San Francisco Garter Snake

Thamnophis sirtalis tetrataenia

Description

The San Francisco garter snake, Thamnophis sirtalis tetrataenia, grows to a length of 51 in (1.3 m). Running down its back is a wide greenish yellow stripe edged in black and bordered on each side by a broad red stripe, which may be broken or divided. Parallel with this is a third black stripe. The belly is greenish blue, and the top of the head is red. The eyes are large.

Behavior

The harmless San Francisco garter snake is extremely wary and will flee into water or under-growth if disturbed. It mates during the first few warm days in March. Ovulation occurs in spring, pregnancy in early summer, and birth sometime in July or August. The female plays a passive role, allowing several males to court her. Males seek out the female by scent. Like most garter snakes, the San Francisco snake is ovoviviparous (eggs are hatched inside the body). Average litter sizes of closely related species are from 12 to 24. This species is known to feed on red-legged frogs, Pacific tree frogs, immature California newts, western toads, threespine stickleback, and mosquito fish. Small mammals may occasionally be eaten as well. The garter snake is diurnal and is active during spring, summer, and fall, with a hibernation period during the winter.

Habitat

The San Francisco garter snake is seen most often near ponds, lakes, marshes, and sloughs. For cover it uses bankside vegetation, such as cattails, bulrushes, and spike rushes. It sometimes shelters in rodent burrows. Lower-lying marsh areas are used for foraging and breeding. The snake often basks on floating algae or rush mats or on grassy hillsides near drainages and ponds. The garter snake may use different areas of its habitat for different functions. Upland sites such as grassy hillsides near ponds may be used for basking; rodent burrows away from standing water for shelter and escape cover; and low-lying marsh areas for feeding and reproduction. Research conducted in 1987, using implanted radio transmitters, has shown that the snake ranges much further from water than originally supposed. It has been found up to 600 ft (182.9 m) away from water in rodent burrows on dry, grassy hillsides.

Distribution

Historically, the San Francisco garter snake was found on the San Francisco Peninsula from the San Francisco County line south through San Mateo County to Ano Nuevo Point. It inhabited lowlands along both the western and eastern foothills of the Santa Cruz Mountains.

The San Francisco garter snake survives at about 20 locations within its historic range. Significant populations are at Ano Nuevo State Reserve, Pescadero Marsh Natural Preserve, San Francisco State Fish and Game Refuge, Sharp Park Golf Course, Cascade Ranch, and Milbrae at San Francisco Airport. The largest population is believed to have more than 600 individuals.

Threats

Alteration and fragmentation of habitat are to blame for the decline of the San Francisco garter snake. Wetlands have been filled in or converted for recreation or residential sites. Streams have been diverted, streambank vegetation eliminated, and large areas brought into cultivation or developed for housing and industry. All this has driven the snake into diminishing pockets of habitat.

Because of the snakes' beautiful coloration, reptile dealers and fanciers pose a threat. Recently, U.S. Fish and Wildlife Service (FWS) agents arrested several dealers and collectors for possession of these snakes. Diligent enforcement of the laws protecting the species has helped reduce collecting.

Conservation and Recovery

Four areas managed for other species also offer incidental protection to populations of the garter snake. These are the Pescadero Marsh Natural Preserve, Ano Nuevo State Reserve, Laguna Salada at Sharp Park, and the San Francisco State Fish and Game Refuge.

The goal of the Recovery Plan is to expand protection to at least ten populations, each consisting of about 200 adult snakes. Current populations are being protected through monitoring, reducing habitat degradation caused by diversion of water flow, salinity, agricultural practices, grazing, timber harvesting and pollution caused by chemical treatment. The FWS is attempting to rehabilitate habitat at some colonies by managing ground water, installing flood control structures, creating habitat ponds, and controlling stream bank channel modification and aggressive vegetation.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

References

Fitch, H. S. 1965. "An Ecological Study of the Garter Snake, Thamnophis sirtalis." University of Kansas Publication of the Museum of Natural History 15: 493-564.

McGinnis, S. M. 1984. "The Current Distribution and Habitat Requirements of the San Francisco Garter Snake in Coastal San Mateo County." Report C-673. California Department of Fish and Game, Sacramento.

U.S. Fish and Wildlife Service. 1985. "Recovery Plan for the San Francisco Garter Snake." U.S. Fish and Wildlife Service, Portland.