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Thamnophis sirtalis (Linnaeus, 1766)
Common Garter Snake
Family: Colubridae

Species account author: Krysia Tuttle

Distribution of Thamnophis sirtalis in British Columbia in British Columbia

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The Common Garter Snake is a medium-sized, diurnal snake. The head is large and distinct from the neck; eyes are large with round pupils. There are usually 7 yellow upper labials, often with red to black blotch markings. Male and female snakes are dark bodied with three distinct straight yellow to greenish-yellow dorsal/lateral stripes and generally have red hatching or bars along their sides (except on Vancouver Island where the red colouration is often absent). Dorsal body scales are strongly keeled and have up to 19 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. Ventral surface ranges from light yellow to black, with the chin and neck lightest in colour. When a snake is nearing ecdysis (shedding their skin), the body color will be dull and eyes will appear blue-grey in colour as fluid lymph fills the area between the new and old layers of skin. Garter snakes range in size from under 200 mm when born to over a metre in total length when adult (Matsuda et al. 2006); however, significant size variation exists between populations. Males are smaller in head dimensions and body size than females, although they have longer tails relative to body length. Such sexual dimorphism has been attributed to reproductive investment in females, as fecundity increased with body size.

Reproduction

Reproduction occurs in the spring (usually end of April – early May) in the vicinity of the den site (Shine et al. 2001). Males emerge first and wait to court females, and both sexes may have multiple pairings each year. Males court females by physical stimulation along the sides of her body; females will often have more than one suitor at a time, leading to a mating ball, where multiple males will coil around her. Males may insert a copulatory plug into a female to prevent further mating, but often females will mate with more than one male, leading to a litter offspring having multiple fathers. Females usually start to reproduce in their second or third year, and may have litters every one to three years, depending on resources (Gregory 2006). Litter sizes range from 2 to 22 (max 85), and female fertility typically increases with size. Lower reproductive output is associated with high-latitude females compared with southern populations (Larsen 1986). Common Garter Snakes are viviparous; females carry developing eggs for 12-16 weeks, and young are born from mid-July through to early September. Neonates are small (145 to 220 mm SVL), slender and independent at birth (some still have partial yolk sac attached).

Diet

Garter snakes are active foragers and tend to occupy areas of high prey density, thus their habitat selection is often correlated with habitat selection by their prey (Gregory 1984; Gregory and Nelson 1991). They are commonly associated with wetland habitats, as their primary prey items are amphibians. Adult snakes are dietary generalists and will prey upon many different species, including earthworms, frogs, toads, salamanders, fish, fledgling birds and occasionally small mammals, depending on seasonal availability of prey and location. Common Garter Snakes are one of a very few species capable of eating the toxic Rough-skinned Newt (Taricha granulosa). Snakes have short, sharp, backwards curving teeth, designed to hold prey which are swallowed whole. Due to their kinetic skull (i.e., moveable joints), garter snakes can swallow large food items. Young snakes feed directly after birth, and are more restricted in diet due to gap-limitations, eating primarily small worms and recently metamorphosized frogs.

Behaviour

Garter snakes are more active during the day and can frequently be seen foraging in aquatic or terrestrial environments, as well as basking in vegetation or on rocky slopes. Gravid females are less active and typically spend most of their day thermoregulating. Male Common Garter Snakes do not exhibit male-to-male combat when competing for females; however large mating aggregations of snakes in the spring are a spectacular sight (Shine et al. 2001). Snakes that are undergoing ecdysis (i.e. shedding their skin) will also generally be less active and will often remain under cover.

Seasonal Charactertistics

Common Garter Snakes are active from March to November, depending on the region, and must find suitably deep hibernating sites for the winter months (Shine and Mason 2004). Northern populations of this species are noteworthy for their communal hibernation behaviour and the extensive migrations (up to 15 km) they make between summer wetland habitats and winter den sites (Gregory 1977; Larsen 1987). Den sites are usually rock sinks or mammal burrows that allow snakes to get below freezing winter temperatures. Hundreds of individuals, sometimes of multiple species of snake, have been found to use the same dens, usually in areas where such sites are limiting. On Vancouver Island where winters are generally milder, snakes will hibernate alone or in smaller groups; den sites do not need to be as deep underground due to lack of snow and warmer winter temperatures.

These snakes have broad habitat preferences, including grasslands and forests, and are commonly associated with aquatic environments (e.g., wetlands, ponds, marshes, river valleys). They are commonly found in cities (parks and waterways) and, because of their abundance and frequent day time activity, are the most familiar snake in British Columbia. Cover is in important component of the habitats for garter snakes, providing shelter from both direct sunlight and predators.

