Ecology of the common garter snake, Thamnophis sirtalis, at the northern limit of its range
Date
1986
Authors
Larsen, Karl W.
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Abstract
Population ecologists concern themselves with two fundamental, and related, kinds of questions. How (and why) do life history traits vary in different environments? What factors limit the distribution and abundance of a species? The answers to such questions depend, on part, on an understanding of patterns in nature, but patterns need to be described before they can be analyzed. The aim of this study was to document the annual cycle and life history characteristics of a wide-ranging species at the edge of its range and compare them with the ecology of the same species in contrasting environments elsewhere.
The common garter snake, Thamnophis sirtalis, is the most widely distributed snake species in North America, ranging further north than any other reptile on the continent. This species has been well studied in several geographic locations, but never at its northern limit. From 1983 to 1985, the ecology of T. sirtalis was examined near Fort Smith, Northwest Territories, the known northern limit of the species. The study centred around the Salt River den, in Wood Buffalo National Park (WBNP).
The active season in WBNP is short, and weather conditions during that period are sometimes harsh. Garter snakes in the Salt River den begin emerging very early in the spring (mid to late April), when snow cover is still prevalent. Body temperatures as low as 0.5 °C were recorded for emerging snakes. The total period of activity at the den (emergence and mating) was quite short, with the vast majority of the population having dispersed by the second week in May. The distance from the Salt River den to the nearest summer habitat (marshes) is approximately 3. 75 km. Mark-recapture and biotelemetry studies indicated that snakes undertake an annual migration to and from this area, presumably in search of food (which is predominantly frogs). One snake completed a round-trip of over 15 km in 1984. Migrations such as this may be costly, both in terms of the time taken to complete the journey between the den and marsh (9 to 13 days for two radio-tracked snakes) and the energy expended. The time taken to migrate to the marshes in the summer is also affected by variable weather conditions.
Growth of snakes was measured by changes in length of recaptured snakes. A paucity of growth records for immature snakes prevented the construction of formal growth curves. However, analysis revealed that growth of adults was similar to that of the same species in Manitoba, but considerably less than that of Kansas snakes. Male and female snakes in WBNP reach sizes that, on average, are larger than those reported elsewhere. The reason for this is unknown.
Only five mature females were recaptured frequently enough to permit interpretation of their reproductive history. However, these few data suggest that females do not reproduce annually, in contrast to the situation in warmer environments. Females may fail to reproduce every year because of the shortness of the season. Birth occurs late in the summer (late August), and most postparturient females are probably unable to feed prior to hibernation. Hence, one (or possibly more) summer(s) may be spent re-attaining reproductive condition, and low annual recruitment is a possible consequence of the short active season. Although gravid females in Wood Buffalo National Park were much larger than those in Manitoba and British Columbia populations, there was not a corresponding increase in litter size (mean=l 1.6 in WBNP). No clear interpretation of this is possible at present.
Surface and subsurface temperatures were measured in the Salt River den, and in a nearby control. Temperatures in the former were warmer and more stable through winter, implying that only certain sites can function as overwintering facilities. The temperature of a hibernating snake (measured through telemetry) remained in the Z-6 °C range during winter. Emergence patterns of the snakes in the spring were not strongly correlated with a reversal in the surface/subsurface temperature gradient.
It is not possible to conclude, from this study, what determines the northern limit of T. sirtalis, but the leading potential factors are length of active season and availability of hibernating sites.