Common garter snake (Thamnophis sirtalis)

Description

Common garter snake fact file

• Description
• Biology
• Range
• Habitat
• Status
• Threats
• Conservation
• Find out more
• Glossary
• References
• Print factsheet

Common garter snake description

The common garter snake (Thamnophis sirtalis) is a long, slender North American snake with a colourful and extremely variable appearance. Its upperparts may range from black, brown or grey to green, olive or red, and there are normally three light stripes running along its length. These stripes can vary from yellowish to brown, green, blue, orange, grey or whitish, and are normally well defined, although they may occasionally be indistinct or even absent ^{(2) (3) (4) (5) (6)}.

The underparts of the common garter snake are usually greenish, yellow, blue-grey or whitish, and are sometimes marked with small black spots ^{(2) (3) (4) (5) (6)}. The top of the head is usually dark, with a pair of small, white or yellowish, dash-like spots ^{(3) (4) (5)}, and there are often dark vertical bars on the upper lip scales ^{(3) (5)}. The eyes of the common garter snake are quite large ⁽⁵⁾, and its tongue is red, with a black tip ⁽⁶⁾.

Many common garter snakes have two alternating rows of black spots between the stripes, which can be fused vertically to form dark bars, or horizontally to form lines. These spots can give some individuals a chequered appearance ^{(2) (3) (5) (6)}. In others, the area between the stripes may be so dark that no pattern is visible ^{(2) (5)}, or there may be red spots or bars ^{(3) (5)}. Melanistic common garter snakes also frequently occur ^{(2) (4) (6) (7)}.

In some parts of its range, the common garter snake has red or orange skin between the scales on its upperparts. This colouration can extend onto the edges of the scales and is most obvious when the snake’s skin is stretched, such as after a large meal. At these times, many white, dash-like marks are also visible on the skin ⁽³⁾.

The male common garter snake is generally smaller and more slender than the female, but has a proportionately longer tail ⁽⁶⁾.

Based on the common garter snake’s highly variable colouration and patterning, a number of subspecies have been described ^{(2) (4) (5) (8)}. However, more recent studies have found extreme variation in appearance even within populations, suggesting that subspecies based on colouration may be questionable ⁽⁷⁾.

Also known as

bluestripe garter snake, blue-striped garter snake, California red-sided garter snake, Chicago gartersnake, eastern garter snake, Maritime garter snake, New Mexico garter snake, Puget Sound garter snake, red-sided garter snake, red-spotted garter snake, San Francisco garter snake, Texas garter snake, valley garter snake.

Synonyms

Coluber sirtalis, Eutaenia sirtalis, Eutaenia sirtalis graminea, Eutaenia sirtalis semifasciata, Eutaenia sirtalis trilineata, Eutainia concinna, Eutainia dorsalis, Eutainia infernalis, Eutainia pickeringii, Eutainia sirtalis, Thamnophis dorsalis, Thamnophis sirtalis pallidula, Tropidonotus bi-punctatus, Tropidonotus concinnus, Tropidonotus obalskii, Tropidonotus ordinatus var. sirtalis.

Size

Length: 46 - 137 cm ^{(2) (3)}

Common garter snake biology

Although usually active during the day, the common garter snake may restrict its hunting activity to the early morning and evening on hot days, and it may also be partially nocturnal at times ^{(2) (3) (8)}. An excellent swimmer ⁽⁵⁾, the common garter snake frequently hunts by swimming slowly along the margins of pools, often sweeping its open mouth from side to side and seizing any prey it encounters ^{(2) (3)}. It may also track prey on land by scent and by sight ^{(3) (6)}, or catch earthworms by locating their casts and thrusting its snout into the tunnel below ⁽²⁾. The saliva of the common garter snake is slightly toxic, possibly helping to immobilise prey. However, its bite is generally harmless to humans ^{(3) (6)}.

The common garter snake takes a diverse range of prey, including fish, amphibians and their tadpoles, earthworms, leeches, and other aquatic and terrestrial invertebrates, such as insects, slugs and crayfish. It also hunts small mammals and birds, and occasionally eats carrion ^{(2) (3) (4) (5) (6)}. Earthworms and amphibians often make up the majority of its diet ^{(3) (4) (6)}.

