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NATURAL HISTORY

SAN FRANCISCO GARTER SNAKE } Thamnophis sirtalis tetrataenia
FAMILY: Colubridae

DESCRIPTION: The San Francisco garter snake is a slender, three-foot-long snake with an orange head, a greenish-yellow back bordered by red and black stripes, and a bright greenish-blue or turquoise belly.

HABITAT: The garter snake lives in freshwater marshes, ponds, canals, and slow-moving streams with emergent vegetation and frog populations, as well as connected grassy uplands with brushy cover.

RANGE: The San Francisco garter snake is found in coastal San Mateo County from Lake Merced south along the foothills of the Santa Cruz Mountains to Point Año Nuevo and along San Mateo County baylands. The only remaining viable populations of San Francisco garter snakes occur at Pescadero Marsh Natural Preserve, Año Nuevo State Reserve, Laguna Salada (Sharp Park) and Mori Point, San Francisco State Fish and Game Refuge (Crystal Springs and San Andreas Reservoirs), Cascade Ranch, and wetlands near the San Francisco International Airport.

MIGRATION: San Francisco garter snakes do not travel far from wetlands habitat, but will migrate up to one kilometer from pond foraging habitats and uplands wintering sites.

BREEDING: Garter snakes mate primarily during the spring.

LIFE CYCLE: Females bear 16 or so live babies in July or August. Peak snake activity occurs from March through July, with snakes feeding within or near ephemeral ponds. Snakes move in pursuit of amphibian prey dispersing from wetlands. San Francisco garter snakes retreat to uplands and are dormant for much of the winter.

FEEDING: San Francisco garter snakes eat California red-legged frogs, Pacific tree frogs, western toads, mosquito and other fish, worms, newts, and salamanders.

THREATS: The San Francisco garter snake is threatened by urban development, wetland habitat loss, fragmentation of habitat by development and roads, agricultural development, dredging of waterways, disturbance by recreational activities, pesticides, poaching, overgrazing, and nonnative bullfrogs.

POPULATION TREND: There are only six known significant populations of the San Francisco garter snake remaining, and at least four of these populations have declined in recent decades. There may be only 1,000 to 2,000 individual San Francisco garter snakes remaining in the wild today.

Photo © Gary Nafis, CaliforniaHerps.com