The Northwestern Garter Snake is the smallest garter snake in British Columbia and can range in size from 355 to 750 mm in snout vent length, although they are usually less than 600 mm (Matsuda et al. 2006). Males are smaller in head dimensions and body size than females, although they have longer tails relative to body length.
Northwestern Garter Snakes exhibit great colour variation; however, most are non-shiny brown to grey in colour, often with a slight checkering on the dorsal surface. Some individuals are melanistic and are uniformly black, and albino individuals (all white/cream in colour) have been found. Dorsal and lateral body stripes range in colour from pale yellow to bright orange, and may be partial or indistinct in many individuals. Ventral scutes range in colour from white to light yellow to black and sometimes have red to orange blotches on the surface. The head is small and not distinct from the neck, eyes are small with round pupils, and the snout is often referred to as blunt. There are usually 7 white upper labials, often with red to black blotch markings. Dorsal body scales are lightly keeled and have up to 17 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. 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.
Biology
Reproduction
Northwestern Garter Snakes are viviparous (i.e., live bearing); reproduction occurs in the spring (usually at the end of April to early May) or fall depending on the population. Females may breed annually, biennially, or even less frequently depending on body condition (Gignac and Gregory 2005). Not much is known about mating in these snakes as they have not been studied as frequently as other species of garter snakes. Females give birth to live young in mid to late summer (July to September); litter sizes are generally under 20 offspring. Neonates range in size from 110 to 150 mm in length.
Diet
Northwestern Garter Snakes are less generalized in their diets than other species of garter snakes as they forage mainly within the terrestrial environment. Common prey items include slugs, worms, and small frogs (Matsuda et al. 2006).
Behaviour
Northwestern Garter Snakes are more active during the day and can frequently be seen foraging in 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 under or near cover. Snakes that are undergoing ecdysis (i.e. shedding their skin) will also generally be less active and will often remain under cover.
Seasonal Charactertistics
Like other species of garter snake in B.C., Northwestern Garter Snakes are ectothermic and are inactive in the winter season (November-March). During this period, they must find over-wintering sites (i.e. hibernacula) that are below the frost level. Den sites for this species may include deep rock crevices, crevices, or existing mammal burrows.
Habitat
Northwestern Garter Snakes are primarily terrestrial, occurring in a variety of habitats including meadows, forest clearings, and along the edges of thickets (Rossman et al. 1996, Matsuda et al. 2006). In coastal areas, these snakes will often occur at the edges of estuaries or beaches. Garter snakes rely on adequate cover from predators and are likely to spend much of their time thermoregulating under various types of cover (e.g., coarse woody debris, dense bushes, rocks). Like other garter snakes, this species will hibernate singly or communally during the winter months. Hibernation sites typically include talus slopes or deep rock crevices.
Distribution
Northwestern Garter Snakes have the smallest distribution of the three garter snakes that occur in British Columbia, ranging from southwest British Columbia down through western Washington, Oregon and California. In British Columbia, these snakes are found on Vancouver Island (including many Gulf Islands), and in the lower mainland and Thompson regions, with the largest populations occurring in the immediate coastal areas (southern Vancouver Island and the Fraser Valley). There is one isolated record of this species occurring near Kimsquit, on the Dean Channel (Matsuda et al. 2006).
Conservation
This species is listed as being common in most parts of its range and is yellow listed in British Columbia (G5 – Secure; S4; Not at Risk COSEWIC 2003).
Taxonomy
This snake has no recognized subspecies (Rossman et al. 1996).
Brodie III, E.D. 1989. Genetic correlations between morphology and antipredator behaviour in natural populations of the garter snake Thamnophis ordinoides. Nature 342:542-543.
Brodie III, E.D. 1993. Consistency of individual differences in anti-predator behaviour and colour pattern in the garter snake, Thamnophis ordinoides. Animal Behavior 45:851-861.
Gignac, A., and P.T. Gregory. 2005. The effects of body size, age, and food intake during pregnancy on reproductive traits of a viviparous snake, Thamnophis ordinoides. Ecoscience 12:236-243.
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.
Rossman, D.A., N.B. Ford, and R.A. Seigel. 1996. The Garter Snakes: Evolution and Ecology. University of Oklahoma Press, London.
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:
2024-10-14 5:38:59 PM]
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