Common Garter Snakes are the most widespread and northerly occurring species of garter snake, ranging from the southern U.S. to slightly north of 60 degrees latitude in the Northwest Territories (Larsen and Gregory 1989). This species often occurs in high densities (Matsuda et al. 2006) making it a widely recognizable garter snake. In British Columbia, this species is widespread throughout the mid- to southern province, with the most northern populations occurring in the Bulkley, Nass River and Peace River regions. It has even been recorded in the northern Rocky Mountains in extreme northern B.C. The highest densities in the province occur across the southern interior and on the south coast (including on Vancouver Island).

Common Garter Snakes are yellow-listed in British Columbia (G5, S5) and not at great risk for population decline as long as hibernation sites and marsh areas for foraging are available. A growing area of concern for reptile conservation in Canada is the effect of road mortality rates on garter snake populations (Shine et al. 2004).

Because of their wide distribution and local abundance, most research on the ecology and life history of high-latitude temperate-zone garter snakes have focused on Common Garter Snakes (Gregory 1977; Larsen 1986). There have been several studies within British Columbia on this species including body size, food habits (Gregory 1984; Gregory and Nelson 1991), defensive behaviours (Isaac 2010), and hibernation (Macartney et al. 1989).

Numerous subspecies of T.sirtalis are recognized throughout North America (Rossman et al. 1996), several of which have been recognized in the past as separate species. There are three subspecies occurring in British Columbia:

1. Valley Garter Snake (T. s. fitchi) in the interior regions
2. Common Red-sided Garter Snake (T. s. parietalis) along the eastern region in the Rockies;
3. Puget Sound Garter Snake (T. s. pickeringii) in coastal areas, including on Vancouver Island.

Origin Status	Provincial Status	BC List (Red Blue List)	COSEWIC
Native	S5	Yellow	Not Listed

BC Ministry of Environment: BC Species and Ecosystems Explorer--the authoritative source for conservation information in British Columbia.

Washington Dept. of Fish and Wildlife Predictive Range Maps
Reptiles of British Columbia

Gregory, P.T. 1977. Life-history parameters of the Red-sided Garter Snake (Thamnophis sirtalis parietalis) in an extreme environment, the Interlake region of Manitoba. Natural Museums of Canada Publications in Zoology 13:1-44.

Gregory, P.T. 1978. Feeding habits and diet overlap of three species of garter snakes (Thamnophis) on Vancouver Island. Canadian Journal of Zoology 56:1967-1974.

Gregory, P.T. 1984. Habitat, diet, and composition of assemblages of garter snakes (Thamnophis) at eight sites on Vancouver Island. Canadian Journal of Zoology 62:2013-2022.

Gregory, P.T. 2006. Influence of income and capital on a viviparous snake: direct and indirect effects. Journal of Zoology 270:414-419.

Gregory, P.T. 2007. Biology and conservation of a cold-climate snake fauna. In C. Seburn and C. Bishop, editors. Ecology, Conservation and Status of Reptiles in Canada. SSAR, Herpetological Conservation 2.

Gregory, P.T., and K.J. Nelson. 1991. Predation on fish and intersite variation in the diet of Common Garter Snakes, Thamnophis sirtalis, on Vancouver Island. Canadian Journal of Zoology 69:988-994.

Gregory, P.T., and K.W. Larsen. 1993. Geographic variation in reproductive characterstics among Canadian populations of the common garter snake (Thamnophis sirtalis). Copeia 1993:946-958.

Larsen, K.W. 1986. Ecology of the Common Garter Snake, Thamnophis sirtalis, at the northern limit of its range. MSc Thesis. University of Victoria, Victoria. Larsen, K.W. 1987. Movements and behavior of migratory garter snakes, Thamnophis sirtalis. Canadian Journal of Zoology 65:2241-2247.

Larsen, K.W., and P.T. Gregory. 1989. Population size and survivorship of the Common Garter Snake, Thamnophis sirtalis, near the northern limit of its distribution. Holarctic Ecology 12:81-86.

Rossman, D.A., N.B. Ford, and R.A. Seigel. 1996. The Garter Snakes: Evolution and Ecology. University of Oklahoma Press, London.

Shine, R., and R.T. Mason. 2004. Patterns of mortality in a cold-climate population of garter snakes (Thamnophis sirtalis parietalis). Biological Conservation 120:201-210.

Shine, R., M.M. Olsson, M.P. LeMaster, I.T. Moore, and R.T. Mason. 2000. Effects of sex, body size, temperature, and location on the antipredator tactics of free-ranging garter snakes (Thamnophis sirtalis, Colubridae). Behavioral Ecology 11:239-245.

Shine, R., M.J. Elphick, P.S. Harlow, I.T. Moore, M.P. LeMaster, and R.T. Mason. 2001. Movements, mating, and dispersal of Red-sided Garter Snakes (Thamnophis sirtalis parietalis) from a communal den in Manitoba. Copeia 2001:82-91.

Shine, R., M.P. LeMaster, M. Wall, T. Langkilde, and R.T. Mason. 2004. Why did the snake cross the road? Effects of roads on movement and location of mates by garter snakes Thamnophis sirtalis parietalis). {i{Ecology and Society 9.

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