Giant Garter Snake

views updated

Giant Garter Snake

Thamnophis gigas

ListedOctober 20, 1993
DescriptionLarge brownish snake with black spots, separated by a yellow dorsal stripe.
HabitatMarshes, sloughs, ponds, small lakes, low gradient streams, drainage canals and rice fields.
FoodSmall fishes, tadpoles, and frogs.
ReproductionBrood of 10-46 young.
ThreatsUrbanization, flooding, contaminants, agricultural and maintenance activities, and introduced predators.


The giant garter snake, Thamnophis gigas, reaches a total length of a least 64 in (162 cm). Females are slightly longer and heavier than males. Viewed dorsally, the background coloration varies from brown to olivaceous with a checkered pattern of black spots, separated by a yellow dorsal stripe and two light colored lateral stripes. Individuals in the northern Sacramento Valley tend to be darker with more pronounced mid-dorsal and lateral stripes. The ventral surface is cream to olivaceous or brownish and is sometimes infused with orange, especially in northern populations.


Upon emergence from the burrow after the dormancy period, males immediately search for a mate. The breeding season extends through March and April, and females give birth to live young from late July through early September. Brood size is variable, ranging from 10 to 46 young. At birth, young average about 8 in (21 cm) snout-vent length and 11 oz (305 g). Young immediately scatter into dense cover and absorb their old sacs, after which they begin feeding on their own. Although growth rates are variable, young typically more than double in size by one year. Sexual maturity occurs about three years in males and five years in females.

The giant garter snake feeds on small fishes, tad-poles, and frogs.

This species is active in the early-spring through mid-fall; in the winter the giant garter snake experiences dormancy.


Endemic to valley floor wetlands in the Sacramento and San Joaquin Valleys of California, the giant garter snake inhabits marshes, sloughs, ponds, small lakes, low gradient streams, and other waterways and agricultural wetlands, such as irrigation and drainage canals and rice fields.

Habitat requirements consist of:

  1. Adequate water during the snake's active season.
  2. Emergent, herbaceous wetland vegetation, such as cattails and bulrushes, for escape cover and foraging habitat during the active season.
  3. Grassy banks and openings in water, side vegetation for basking.

4) Higher elevation uplands for cover and refuge from flood waters during the snake's dormant season in the winter.

Throughout its winter dormancy period, the giant garter snake inhabits small mammal burrows and other crevices above flood elevations. This species is known to select burrows with sunny aspects along south and west facing slopes.


Historically, this species extended from Sacramento and Contra Costa Counties southward to Buena Vista Lake, near Bakersfield in Kern County. Prior to 1970, the giant garter snake was known from 17 locations. Five of the localities were around Los Banos, Merced County. This snake has been extirpated from Buena Vista Lake and Kern Lake in Kern County, and from Tulare Lake and other riparian areas in Kings and Tulare Counties.

In 1970, the range of the giant garter snake extended from Fresno County northward to Butte County. There have been no sightings of the species from Fresno County northward to San Joaquin County after 1980.

Giant garter snake populations currently range from rice production zones in Sacramento, Sutter, Butte, Colusa, and Glenn Counties; along the western border of the Yolo Bypass in Yolo County; and along the eastern fringes of the Sacramento-San Joaquin River delta from the Laguna Creek-Elk Grove region of central Sacramento County southward to the Stockton area of San Joaquin County. Thirteen populations have been identified using locality records collected since the mid-1970s.


The giant garter snake is currently threatened by habitat loss and effects from urbanization, flooding, contaminants, agricultural and maintenance activities, and introduced predators.

