B.C. Reptiles & Amphibians

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Credit: Laura Matthias

Common Gartersnake

Thamnophis sirtalis

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Description

Other names: Rana aurora aurora

The Common Gartersnake is a medium-sized, diurnal snake that will grow to a mature size from 46 – 130 cm; snakes that occupy more northerly habitats tend to grow larger than snakes in the south. Males are typically a fair amount smaller than females, as female fecundity increases with body size.

They have a large head with 7 yellow upper labial scales and distinct eyes that have large, round pupils. The body is long and slender and is black or gray-green with a bright yellow-green dorsal stripe and red barring along the sides with lateral stripes just below. The scales on the dorsal body are strongly keeled. The belly is typically pale yellow but may also be dark or black, with the chin and neck having the lightest colour.

When preparing for ecdysis the colouration will become dull, and the eyes will glaze over blue-gray. Juvenile Common Gartersnakes have the same colouration and markings as adults, although they are typically brighter and around 20 cm at birth.

Listen to the Indigenous words for “snake” here!

Similar Species

The Common Gartersnake may be easily confused with the other species of Garter Snake in British Columbia; the Western Terrestrial Gartersnake and the Northwestern Gartersnake. Common and Northwestern Gartersnakes will typically only have 7 scales on the upper lip, whereas Western Terrestrial Gartersnakes will typically have 8, with one of them being enlarged. The western sub-species of Common Gartersnake that occurs in British Columbia usually has red bars on the sides between the dorsal and lateral stripes. Overall, Western Terrestrial and Northwestern Gartersnakes are generally lighter in colour and tend to lack red bars on the sides.

Western Terrestrial Gartersnake

Common Gartersnake

Northwestern Gartersnake

Distribution

The Common Gartersnake is the most widespread snake in Canada, and reaches more northerly latitudes than any other snake species in North America. In North America it ranges very broadly from the southern United States to slightly north of 60 degrees latitude in the Northwest Territories. In British Columbia, the Common Gartersnake is abundant and widespread in the central and southern portions of the province and reaches as far north as the Bulkley, Nass, and Peace River regions. Common Gartersnakes occur at their highest densities in the southern interior and the south coast of the mainland and Vancouver Island.

There are three recognized subspecies of the Common Gartersnake in British Columbia.

The Puget Sound Gartersnake (Thamnophis sirtalis pickeringi) – found on Vancouver Island and throughout the Lower Mainland.
The Valley Gartersnake (Thamnophis sirtalis fitchi) – found across southern B.C. and the northern coast to Yukon.
The Red-sided Gartersnake (Thamnophis sirtalis parietalis) – found in Eastern B.C. along the Rockies and extends east towards the prairies
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Habitat

Common Gartersnakes spend the winters underground in hibernacula. In Canada, Common Gartersnakes may travel several kilometers to and from their hibernacula which are usually rocky crevices or caverns, or old mammal burrows below the freezing line. In much of Canada, overwintering sites are limited, which will often lead to many individuals (sometimes hundreds!) in the same den. Dens may be shared with other species of Gartersnake, Western Yellow-bellied Racers, Rubber Boas, Great Basin Gophersnakes, and Western Rattlesnakes.

On Vancouver Island where the winter conditions are relatively mild, Common Gartersnakes may hibernate alone or in smaller groups, as there is more available denning habitat due to the warmer temperatures and lack of snow. Common Gartersnakes emerge from hibernacula in the spring and are generally active from March through to November, depending on the region.

Common Gartersnakes are successful in a broad array of habitats including grasslands, forests, subalpine, and urban areas, but in all of these habitats they are most commonly associated with aquatic environments like wetlands, ponds, marshes, river valleys, or urban waterways. In all of these habitats, cover is an important microhabitat feature for providing shelter from direct sunlight and predators.

