U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

Species Information

Photo of Giant Garter Snake

Photo Credit: Westervelt Ecological Services

Giant Garter Snake

Thamnophis gigas

Basic Species Information

STATUS

Threatened. The species is likely to become endangered in the foreseeable future throughout all or a significant portion of its range, but it is not in danger of extinction right now.

DESCRIPTION

The giant garter snake is not dangerous. It is in the family Colubridae, which includes most of the species of snakes found in the western United States.

The giant garter snake is one of the largest garter snakes. It is at least 64 inches. Common garter snakes are only about 18 to 55 inches.

Females tend to be longer and heavier than males, typically weighing about 1 to 1.5 pounds.

The back (dorsal background) of a giant garter snake varies from brownish to olive, with a checkered pattern of black spots. A yellow stripe runs down the center of the back. Along the sides are two light-colored stripes. The underside (ventral surface) is cream to olive or brown and sometimes infused with orange, especially in northern populations. Background coloration and prominence of a black checkered pattern and the three stripes are geographically and individually variable. Common garter snakes have red spots.

FOOD SOURCES

Giant garter snakes primarily feed on small fish, tadpoles, and frogs.

HABITAT

The giant garter snake inhabits agricultural wetlands and other waterways such as irrigation and drainage canals, sloughs, ponds, small lakes, low gradient streams, and adjacent uplands in the Central Valley.

Most of the snake’s natural habitat has been lost, which is why many giant garter snakes live in rice fields. Rice fields provide hundreds of thousands of acres of habitat for the species. See a California Rice Commission webpage for more information about how various species use the fields.

Giant garter snakes require enough water to provide food and cover during the active season, which is early-spring through mid-fall. Wetland plants such as cattails and bulrushes are used for cover and foraging. Grassy banks and openings in vegetation are for sunning.

Higher elevation uplands for cover and refuge from flood waters are required during the snake's inactive season in the winter.

HIBERNATION

Giant garter snakes are dormant during the winter so they inhabit small mammal burrows and other soil crevices above flood elevations during this inactive period. The snakes typically select burrows with sunny exposure along south and west facing slopes.

Around October 1, they start looking for winter retreats. By November 1, they are in winter retreats and mostly stay there until spring. Some may bask in the sun or move short distances on warmer days.

These snakes are most active from early spring through mid-fall. Between April 1 and May 1, they emerge and start hunting for food.

MATING

Males reach sexual maturity in three years, females in five. The breeding season extends through March and April, and females give birth to live young from late July through early September.

Brood size varies, ranging from 10 to 46 young. Young immediately scatter into dense cover and absorb their yolk sacs, after which they begin feeding and foraging on their own. They typically more than double in size in the first year.

RANGE

Because of the direct loss of natural habitat, the giant garter snake relies heavily on rice fields in the Sacramento Valley, but also uses managed marsh areas in Federal National Wildlife Refuges and State Wildlife Areas. There have been only a few recent sightings of giant garter snakes in the San Joaquin Valley.

Giant garter snakes are typically absent from larger rivers due to the lack of suitable habitat and emergent vegetative cover, and from wetlands with sand, gravel, or rock substrates. Riparian (river bank) woodlands typically do not provide suitable habitat because of excessive shade, lack of basking sites, and absence of prey populations. The major rivers have been highly channelized, removing oxbows and backwater areas that at one time likely provided suitable habitat.

PREDATORS

Predators include raccoons, skunks, opossums ("possums"), foxes, hawks, northern harriers, egrets, bitterns, and great blue herons.

THREATS

Habitat loss and fragmentation, flood control activities, changes in agricultural and land management practices, predation from introduced species, road mortalities, water pollution, and continuing threats are the main causes for the decline of this species.

Giant garter snakes can inhabit water bodies that contain predatory fish. When a lot of cover is available, they tend to hold their own, even when numerous predators share the same habitats.

WHAT YOU CAN DO

Learn more about snakes. Visit a local zoo or nature center. If you are lucky enough to see a giant garter snake, just look, don't touch.

All endangered and threatened species need your help. Private citizens can play a critical role in protecting our country’s wildlife and plants. See What You Can Do to Help Wildlife and Plants (201 KB PDF) for more specific ideas about how to help.

Need more specifics? Download the Giant Garter Snake scientific species account.

Last updated: November 30, 2017