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San Francisco garter snake

Related subjects: Insects, Reptiles and Fish

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San Francisco garter snake
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Suborder: Serpentes
Family: Colubridae
Genus: Thamnophis
Species: T. sirtalis
Subspecies: T. s. tetrataenia
Trinomial name
Thamnophis sirtalis tetrataenia
Cope, 1875

The San Francisco garter snake (Thamnophis sirtalis tetrataenia) is a slender multi-colored colubrid snake. Designated as an endangered species since the year 1967, it resides only in San Mateo County, California, and the extreme northern part of coastal Santa Cruz County, California. There are only 1,000 to 2,000 of the subspecies T. s. tetrataenia remaining. This garter snake prefers wet and marshy areas and is elusive to see or capture. It is a subspecies of the common garter snake (Thamnophis sirtalis), a species found across the United States and into southern Canada.

Range and Habitat

This subspecies of garter snake is found in scattered wetland areas on the San Francisco Peninsula from approximately the northern boundary of San Mateo County south along the eastern and western bases of the Santa Cruz Mountains, at least to the Upper Crystal Springs Reservoir, and along the Pacific coast south to Año Nuevo Point, and thence to Waddell Creek in Santa Cruz County. It is difficult to obtain reliable distribution information and population statistics for the San Francisco garter snake, because of the elusive nature of this organism and the fact that much of the remaining suitable habitat is located on private property that has not been surveyed for the presence of the snake. This subspecies is extremely shy, difficult to locate and capture, and quick to flee to water or cover when disturbed. The U.S. Fish and Wildlife Service has stated that many locations that previously had healthy populations of garter snakes are now in decline due to land development pressure and the filling of wetlands in San Mateo County over the last sixty years. This snake is also a rare species in addition to its endangered classification.

The snake’s preferred habitat is a densely vegetated pond near an open hillside where it can sun, feed, and find cover in rodent burrows; however, markedly less suitable habitat can be successfully used. Temporary ponds and other seasonal freshwater bodies are also appropriate. This subspecies avoids brackish marsh areas because its preferred prey, the California red-legged frog (Rana aurora) cannot survive in saline water. Emergent and bankside vegetation such as cattails, ( Typha spp.), bulrushes ( Scirpus spp.) and spike rushes ( Juncus spp. and Eleocharis spp.) apparently are preferred and used for cover. The zone between stream and pond habitats and grasslands or bank sides is characteristically utilized for basking, while nearby dense vegetation or water often provide escape cover. The subspecies occasionally uses floating algal or rush mats, when available.

San Francisco garter snakes forage extensively in aquatic habitats. Adult snakes feed primarily on California red-legged frogs, which is federally listed as endangered. They may also feed on juvenile bullfrogs (Rana catesbeiana), but they are unable to consume adults; in fact, adult bullfrogs prey on juvenile garter snakes, and may be a contributing factor in the population decline of the San Francisco garter snake. Newborn and juvenile San Francisco garter snakes depend heavily upon Pacific treefrogs (Hyla regilla) as prey. If newly metamorphosed Pacific treefrogs are not available, the young garter snakes may not survive. San Francisco garter snakes are one of the few animals capable of ingesting the toxic California newt (Taricha torosa) without incurring sickness or death.

Life Cycle

Adult snakes sometimes estivate (enter a dormant state) in rodent burrows during summer months when ponds dry. Along the Pacific Ocean coast, snakes hibernate during the winter, but further inland, if the weather is suitable, this species is known to be active year-round. Recent studies have documented San Francisco garter snake movement over several hundred meters from wetlands to hibernate in upland small mammal burrows. In spite of being primarily diurnal, captive specimens housed in an exterior setting were observed foraging nocturnally or crepuscularly on warm evenings.

The San Francisco garter snake mates in the spring or autumn, and the females give birth to live young in June through September, numbering up to two dozen, but averaging about 16 offspring. The young are approximately 12 to 18 centimeters in length and mature in two years time.

Taxonomy and Helation to Other Garter Snakes

For a brief period from 1996 to 2000 there was confusion over the differentiation of the San Francisco garter snake from two other subspecies, known as the red-sided garter snake (T. s. infernalis) and the red-spotted garter snake (T. s. concinnus). Barry petitioned the International Commission on Zoological Nomenclature (ICZN) to suppress the changes proposed in 1996 to merge two of these species. In 2000, the ICZN agreed and voted to retain the historical taxonomic arrangement of subspecies within this evolutionary lineage. Accordingly, the subspecies tetrataenia was reaffirmed for the San Francisco garter snake and the races concinnus and infernalis retain their historical definition.

The San Francisco garter snake cohabits ecosystems that host two other species of garter snake: the coast garter snake (Thamnophis elegans terrestris), a subspecies of western terrestrial garter snake (T. elegans), and the Santa Cruz aquatic garter snake (Thamnophis atratus atratus) a subspecies of the aquatic garter snake (T. atratus). These three subspecies are known to prey upon same foods; however, their preferences are slightly different. Herpetologist Sean Barry notes that they divide up the food resource as follows:

  • The San Francisco garter snake eats primarily small frogs;
  • The coast garter snake eats principally slugs, and
  • The Santa Cruz aquatic garter snake eats preferentially minute fish and amphibian larvae.

DNA Analysis

While the findings of the ICZN have given the San Francisco garter snake unique taxonomic standing for now, a molecular study challenges the subspecific status of this population. Janzen analyzed sequences in mitochondrial DNA to determine relationships within the common garter snake (T. sirtalis). Janzen found that molecular evidence differed, often sharply, with the territorial boundaries of subspecies named on phenotypic variation. He further deduced that local environmental forces were more significant in shaping the color patterns shown by the garter snakes than shared common ancestry, and concluded all morphologically based subspecies in the western U.S. to be subject to revision. This result strongly suggests that the colour traits that are diagnostic for (T.s. tetrataenia) are the result of local selection rather than long-term isolation from other races of (T. sirtalis) in central California. On the other hand, the article places the three nearest populations of T. s. infernalis to T.s. tetrataenia in Sonoma County, Contra Costa County, and Santa Clara County into a separate group that exhibits an "elevated rate of molecular evolution". The authors suggest that sequencing nuclear DNA may provide a more precise analytical tool to crack some of the ultimate taxonomic quandaries of the San Francisco garter snake and its relatives.

Outlook for this Subspecies

Many of the factors that led to the listing of the San Francisco garter snake in 1967 continue to impact the organism. These environmental elements include loss of habitat from agricultural, commercial and urban development as well as collection by reptile fanciers and breeders. One of the largest populations of this species is located immediately west of the San Francisco International Airport and has been studied in relation to airport and airport infrastructure expansion plans. These studies have led to guidelines for land development and habitat mitigation to maximize the likelihood of species survival. Ironically, morphological data suggest that the airport population is made up of hybrid individuals.

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