A number of land use practices and other human activities currently threaten the survival of the giant garter snake throughout its remaining range. Although some populations have persisted at low population levels in artificial wetlands associated with agricultural and flood control activities, many of these altered wetlands are now threatened with urban development. Examples of these activities include: a new city proposed in San Joaquin County which would threaten known or potential habitat for the Badger and Willow Creek population; the Sacramento Metropolitan Area Investigation, a 400-year flood protection project proposed by the Corps of Engineers and local governments for over 8,000 acres (3,240 hectares) of agricultural lands and open space threaten an estimated 28 mi (45 km) of small water way habitat potentially inhabited by portions of the Yolo Basin/Willow Slough population of the giant garter snake; in the Laguna Creek-Elk Grove region of Sacramento County, 11 proposed residential developments and associated stream channelization projects would threaten portions of the Sacramento Basin population.

By the 1940s and 1950s, reclamation of wetlands for agriculture and other purposes completely extirpated the species from the southern one-third of its range.

Certain agricultural practices can destroy habitat that supports the giant garter snake. Activities such as intensive vegetation control activities and livestock grazing threatens this species by fragmenting and isolating available habitat. Livestock grazing along the edges of water sources degrades habitat quality by reducing vegetative cover. Overall, grazing has contributed to the elimination and reduction of the quality of available habitat at four known locations.

Collection and harassment associated with recreational activities apparently cause a substantial impact in certain areas. Recreational activity disturbs basking snakes; this in turn disturbs the snake's thermoregulatory processes.

The California Fish and Game Department stopped mark and recapture studies after noting that the snakes were slow to recuperate from injuries sustained during the process.

Unidentified parasitic worms have been found in the American Basin population of the giant garter snake. Snakes infected with the worms seemed to display reduced appetites and growth rates. All infected snakes eventually died after lingering malaise; some reached 12-14 months of age before perishing. The worms taken after death of the snake were 2-3 in (5-8 cm), about 0.01 in (0.25 mm) thick, and colored with alternating narrow rings of red and beige; the worms emerged from noticeable lumps at any location along the ventral or dorsal skin surfaces.

Predation levels on the giant garter snake have increased due to a number of factors. A number of native mammals and birds are likely known predators of the giant garter snake including raccoons, skunks, opossums, foxes, hawks, egrets, and herons. The abundance and diversity of predators and diminishing escape cover in remaining habitat suggest that predation pressure on this species is probably severe. The high fecundity and extremely wary behavior of the giant garter snake provide additional evidence that the species has developed physiological and behavioral adaptations to help withstand predatory pressure. In 1986, it was observed that nearly all the snakes captured and examined possessed scars or recent injuries presumably acquired during attacks by predators.

Domestic cats prey upon the giant garter snake. In addition, introduced aquatic species such as largemouth bass and catfish are opportunistic predators of many species, including the giant garter snake. Introduction of the bullfrog, Rana catesbeianna, further escalates the threat of predation to this snake.

The giant garter snake inhabits water management facilities adjacent to rice fields. The seasonal flooding and draining of rice ponds may provide an adequate forage base and may prevent establishment of populations of large predatory fish. Intensive control of vegetation along water delivery and drainage facilities eliminates remaining habitat and prevents reestablishment of former habitat. Such activities may kill or prove harmful to the giant garter snake.

This snake is vulnerable to changes in water management, due to its dependence on the availability of wetlands. Contaminants, such as fertilizers and pesticides, could adversely affect the giant garter snake populations by degrading water quality and reducing prey populations. Selenium contamination of agricultural drain water appears to pose a severe threat to any population that inhabits the grasslands region of Merced County.

Conservation and Recovery

Populations inhabiting wetlands on private and public lands fall under regulatory jurisdiction of the Corps of Engineers and are protected by the Clean Water and the Rivers and Harbors Acts.


U.S. Fish and Wildlife Service
Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Avenue
Portland, Oregon 97232

Sacramento Ecological Services Field Office
Room E-1803/1823
2800 Cottage Way
Sacramento, California 98525


U.S. Fish and Wildlife Service. 20 October 1993. "Determination of Threatened Status for the giant garter snake, Thamnophis gigas." Federal Register 522(58): 53804.