Giant garter snake (Thamnophis gigas)

Also known as: giant gartersnake
Synonyms: Thamnophis couchii gigas, Thamnophis ordinoides gigas
GenusThamnophis (1)
SizeTotal length: up to 163 cm (2) (3)
Weightup to 0.7 kg (4)

The giant garter snake is classified as Vulnerable (VU) on the IUCN Red List (1).

The largest of the garter snake species (2) (3) (4), the attractively striped giant garter snake (Thamnophis gigas) is brown to olive in colour, with a yellow stripe down the back and two light-coloured lateral stripes down the sides. There may also be a chequered pattern of black spots, although the background colouration and the prominence of the stripes and spots vary both between individuals and between different populations. The underside of the body is cream to olive, greyish or brown, sometimes with an orange tinge, particularly in northern populations (3) (4) (5) (6) (7), and the head is elongated, with a long, narrow snout (3) (6).

The female giant garter snake tends to be slightly longer and heavier than the male (4), and may have a longer snout (3).

The giant garter snake was formerly classified with the western aquatic garter snake, Thamnophis couchii, but is now considered a distinct species (1). It can be distinguished from the western aquatic garter snake mainly by its larger size, and from the common garter snake (Thamnophis sirtalis) by the lack of red markings on the sides (5) (6).

The giant garter snake is found in the western United States, where it historically occurred in the Central Valley (Sacramento and San Joaquin valleys) of California, from Butte County in the north to Kern County in the south, at elevations of up to 122 metres (1) (2) (3) (5) (6). However, it has now been lost from most of its original range in the San Joaquin Valley, with only a few recent sightings in the area (1) (3) (4) (5).

This highly aquatic snake inhabits marshes, sloughs, slow-moving streams, ponds and small lakes, as well as agricultural wetlands, and irrigation and drainage canals. Vegetation, such as cattails and bulrushes, is needed for cover and for basking, and the species is usually absent from larger rivers because of a lack of suitable habitat and emergent vegetation, and possibly because of introduced predatory fish (1) (3) (4) (5) (6).

The loss of much of its natural habitat has led the giant garter snake to rely heavily on rice fields and managed marsh areas (1) (4) (6).

The giant garter snake lives a highly aquatic lifestyle, rarely being found away from water (1) (3) (5), where it is an active hunter (8), foraging mainly for fish, amphibians and their larvae (3) (4) (5) (6). The snake may take advantage of pools which trap and concentrate prey (6), and ground-nesting birds and their young may also be taken (5). The predominant prey may now be introduced species such as carp, mosquitofish and bullfrogs, as many of its former prey species have now disappeared (3). When alarmed, the giant garter snake may expel cloacal secretions that render it unpalatable to potential attackers (6).

The giant garter snake is active from early spring to mid-autumn, spending the winter dormant within an animal burrow or other soil crevice (3) (4) (6). Males immediately begin searching for a mate after emerging in the spring, with the breeding season lasting from March to May. The female gives birth to live young, from late July to early September, with brood size ranging from around 10 to 46. The young garter snakes measure approximately 20 centimetres, and immediately scatter into dense cover after birth (3) (4) (5) (6). The giant garter snake reaches maturity at around three years in males and up to five years in females (3) (4) (6).

The giant garter snake has been lost from much of its former range as a result of the loss, fragmentation and degradation of its wetland habitats, due, for example, to flood control, pollution, changes in agricultural and land use practices, and overgrazing in adjacent habitats (1) (3) (4) (5) (6). Contamination with metals such as mercury and selenium may potentially decrease survival rates (1) (9), and the snake may also fall prey to introduced predators such as cats, bullfrogs, and large predatory fish (1) (3) (4) (6).

The total population of the giant garter snake is currently unknown, but its small, fragmented and declining range makes the species increasingly vulnerable to extinction (1).

A recovery plan drafted for the giant garter snake in 1999 recommended a number of conservation actions for the species, including the protection of existing populations and habitat, restoration of former habitat, population surveys and monitoring, further research into the species, and outreach and incentive programmes (6). The species is listed as Threatened by the U.S. Fish and Wildlife Service (7), and occurs in a number of wildlife refuges (1) (7).

The protection of waterfowl habitats in the Delta and Sacramento valleys may also be important to the giant garter snake's survival (1) (5). It is believed that a number of other threatened species in the area would also benefit from the implementation of the various conservation measures recommended for this beautiful snake (6).

To find out more about the conservation of the giant garter snake and of other reptiles see:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

  1. IUCN Red List (February, 2010)
  2. The Reptile Database (February, 2010)
  3. Rossman, D.A., Ford, N.B. and Seigel, R.A. (1996) The Garter Snakes: Evolution and Ecology. University of Oklahoma Press, Norman.
  4. U.S. Fish and Wildlife Service, Sacramento Fish and Wildlife Office: Species Account - Giant Garter Snake, Thamnophis gigas (February, 2010)
  5. Stebbins, R.C. (2003) A Field Guide to Western Reptiles and Amphibians. Third Edition. Houghton Mifflin Company, New York.
  6. U.S. Fish and Wildlife Service. (1999) Draft Recovery Plan for the Giant Garter Snake (Thamnophis gigas). U.S. Fish and Wildlife Service, Portland, Oregon.
  7. U.S. Fish and Wildlife Service: Species Profile - Giant Garter Snake (Thamnophis gigas) (February, 2010)
  8. Halliday, T. and Adler, K. (2002) The New Encyclopedia of Reptiles and Amphibians. Oxford University Press, Oxford.
  9. Wylie, G.D., Hothem, R.L., Bergen, D.R., Martin, L.L., Taylor, R.J. and Brussee, B.E. (2009) Metals and trace elements in giant garter snakes (Thamnophis gigas) from the Sacramento Valley, California, USA. Archives of Environmental Contamination and Toxicology, 56: 577-587.