Reproduction

Common Gartersnakes generally mate in the spring, and typically very close to the hibernacula. Males will emerge from the overwintering dens first, followed shortly after by the females. Males typically pursue a single female. Attracted to them by their scent, they will often begin courting a female before she has even fully emerged from the den. The male will display his interest by rubbing along the length of the female’s body. In some cases, females may be very in-demand, leading to a ‘mating ball’ of many males attempting to mate with a single female. Once the female has mated, a “copulatory plug” forms in her reproductive tract that is supposed to prevent other males from mating with her. Males appear able to sense the presence of a plug and tend not to pursue females that have already mated. However, some females will still mate with multiple males, leading to litters that may have multiple sires. Common Gartersnakes are ovoviviparous, and they will give birth to fully developed young that are ready to tackle the world. Common Garter Snakes will typically give birth to 10-15 young in mid-summer (July-August). However, Common Garter Snake fecundity is reliant on body size, and particularly large snakes may have as many as 70-80 young, and small snakes as few as 5. Neonatal snakes are very small (14 – 22 cm) but completely independent at birth. Females generally begin to reproduce in their second or third year, and will reproduce every 1-3 years depending on size and available resources.

Diet

Common Gartersnakes are primarily diurnal hunters, but they are also not averse to foraging during the night and their active periods tend to vary with the season. Gartersnakes are very active foragers, and as such, prefer to inhabit areas with a high prey density. Thus, to find Gartersnakes seek out areas with lots of their favourite foods! Amphibians are the favourite prey items of Gartersnakes, giving them an affinity for shallow wetlands. Adult Gartersnakes prey on slugs, frogs, toads, salamanders, tadpoles, small fish, and insects. More terrestrial individuals will seek out small mammals, birds, and other reptiles. Newly born Gartersnakes are capable of feeding immediately after birth, but appear to subsist almost entirely on earthworms or newly metamorphosed frogs until they are large enough to hunt more difficult prey items.

Common Gartersnakes are able to handle and eat prey that are too toxic and unpalatable for other predators. They can eat toxic Rough-skinned Newts and poisonous Western Toads without major adverse effects. However, prey with particularly potent toxins appear to send Common Gartersnakes into a ‘daze’ that makes them so sluggish that they may appear drunk.

Conservation Status

Global: G5 (2016)

Provincial: S5 (2018)

BC List: Yellow

Learn more about conservation status rankings here

Threats

The Common Gartersnake is the most abundant snake in Canada. Due to its large population, wide distribution, high local densities, and apparent adaptability, the Common Gartersnake is thought to be secure and not at high risk of extinction as long as hibernation sites and wetlands for foraging remain available. The top concern for Common Gartersnakes in Canada is road mortality, especially if mortality rates begin to cause large-scale local declines or extirpations. Habitat loss and fragmentation is also a concern, although Common Gartersnakes appear able to persist and tolerate low-moderate human disturbance.
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Did You Know?

Fact #1

Common Gartersnakes can fare quite well in cities, and are often found near parks and waterways. Because of their abundance and diurnal behaviour, they are one the most commonly encountered and familiar snakes in British Columbia.

Fact #2

Birds, toads, and mice seem like massive meals for such a slender snake. Thankfully, Gartersnakes have a kinetic skull (a skull that has movable joints) that allows snakes to adjust the geometry of their head and jaws to swallow very large food items.

Fact #3

Common Gartersnakes appear to be able to orient themselves according to the position of the sun. This may help them navigate between their summer foraging grounds and their hibernacula.

Species Account Author: Marcus Atkins

References

B.C. Conservation Data Centre. 2010. Species Summary: Thamnophis sirtalis. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 14, 2021). 
B.C. Conservation Data Centre. 2011. Conservation Status Report: Thamnophis sirtalis. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 14, 2021). 
B.C. Conservation Data Centre. 2021. BC Species and Ecosystems Explorer. B.C. Minist. of Environ. Victoria, B.C. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 14, 2021). 
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 characteristics 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. 
https://www.bcreptiles.ca/snakes/comGarter.htm 
http://linnet.geog.ubc.ca/efauna/Atlas/Atlas.aspx?sciname=Thamnophis%20sirtalis 
http://canadianherpetology.ca/species/species_page.html?cname=Valley%20Gartersnake