Predators of the common garter snake include large fish, turtles, birds, mammals, bullfrogs, domestic animals and even other snakes ⁽⁶⁾. This species’ first line of defence is to escape into water or move away, but if caught it will try to smear its captor with excrement and a foul-smelling discharge from glands at the base of its tail ^{(3) (5) (6)}.

More tolerant of the cold than any other garter snake ^{(2) (5)}, the common garter snake is one of the last snake species to enter hibernation in cooler regions, and one of the first to emerge again in spring ^{(2) (3)}. In northern parts of its range, the common garter snake may be active from March to November ^{(2) (3) (6)}, but in more southerly locations it may leave the winter den to bask during warmer weather ^{(3) (6)}, or may be active year-round ⁽²⁾.

The winter hibernation sites of this species include rocky hillsides, mammal or crayfish burrows, stream banks, decaying logs, tree stumps, ant mounds, and man-made sites such as dams ^{(3) (6)}. In some areas, particularly in the north, large numbers of common garter snakes may share the same site, sometimes with other snake species ⁽³⁾. The mass emergence of common garter snakes in spring can then be followed by large mating aggregations, in which numerous males and females engage in frenzied courtship and mating behaviour ^{(2) (3)}. In other areas, the common garter snake may not hibernate in such large numbers. Some mating also occurs in the autumn, with the sperm remaining viable in the female until the following spring ^{(2) (3)}.

After mating, common garter snakes disperse to their summer feeding grounds, which can sometimes be some distance away from the hibernation site ^{(2) (3) (6)}. The female common garter snake usually gives birth to around 7 to 20 live young, although litter size can sometimes reach 70 or more ^{(3) (4)}. The young are born between May and November ^{(2) (5)}, and measure between 12.5 and 23 centimetres at birth ^{(3) (4) (6)}. 

The common garter snake reaches maturity at two to four years old, with males maturing at a smaller size than females ^{(2) (6)}. Near the northern limits of the common garter snake’s range, females rarely give birth in two successive years ⁽⁹⁾.

Common garter snake range

The common garter snake is the most widely distributed garter snake (Thamnophis) species, and its range also extends much further north than any other snake in the western hemisphere ⁽²⁾. 

This species occurs from southeast Alaska, across Canada, and south through most of the United States ^{(1) (2) (5) (6) (8)}. Isolated populations also occur in New Mexico in the U.S. and north-western Chihuahua in Mexico ^{(1) (2) (8)}.

Common garter snake habitat

Across its vast range, the common garter snake occupies a wide variety of habitats, although it generally prefers moist areas such as the margins of ponds, marshes, streams, lakes and ditches ^{(1) (2) (4) (5) (6)}. It can also be found in woodland, prairies, damp meadows, grassland, fallow fields, hillsides and even some urban habitats, such as parks, gardens and vacant lots ^{(2) (3) (4) (5) (6)}. The common garter snake has been recorded up to elevations of about 2,540 metres ⁽²⁾.

The habitat preferences of the common garter snake may vary between different regions ⁽¹⁾; for example, some western subspecies are more aquatic than those in the east ⁽²⁾. This species tends to stay close to water, and often escapes into it when alarmed ^{(3) (5)}. At other times, it takes shelter under a variety of objects, including logs, rocks, boards, metal sheets, piles of plant debris, or loose tree bark ^{(3) (6)}.

Common garter snake status

The common garter snake is classified as Least Concern (LC) on the IUCN Red List ⁽¹⁾.

Common garter snake threats

The common garter snake is a widespread and abundant species, and is not currently considered at risk of extinction ⁽¹⁾. There are not known to be any major threats to this species ⁽¹⁾, and it is able to adapt quite well to urban and suburban habitats ⁽⁶⁾.

However, although not threatened on a global scale, the common garter snake may face a number of localised impacts from human activities. Intensive urbanisation can result in the loss and alteration of its wetland habitats, and individuals are often killed by domestic animals such as dogs and cats, or by lawnmowers and traffic ^{(6) (10) (11)}. The use of pesticides and other chemicals may also affect the common garter snake ^{(6) (11)}, and its habitat can be further degraded by changes in water levels ^{(1) (11)}. This species is also commonly kept in captivity, and some populations may have been affected by collection for the pet and scientific trades ⁽²⁾.

