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Canadian Herpetological Society Société d'Herpétologie du Canada |
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Northwestern Gartersnake
Thamnophis ordinoides
Family: Colubridae
COSEWIC status:- Not at Risk
- No Status
- Least Concern
Description
The Northwestern Gartersnake is highly variable in colour and pattern but is usually brown, black or olive-coloured with three yellow stripes, one down the back (dorsal stripe) and one on each side (lateral stripes). The lateral stripes are on the second and third scale rows. The dorsal or side stripes may also be red, white or bluish or may be absent on some individuals. There are usually two rows of small black spots or blotches running down each side between the dorsal and side stripes; these spots do not invade the dorsal stripe, resulting in a smooth-edged dorsal stripe. The snout is relatively blunt and the upper lip is pale or white and usually has seven scales. The belly is pale, usually yellowish-white or gray, often with red blotches. The Northwestern Gartersnake is relatively small, with adult lengths typically less than 60 cm, though they have been known to reach 96 cm in total length.
Similar Species
The Terrestrial Gartersnake usually has eight scales on its upper lip, two of which are enlarged, and rows of black spots that invade the dorsal stripe, often causing the dorsal stripe to have a jagged or wavy edge. The two subspecies of the Common Gartersnake in British Columbia that overlap with the range of the Northwestern Gartersnake are the Valley Gartersnake and the Puget Sound Gartersnake. These species are much larger, are more apt to be black in background colour and often have red bars on the sides between the dorsal and side stripes.
Distribution
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In Canada the Northwestern Gartersnake occurs along the south coast of British Columbia, as well as most of Vancouver Island and many of the intervening Gulf Islands. In the United States it occurs along the West coast from Canada to northern California.
Habitat
Northwestern Gartersnakes occur in a variety of terrestrial habitats but tend to be associated with dense thickets, forest clearings and meadows. This species may be found near water but it rarely uses aquatic habitats. Northwestern Gartersnakes are commonly found under cover objects, such as rocks and logs, which provide important microhabitat for shelter and thermoregulation. They overwinter — often communally — below the frost line in mammal burrows, rock crevices, talus slopes and anthropogenic structures (e.g. old foundations, cisterns).
Biology
Breeding occurs either in the spring or fall. As in all Gartersnakes, females no not lay eggs but give birth to their young. From 3–20 young are born during the summer. The young are 15–18 cm in total length at birth. The Northwestern Gartersnake primarily eats slugs and earthworms, but may also eat fish, amphibians, small mammals and eggs. This species can live for more than twenty years.
Threats
Road mortality can be a significant threat to Northwestern Gartersnake populations located near busy roads. Although intensive habitat loss is a threat to all snakes, this species is able to persist in areas with low to moderate human disturbance. Human persecution and subsidized predation may also present a risk to this species in areas of high human density.