialized almost completely on small mammals. This may maximize feeding efficiency, and may be associated with adaptations for handling large, active prey.
The extent of ecological variation among populations of each species is at least as great as the variation between each species. Both T. sirtalis and T. elegans exhibit flexibility in their life histories: Thamnophis sirtalis, in particular, may be able to make phenotypic or genotypic (or both) adjustments to local conditions, thereby allowing it to persist under a wide range of environmental conditions.