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Last updated on April 18, 2014 at 8:42 EDT

San Francisco Garter Snake

The San Francisco Garter Snake, Thamnophis sirtalis tetrataenia, is a species of colubrid snake resident only in San Mateo County and extreme northern Santa Cruz County in the state of California. There are only 1,000 to 2,000 estimated numbers of this subspecies of the common garter snake. They prefer wet, marshy areas and are very elusive to see or capture.

This particular garter snake presents a burnt orange head, greenish-yellow dorsal stripe edged with black, bordered by a red stripe (which may be continuous or broken by black blotches), and black side stripe below. The underbelly color ranges from greenish-blue to blue. Adults can grow to about 40 inches in length.

San Francisco garters forage extensively in aquatic habitats. Adults feed primarily on California red-legged frogs which are listed as an endangered species. They may also feed on juvenile bullfrogs, but are unable to consume adults – in fact adult bullfrogs prey on juvenile garter snakes. Juvenile San Francisco garters depend heavily on Pacific tree frogs as prey. If these tree frogs are not readily available to the newborn garter, they may not survive. This species is also one of the few animals capable of eating the toxic California Newt without incurring sickness or death.

Where the San Francisco garter is largely an aquatic species, it may sometimes enter a dormant hibernation-like state in a rodent burrow during summer months when ponds and other water sources become dry. Along coastal areas, snakes hibernate in the winter months, but further inland, if the weather is suitable, the species is active year-round. They mate in spring or autumn, and the females give birth to living young in June through September. Offspring numbers up to two dozen, but average about 16. The young are between 4 and 7 inches long and mature in two years.

San Francisco Garter Snake