Not Dangerous (Non-poisonous) - This snake does not have venom that is dangerous to most humans.
Gartersnakes have toxins in their saliva which can be deadly to their prey and their bite might produce an unpleasant reaction in humans, but they are not considered dangerous to humans.
The largest species of gartersnake, adults from 36 - 65 inches long (91 - 165 cm). Snakes encountered are typically 36 - 48 inches in length (91 - 122 cm).
Neonates are 8.5 - 11.5 inches long (22 - 29cm).
A large snake with with keeled dorsal scales and a head slightly wider than the neck.
Color and Pattern
Ground color is brown or olive to black.
The underside is light brown or light grayish.
There is typically a yellowish dorsal stripe, a light yellowish stripe on each side, and two rows of dark blotches on the sides.
Snakes at the northern end of the range in the upper Sacramento Valley tend to have distinct stripes and a dark ground color.
Snakes in the San Joaquin Valley may also have indistinct stripes or no stripes, creating a checkered appearance.
Active during daylight, and at night in hot weather.
Secretive and difficult to approach, this snake will quickly drop into the water from its basking site and dive to the bottom before the observer can get close.
This behavior probably derives from the habitat of this snake which is often open and treeless and the presence of many airborne predators such as egrets, herons, and hawks.
Emerges from overwintering sites in March.
Basks on vegetation near water in spring, and utilizes animal burrows and vegetation piles during hotter weather.
Some snakes active until October.
Overwinters in animal burrows.
When threatened or picked up, this snake will release its cloacal contents and excrete a foul-smelling musk.
Diet and Feeding
Feeds primarily on aquatic fish, frogs and tadpoles.
Historical prey has been extirpated in much of this snake's range, leaving it to consume introduced fish and bullfrogs.
Mating takes place soon after emergence in the Spring.
Females bear live young from July through early September.
Found primarily in marshes, sloughs, drainage canals, and irrigation ditches, especially around rice fields, and occasionally in slow-moving creeks.
Prefers locations with vegetation close to the water for basking.
Endemic to California.
Historically, this snake ranged from Kern County north along the Central Valley to Butte County, with a gap in the central part of the valley. Currently, ranges from Glenn County to the southern edge of the San Francisco Bay Delta, and from Merced County to northern Fresno County, apparently no longer occurring from south of northern Fresno County.
From sea level to 400 ft. (122 m).
Notes on Taxonomy
Formerly classified as a subspecies of Thamnophis ordinoides, and later of Thamnophis couchii.
Recognized as a full species by Rossman and Stewart in 1987.
Apparently absent from an estimated 98 percent of former habitat in the San Joaquin Valley. Listed as a threatened species due to loss of habitat and introduced predatory fish. This snake's habitat has been destroyed and seriously fragmented largely due to the loss of or degredation of wetlands in the Central Valley due to an extensive system of dams in the Sierra Nevada Mountains, and dikes and draining for agriculture.
Protected waterfowl habitats in wildlife refuges are an important source of habitat for this snake, but they do not necessarily provide good habitat for this snake when they are flooded in winter and drained in summer, the opposite of this snake's needs, which resulted from the summer flooding of valley wetlands due to snow melt from the Sierra Nevada Mountains, and their drying up in winter. Rice fields and irrigation ditches, which are both flooded in summer, are now providing good habitat for this snake.
Pesticide and fertilizer runoff from agriculture are also responsible for killing some of this snake's prey, including native Red-legged frogs. Grazing of vegetation along water sources also threatens this snake. Introduced watersnakes (Nerodia) in the Folsom area could possibly threaten this snake if they were to spread downriver into the valley.
North American Gartersnakes
Thamnophis gigas - Fitch, 1940 - Univ. California Publ. Zool., Vol. 44, p. 69
Rossman, Douglas A., Neil B, Ford, & Richard A. Siegel. The Garter Snakes - Evolution and Ecology. University of Oklahoma press, 1996.
Thelander, C. G., ed. Life on the Edge: A Guide to California's Endangered Natural Resources Volume I: Wildlife.
Santa Cruz, California. Biosystems Books, 1994.
Stebbins, Robert C., and McGinnis, Samuel M. Field Guide to Amphibians and Reptiles of California: Revised Edition (California Natural History Guides) University of California Press, 2012.
Stebbins, Robert C. California Amphibians and Reptiles. The University of California Press, 1972.
Samuel M. McGinnis and Robert C. Stebbins. Peterson Field Guide to Western Reptiles & Amphibians. 4th Edition. Houghton Mifflin Harcourt Publishing Company, 2018.
Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. 3rd Edition. Houghton Mifflin Company, 2003.
Behler, John L., and F. Wayne King. The Audubon Society Field Guide to North American Reptiles and Amphibians. Alfred A. Knopf, 1992.
Powell, Robert., Joseph T. Collins, and Errol D. Hooper Jr. A Key to Amphibians and Reptiles of the Continental United States and Canada. The University Press of Kansas, 1998.
Bartlett, R. D. & Patricia P. Bartlett. Guide and Reference to the Snakes of Western North America (North of Mexico) and Hawaii. University Press of Florida, 2009.
Bartlett, R. D. & Alan Tennant. Snakes of North America - Western Region. Gulf Publishing Co., 2000.
Brown, Philip R. A Field Guide to Snakes of California. Gulf Publishing Co., 1997.
Ernst, Carl H., Evelyn M. Ernst, & Robert M. Corker. Snakes of the United States and Canada. Smithsonian Institution Press, 2003.
Wright, Albert Hazen & Anna Allen Wright. Handbook of Snakes of the United States and Canada. Cornell University Press, 1957.
Karen J. Miller, Kelly Hornaday, U.S. Fish and Wildlife Service Sacramento Fish and Wildlife Office and
The Giant Garter Snake Recovery Team. DRAFT RECOVERY PLAN FOR THE GIANT GARTER SNAKE (Thamnopsis gigas). U.S. Fish and Wildlife Service (1999)
Glenn D. Wylie and Michael L. Casazza. Investigations of Giant Garter Snakes in the Natomas Basin: 2001 Field Season.
USGS-BRD, Western Ecological Research Center Dixon Field Station
The following conservation status listings for this animal are taken from the November 2020 California "Special Animals List" and the November 2020 "State and Federally Listed Endangered and Threatened Animals of California" list, both of which are produced by multiple agencies and available here: https://www.wildlife.ca.gov/Data/CNDDB/Plants-and-Animals. You can check the link to see if there are more recent lists.
If no status is listed here, the animal is not included on either list. This most likely indicates that there are no serious conservation concerns for the animal. To find out more about an animal's status you can go to the NatureServe and IUCN websites to check their rankings.
The 2019 Threatened and Endangered List shows this comment: Listed by State of California as Thamnophis couchi gigas.
NatureServe Global Ranking
Imperiled - At high risk of extinctiion due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors.
NatureServe State Ranking
Imperiled in the state because of rarity due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors making it very vulnerable to extirpation from the state.