Western Terrestrial Garter Snakes are medium-sized, diurnal snakes. Due to the wide colour variation exhibited by this species, Western Terrestrial Garter Snakes are the most difficult garter snake to identify and are frequently mixed up with Northwestern and Common Garter Snakes, as well as smaller Gopher Snakes in the Okanagan. Adults have a large heads relative to neck size, and medium to large eyes with round pupils. Lips have 10 lower and 8 upper labials, often with vertical black markings on the front edges of the scales. Dorsal body scales are strongly keeled and have up to 21 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. In British Columbia, Western Terrestrial Garter Snakes can range in size from 150 to 750 mm in snout-vent length (Waye 1999; Isaac 2010). Garter snakes exhibit sexual size dimorphism; 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.
There are two distinct colour morph variations of this species in British Columbia: a darker coastal type and a lighter interior type. Both morphs typically have three stripes running the length of their bodies (one dorsal and two lateral); however, there is noticeable variation in colouration, length and visibility of each stripe between individuals and populations (Isaac 2010). The interior colour morph, which is the predominant form, is lighter in dorsal body colour (dusty brown to beige) and usually has a wavy yellow dorsal line, lined by two rows of dark spots. Lateral stripes, located on the second and third scale rows on each side of the body, are yellow and often lighter and less distinct that the dorsal strip. The darker coastal morph typically has a dark body (similar to Common Garter Snake colouration), with a wavy or straight orange-to-yellow dorsal strip and two yellow lateral stripes. The ventral scutes can range in colour from light blue/grey to dark black, and may have irregular markings. When a snake is nearing ecdysis (shedding their skin) the body color will be dull and eyes will appear bluish in colour as fluid lymph fills the area between the new and old layers of skin.
Western Terrestrial Garter Snakes are viviparous snakes (i.e., live bearing). Environmental temperatures have a great influence on growth and reproduction in reptiles, and female garter snakes often delay sexual maturity and do not reproduce on an annual basis--especially at northern latitudes (Charland 1995). Mating occurs in the spring shortly after emergence from overwintering sites. Males emerge earlier in the spring than females, and will either actively search for mates or congregate near den areas to mate with emerging females. Males sometimes insert a copulatory plug post-mating to ensure fertilization of his progeny and prevent further mating by the female. In viviparous species, such as garter snakes, larger females have an increased ability to accommodate more eggs in their body cavity; thus their body size is linked to litter size. (Gregory and Skebo 1998). Females give birth to between 3 to 24 live young in mid to late summer (July to September). Newborns usually measure 120 to 220 mm in length and can be a variety of solid, checkered, or striped patterns. Neonate snakes are independent at birth and grow rapidly in the first few years of life (Gregory and Prelypchan 1994).
Western Terrestrial Garter Snakes are gape-limited carnivores that actively forage in both aquatic and terrestrial environments. Like many species of garter snakes, Western Terrestrial Garter Snakes are generalist foragers and commonly include worms, slugs, leeches, frogs, small mammals, birds, and fish in their diets (Gregory 1978; Matthews et al., 2002). In coastal and freshwater riparian areas, large snakes will often be dominantly piscivorous (i.e., fish eating). Snakes have short, sharp, backwards curving teeth, designed to hold prey. Prey are swallowed whole and, due to their kinetic skull (i.e., moveable joints), garter snakes can swallow large food items. Coiling around and constricting prey items have been observed in a few cases (especially when preying on small mammals) and these snakes may also secrete fluids that aid in the digestion of tissues (Gregory et al. 1980).
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 than other snakes and typically spend most of their day thermoregulating. Snakes that are undergoing ecdysis (i.e., shedding their skin) will also generally be less active and will often remain under cover.
Both young and adult snakes are preyed upon mainly by birds (e.g., corvids, great blue herons, hawks) and occasionally by mammals (e.g., skunks, raccoons, dogs). When captured, these snakes will often emit a pungent-smelling fluid with excrement from the cloaca as an anti-predator strategy (Gregory and Gregory 2006). Snakes may bite or play dead when handled, but more frequently will either flail or whip their body in order to escape, or coil into a ball with the body protecting the head. These snakes are not venomous, but significant swelling of the area may occur after a prolonged bite.
Being ectothermic, Western Terrestrial Garter Snakes are inactive in the winter season (November-March) and must find over-wintering sites (i.e. hibernacula) that are below the frost level. Den sites for this species may include rock sinks, crevices, or existing mammal burrows. These snakes often hibernate communally with conspecifics and occasionally with other species, depending on the availability of over-wintering sites.
