Thamnophis gigas - Giant Gartersnake

Nonvenomous

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.

Size

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).

Appearance

A large snake with with keeled dorsal scales and a head slightly wider than the neck. Ground color is brown or olive to black. 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. The underside is light brown or light grayish.

Behavior

HIghly aquatic. 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

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.

Reproduction

Mating takes place soon after emergence in the Spring. Females bear live young from July through early September.

Range

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 occuring from south of northern Fresno County.

Habitat

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. From sea level to 400 ft. (122 m).

Taxonomic Notes

Formerly classified as a subspecies of Thamnophis ordinoides, and later of Thamnophis couchii. Formerly recognized as a full species by Rossman and Stewart in 1987.

Conservation Issues (Conservation Status)

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.