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Photo Credit: Sheila Larsen / USFWS
San Francisco Garter Snake
Thamnophis sirtalis tetrataenia
Basic Species Information
Endangered. This species is in danger of extinction throughout all or a significant portion of its range.
The San Francisco garter snake is often called the most beautiful snake in the United States. The first things you notice are the burnt-orange head, the slender, turquoise-blue body and bold stripes. The stripe pattern that runs along the snake's "shoulders" is black, red-orange, black. Large adults can reach a meter (3 feet) or more in length.
San Francisco garter snakes are primarily active during the day. They may hunt after dark on warm evenings. The snakes are extremely shy, difficult to locate and capture, and quick to flee to water or cover when disturbed.
Garter snakes are not dangerous. In California, only rattlesnakes have venom that is dangerous to humans.
Adult San Francisco garter snakes feed primarily on California red-legged frogs (143 KB PDF) (which are Federally listed as threatened). They may also feed on juvenile bullfrogs, but they are unable to feed on the larger adults.
Newborn and juvenile San Francisco garter snakes depend heavily upon Pacific tree frogs as prey. If newly metamorphosed Pacific tree frogs are not available, the young may not survive.
San Francisco garter snakes are one of the few animals able to eat the toxic California newt without suffering serious side effects.
The snakes forage extensively in aquatic habitats. Captive snakes housed in an outside enclosure were observed foraging after dark on warm evenings.
The snakes' preferred habitat is a densely vegetated pond near an open hillside where they can sun themselves, feed, and find cover in rodent burrows; however, considerably less ideal habitats can be successfully occupied. Temporary ponds and other seasonal freshwater bodies are also used. The snakes avoid brackish marsh areas because their preferred prey (California red-legged frogs) cannot survive in saline water.
Emergent and bankside vegetation such as cattails, bulrushes and spike rushes are preferred and used for cover. The area between stream and pond habitats and grasslands or bank sides is used for basking, while nearby dense vegetation or water often provide escape cover. The snakes also use floating algal or rush mats, if available.
Adult snakes sometimes estivate (enter a dormant state) in rodent burrows during summer months when ponds dry. On the coast, snakes hibernate during the winter, but further inland, if the weather is suitable, snakes may be active year-round. Recent studies have documented San Francisco garter snake movement over several hundred yards away from wetlands to hibernate in upland small mammal burrows.
Females give live birth from June through September, with litters averaging 16 babies.
Historically, San Francisco garter snakes occurred in scattered wetland areas on the San Francisco Peninsula from approximately the San Francisco County line south along the eastern and western bases of the Santa Cruz Mountains, at least to the Upper Crystal Springs Reservoir, and along the coast south to Año Nuevo Point, San Mateo County, and Waddell Creek, Santa Cruz County.
Currently, although the geographical distribution may remain the same, reliable information regarding specific locations and population status is not available. Much of the remaining suitable habitat is located on private property that has not been surveyed for the presence of the snake. Many locations that previously had healthy populations of garter snakes are now in decline.
Birds such as hawks and herons and other snakes are considered predators. This includes domestic cats and other small mammals. Adult bullfrogs prey on smaller San Francisco garter snakes, which is a factor in their decline.
Loss of habitat from agricultural, commercial and urban development and illegal collection (because of their beauty) led to the listing of the San Francisco garter snake as "threatened" in 1967. These historical threats to the species remain, but there are now additional threats, such as the documented decline of the California red-legged frog (an essential prey species) and the introduction of bullfrogs into San Francisco garter snake habitat. Bullfrogs prey on both San Francisco garter snakes and California red-legged frogs.
Read a San Francisco Chronicle story about how one of our biologists took her 10 year-old daughter to try to find them in the wild. There is also a story about how we obtained 10 of these snakes that are at the San Francisco Zoo.
WHAT YOU CAN DO
See What You Can Do to Help Wildlife and Plants (201 KB PDF) for ideas about protecting the environment.
If you are lucky enough to see a San Francisco garter snake, it will probably slither away quickly. Do not try to pick it up. It may bite you or poop on you. Remember, it is against the law to mess with endangered species.
Wherever you live in California, there are zoos and nature centers where you can see and learn about snakes.
Need more specifics? Download the San Francisco Garter Snake scientific species account.
Photo Credit: Shelia Larsen / USFWS
Photos & More
News Releases and Featured Stories
- San Francisco garter snake returns to its namesake city
- November 21, 2012 - Stanford University's Habitat Conservation Plan Final Environmental Impact Statement Available for Public Review
- August 30, 2012 - Meeting Set to Explain Pescadero Lagoon Breach
- May 17, 2010 - Habitat Conservation Plan proposed for Stanford University
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Last updated: December 6, 2017