San Francisco Garter Snake

(Thamnophis sirtalis tetrataenia)

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A close up of the San Francisco Garter Snake

Threatened or Endangered

The San Francisco garter snake (Thamnophis sirtalis tetrataenia) is federally listed as endangered.

(Summary below adapted from text by from Don Roberson)

Description

Most who appreciate wildlife will agree that the San Francisco garter snake is California’s most beautiful snake. The bright orange head, combined with dazzling black and red stripes, is impressive enough, but the pale stripes and belly are washed with the most delicate turquoise. It is just a wonderful serpent.

Population

It is also among California’s rarest snakes. It has been official designated as “endangered” on State lists since 1966, and was on the first Federal Endangered Species List established in 1973. Once common in stock ponds and small marshes in San Mateo County on the San Francisco Peninsula, it has been reduced to a mere handful due to urbanization, the draining and pollution of wetlands, and because its beauty makes it popular with illegal collectors. A recent population estimate was of ~1,500 snakes. It has been completely extirpated from many areas it was once common.

Food, Habitat, & Behavior

This garter snake is primarily a frog-eater, and thus it sticks close to water. Much of its remaining habitat is in small reedy marsh-edges and ponds. Among a kaleidoscope of green, rust, and pale tan reeds, the smallish snake (1 to 3 feet typically; maximum 4.25 feet) is amazingly well camouflaged. While the color pattern can be discerned among the grass at close range, at any distance it just blends into the habitat. Eaton (2002) thought the bright patterns might be a warning to predators, but perhaps the opposite is true: the crisply patterned colors fit the snake’s preferred habitat and hide it from predators. Only when the ponds dry up in late summer does it move into uplands, spending the winter in small mammal burrows.

Range

Contrary to its name, the San Francisco garter snake was historically found wholly within San Mateo County lines. This small range has been steadily reduced over the years by human development and habitat reduction. One of the larger blows to its population was the development of the Skyline Ponds “sag” ponds along Skyline Boulevard in the post World War II housing boom. In the 1940’s these ponds were home to “hundreds” of individuals. By the 1960’s nearly all of these ponds had been drained and developed. By 1966, scientists were concerned enough to list it as one of the first endangered species. Currently, San Francisco garter snake’s are found only in a few isolated locations within their former range. Laguna Salada, owned by the City of San Francisco’s Sharp Park golf course and adjacent Mori Point are two of them.

Other Facts

Although the San Francisco garter snake is considered beneficial to humans, preying on pests such as small mammals, reptiles, and amphibians, they are not to be approached upon sighting. When threatened, they excrete foul smells from their anal glands and often empty their bowels on unsuspecting captors. Nonetheless, these endangered creatures are quickly losing ground, and with only five populations left, they need the help of humans to protect their few remaining habitats.