In addition to these direct threats, the common garter snake may be indirectly affected by declines in its amphibian prey ^{(2) (11)}, due to habitat changes, disease, and predation by and competition with invasive species ⁽¹¹⁾.

One subspecies of the common garter snake, the San Francisco garter snake (Thamnophis sirtalis tetrataenia), is listed as ‘Endangered’ by the U.S. Fish and Wildlife Service ⁽¹²⁾. Restricted to the San Francisco Peninsula, this subspecies is threatened mainly by habitat loss and over-collection ^{(2) (10) (11)}.

Common garter snake conservation

The common garter snake occurs in many protected areas across its large range ⁽¹⁾. There are not known to be any specific conservation measures currently in place for the species as a whole, but a number of actions are underway for the threatened San Francisco garter snake (T. s. tetrataenia) ^{(10) (11)}.

A recovery plan is in place for this subspecies, and recommended conservation measures include habitat protection and restoration, studies into its life history and population trends, public awareness programmes and improved law enforcement ^{(10) (11)}. Captive breeding and ‘head starting’ programmes, in which young San Francisco garter snakes are temporarily reared in captivity during their most vulnerable life stage, may also help to enhance the wild populations of this attractive snake ⁽¹¹⁾.

Find out more

Read more about the common garter snake and other garter snake species:

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

Find out more about reptile conservation:

• International Reptile Conservation Foundation:
  http://www.ircf.org/

Authentication

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Glossary

Carrion

The flesh of a dead animal.

Gland

An organ that makes and secretes substances used by the body.

Hibernation

A winter survival strategy in which the animal passes the winter in a resting state. This period of inactivity is characterised by specific biological and biochemical changes including lowered blood pressure and respiration rate. In reptiles, this is also known as brumation.

Invertebrates

Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.

Melanism

In animals, the condition of having darkened skin, hair or feathers due to excessive production of the pigment melanin, usually resulting in the animal being entirely black.

Nocturnal

Active at night.

Subspecies

A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.

References

1. IUCN Red List (August, 2011)
   http://www.iucnredlist.org/
2. Rossman, D.A., Ford, N.B. and Seigel, R.A. (1996) The Garter Snakes: Evolution and Ecology. University of Oklahoma Press, Norman.
3. Werler, J.E. and Dixon, J.R. (2000) Texas Snakes: Identification, Distribution, and Natural History. University of Texas Press, Austin, Texas.
4. Conant, R. and Collins, J.T. (1998) A Field Guide to Reptiles and Amphibians of Eastern and Central North America. Third Edition, Expanded. Houghton Mifflin Company, New York.
5. Stebbins, R.C. (2003) A Field Guide to Western Reptiles and Amphibians. Third Edition. Houghton Mifflin Company, New York.
6. Harding, J.H. (1997) Amphibians and Reptiles of the Great Lakes Region. University of Michigan Press, Michigan.
7. Mooi, R.D., Wiens, J.P. and Casper, G.S. (2011) Extreme color variation within populations of the common gartersnake, Thamnophis sirtalis, in central North America, with implications for subspecies status. Copeia, 2011(2): 187-200.
8. The Reptile Database (August, 2011)
   http://reptile-database.reptarium.cz/search.php
9. Larsen, K.W., Gregory, P.T. and Antoniak, R. (1993) Reproductive ecology of the common garter snake Thamnophis sirtalis at the northern limit of its range. American Midland Naturalist, 129(2): 336-345.
10. U.S. Fish and Wildlife Service (1985) Recovery Plan for the San Francisco Garter Snake (Thamnophis sirtalis tetrataenia). U.S. Fish and Wildlife Service, Portland, Oregon. Available at:
    http://ecos.fws.gov/docs/recovery_plan/850911.pdf
11. U.S. Fish and Wildlife Service (2006) San Francisco Garter Snake (Thamnophis sirtalis tetrataenia). 5-Year Review: Summary and Evaluation. U.S. Fish and Wildlife Service, Sacramento, California. Available at:
    http://ecos.fws.gov/docs/five_year_review/doc774.pdf
12. U.S. Fish and Wildlife Service: Species Profile - San Francisco garter snake (Thamnophis sirtalis tetrataenia) (August, 2011)
    http://ecos.fws.gov/speciesProfile/profile/speciesProfile.action?spcode=C002