Snake, San Francisco garter

Thamnophis sirtalis tetrataenia

phylum: Chordata

class: Reptilia

order: Serpentes

family: Colubridae

status: Endangered, ESA

range: USA (California)

Description and biology

For those who love snakes, the San Francisco garter snake is thought to be one of the most beautiful varieties. Adult San Francisco garter snakes grow to be about 51 inches (1.2 meters) long and are vividly colored. They have a large bright greenish-yellow stripe, bordered in black, running the length of their body. To either side of the stripe there are red or reddish-orange stripes. The top of their heads are red, and their bellies are greenish-blue.

San Francisco garter snakes are diurnal (active during the day). They like to be warm, and during cool weather can often be found seeking out the sun. Each adult snake has its own hibernating burrow—a hole dug out by another animal or a natural crevice. Some San Francisco garter snakes estivate

(enter a dormant, or sleep-like, state) in their burrows at times in the summer when ponds have gone dry. In cooler parts of their range, particularly near the coast, the snakes hibernate in winter. Even during its periods of hibernation, if the weather is nice, the snake will come out of its burrow to lie in the sun. Where it is warmer, the snakes remain active through the seasons.

The main food of the San Francisco garter snake is the California red-legged frog (see entry in Volume 2). They also eat other frogs and toads, as well as other prey, such as earthworms and insects, which they find by foraging in the grass and leaves on the forest floors, and small fish they find in the water. These snakes are shy. When they are disturbed they tend to hide in water or under cover in the woods.

San Francisco garter snakes mate from about late March to early June. Females of this species can store the male's sperm throughout the winter. They will give birth to anywhere from 7 to 85 live infants from late June to early October.

Habitat and current distribution

San Francisco garter snakes live in moist habitats like grasslands or meadows, marshy and damp woodlands, and farms or parks that are near ponds and marshes. They are most often found around ponds that have large frog populations. They live within small areas in San Mateo County, California. Currently, there is little information about where the snakes occur, largely because they are scattered through privately owned land, although they have also been observed on county lands. Their population numbers are not known.

History and conservation measures

The San Francisco garter snake's range has always been in San Mateo County. However, it was once found in a wide sweep up and down the county and now is found only in isolated places.

The San Francisco garter snake is threatened because of habitat destruction from farming and urban development. For years it has been captured for sale to reptile collectors and traders. One of the threats facing the species is the grave population decline of the snake's main food source, the California red-legged frog. At the end of the nineteenth century, bullfrogs were introduced into the areas shared by the California red-legged frog and the San Francisco garter snake. Bullfrogs prey on California red-legged frogs and have reduced the once-abundant population to the brink of extinction, thus depriving the snakes of a vital food supply. Bullfrogs also prey upon young San Francisco garter snakes.