Monday's Specie in Peril---San Francisco Garter Snake



This strikingly beautiful snake is the San Francisco Garter Snake (Thamnphis sirtalis tetrataenia) and is considered one of the most critically endangered animals in North America. They occur in a very small range in the San Francisco Peninsula and south near the San Francisco-San Mateo county border and along the eastern and western base of the Santa Cruz Mountains. In 1966 they became the first animal to be listed with the government enacted Federal Endangered Species Act. Several years later in 1971 they were listed on the California endangered species list.

Because of their limited range and low litter size they fell victim to population crashes due to pollution, habitat destruction by land development, agriculture, commercial building, residential building and recreational pursuits. Another factor that has hurt this species population is its preferred prey item. The Red-Legged Frog is the primary diet of these garter snakes and they are also listed as threatened or endangered in nearly all their range. So we have a case of an endangered species, feeding on an endangered species. They will also feed on other amphibians such as the non-native (To California) bullfrog....however, they are unable to swallow a fully grown bullfrog instead they may find themselves on the menu as bullfrogs often feed on small snakes. Unlike most animals this species of snake is even able to feed on the Pacific newt which is fatally toxic to other predators which may try to consume it. These snakes will also consume small birds, reptiles, slugs, earthworms, leeches and fish.

We also have the pet trade to thank for the descrease in numbers for this species. Many collectors in Europe have these snakes in their personal collections and there is even some speculation that there may be more in captivity in Europe than exist in their native range!

Mating occurs either in the fall or spring. If females are bred in autumn they will retain the males sperm until the following spring. Live young are born in the autumn. The juveniles are born looking nearly identical to the adults and measure up to 9 inches in length. The adults are capable of reaching lengths up to 51 inches. Like most garter snakes this species is most active during daylight hours where it may be seen on roadways basking. Please give them the right of way by slowing down and driving around them. There are an estimated 1,000-2,000 individual snakes in the wild.

They are a semi-aquatic species and can be found near ponds, marshes and sloughs. They are a secretive snake and quick to flee if approached. In the late summer or early fall they may head to higher elevations in search of prey items and hibernation locations. They will often utilize rodent burrows for hiding places and hibernation dens. 

Much research still needs to be done on this species as much of its habitat occurs on privately owned land and has not been fully studied. Anyone coming across this snake in its native home should leave it be. If you see one on the road, instead of running over it, drive around it. It would be a sad day indeed to lose such a gorgeous snake to extinction.