Giant Garter Snake

Status

Giant garter snake (Thamnophis gigas) is Federally-listed as Threatened under the Federal Endangered Species Act (October 20, 1993) and State-listed as Threatened under the California Endangered Species Act (June 27, 1971). This species historically ranged from Kern County north along the Central Valley to Butte County, with a gap in the central part of the valley. Habitat for giant garter snake has been substantially reduced throughout the Central Valley as a result of the loss or degradation of wetlands and introduction of predatory fish.

Description

Giant garter snake is the largest species of garter snake and adults range from 36 to 65 inches in length. Females tend to be slightly longer and proportionately heavier that males and typically weigh between 1.0 and 1.5 pounds. Adult giant garter snakes have keeled dorsal scales and a head that is slightly wider than the neck. Dorsal background color varies from brownish to olive with a checkered pattern of black spots, separated by a yellow dorsal stripe and two light colored lateral stripes. Background coloration and prominence of the three light stripes varies geographically and between individuals. The underside of the snake is cream to olive or brown and sometimes includes a small amount of orange coloration, especially in northern populations. Snakes at the northern end of their range in the upper Sacramento Valley tend to have distinct stripes and a darker overall color. Snakes in the San Joaquin Valley may also have indistinct stripes or no stripes, creating a checkered appearance.

Habitat Requirements

Giant garter snake is a highly aquatic snake that is active during the daytime and at night during periods of hot weather. This species occurs in a variety of freshwater marshes, slow-moving creeks and sloughs and has adapted to agricultural wetlands and various other man-made waterways including irrigation ditches and drainage canals with mud bottoms. Because of the direct loss of natural habitat, the giant garter snake relies heavily on rice fields of the Sacramento Valley, but also uses managed marsh areas in Federal Wildlife Refuges and State Wildlife Areas. Habitat requirements include the presence of adequate surface water and emergent vegetation, such as cattails and bulrush, throughout the active season (early-spring through mid-fall), openings in the vegetation for basking, and adequate nearby upland vegetation for use during the dormant season. This species will typically inhabit small mammal burrows and other soil crevices, located on west and south facing slopes, during the rainy season and dormancy period. Breeding activity usually occurs during March and April. Live young are born from late July through early September in secluded sites such as under loose bark, rotting logs, or in dense vegetation adjacent to water. Following birth, young immediately disperse to nearby dense cover.

The diet of giant garter snake includes small fish, tadpoles and frogs. Natural predators include raccoons, skunks, foxes, northern harriers, hawks and a variety of wading birds. The non-native bullfrog is also known to prey on young garter snakes.

Local Occurrence

WHF preserves that support giant garter snakes include Gilsizer Slough, Ridgecut, South Stone Lakes, and Tule Basin.