As this species is found across the southern parts of British Columbia, Western Terrestrial Garter Snakes are found in a wide variety of habitats from coastal forests and meadow edges on Vancouver Island, dry grassland habitats in the interior, to large wetland complexes in the Kootenays (Matsuda et al. 2006). This species is considered highly aquatic and typically occurs near water bodies such as marshes, streams, lakes, and estuaries. This species is less likely to be found in forests than the other two species of garter snake found in British Columbia. Unlike other garter snakes in the province, this species regularly forages in intertidal habitats and will even traverse marine environments (i.e., when moving between islands, etc.).
The range of Western Terrestrial Garter Snakes extends from central western Canada (British Columbia, Alberta, Saskatchewan, south through all of the western United States (South Dakota, Nebraska, Colorado, Oklahoma, and New Mexico) to northern California (Rossman et al. 1996; Stebbins 2003). Elevational range extends from sea level to 3,355 metres or higher in some locations. In British Columbia, these snakes are found in the southern half of the province, from the Rockies to the coast and north to the Nass River and Peace River areas, including southern and eastern Vancouver Island and the Gulf Islands. The highest densities in the province are found across the southern interior (north to the Williams Lake area) and in the Georgia Depression on the south coast.
This species is yellow-listed in British Columbia (S5, Dec. 2007) and not at great risk for population decline as long as hibernation sites and foraging areas are available (Gregory 2007). There have been several studies within British Columbia on Western Terrestrial Garter Snakes including body size (Farr and Gregory 1991), reproduction (Charland 1995; Gregory and Skebo 1998), food habits (Gregory 1978, 1984), population dynamics (Waye 1999), defensive behaviours (Gregory and Gregory 2006; Isaac 2010), and habitat use (Charland and Gregory 1995).
There are four recognized subspecies of Western Terrestrial Garter Snake in North America (Rossman et al. 1996), but the Wandering Garter Snake (Thamnophis elegans vagrans) is the only subspecies that occurs in British Columbia (Matsuda et al. 2006); however, there is some debate over the Vancouver Island populations belonging to a different subspecies (T. e. nigrescens).
Charland, M.B. 1995. Thermal consequences of reptilian viviparity: thermoregulation in gravid and nongravid garter snakes (Thamnophis). Journal of Herpetology 29:383-390.
Charland, M.B., and P.T. Gregory. 1995. Movements and habitat use in gravid and nongravid female garter snakes (Colubridae: Thamnophis). Journal of Zoology of London 236:543-561.
Farr, D.R., and P.T. Gregory. 1991. Sources of variation in estimating litter characteristics of the garter snake, Thamnophis elegans. Journal of Herpetology 25:261-268.
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 62:2013-2002.
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 C.J. Prelypchan. 1994. Analysis of variance of first-year growth in captive garter snakes (Thamnophis elegans) by family and sex. Journal of Zoology London 232:313-322.
Gregory, P.T., and K.M. Skebo. 1998. Trade-offs between reproductive traits and the influence of food intake during pregnancy in the garter snake, Thamnophis elegans. The American Naturalist 151:477-486.
Gregory, P.T., and L.A. Gregory. 2006. Immobility and supination in garter snakes (Thamnophis elegans) following handling by human predators. Journal of Comparative Psychology 120:262-268.
Gregory, P.T., J.M. Macartney, and D.H. Rivard. 1980. Small mammal predation and prey handling behavior in the garter snake, Thamnophis elegans. Herpetologica 36:87-93.
Matthews, K.R., R.A. Knapp, and K.L. Pope. 2002. Garter snake distributions in high-elevation aquatic ecosystems: is there a link with declining amphibian populations and nonnative trout introductions? Journal of Herpetology 36:16-22.
Macartney, J.M., K.W. Larsen, and P.T. Gregory. 1989. Body temperatures and movements of hibernating snakes (Crotalus and Thamnophis) and thermal gradients of natural hibernacula. Canadian Journal of Zoology 67:108-114.
Matsuda, B.M., D.M. Green, and P.T. Gregory. 2006. Amphibians and Reptiles of British Columbia. Royal BC Museum Handbook, Victoria, Canada. 266pp.
Rossman, D.A., N.B. Ford, and R.A. Seigel. 1996. The garter snakes: evolution and ecology University of Oklahoma Press, Norman. xx + 332 pp.
Stebbins, R.C. 1985. A field guide to western reptiles and amphibians. Second edition. Houghton Mifflin Company, Boston, Massachusetts. xiv + 336 pp.
Waye, H. 1999. Size and age structure of a population of Western Terrestrial Garter Snakes (Thamnophis elegans). Copeia 1999:819-823.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2022-08-11 12:57:28 PM]
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