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San Francisco garter snake

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

           iSan Francisco garter snake

                             Conservation status

   Endangered (EN)
            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. This garter snake prefers wet and marshy areas and is
   elusive to see or capture. There are only 1,000 to 2,000 of the
   subspecies T. s. tetrataenia remaining. It is a subspecies of the
   common garter snake (Thamnophis sirtalis): a species found across the
   United States and into southern Canada.

Description

   The San Francisco garter snake presents a burnt orange head, greenish-
   yellow dorsal stripe edged in black, bordered by a red stripe, which
   may be continuous or broken with black blotches, below which is a black
   side stripe. Pigmentation of the underside ranges from greenish- blue
   to blue. Large adults can attain a length of one meter. The scales are
   keeled in nineteen rows, like other garter snakes, and there is a
   single anal plate. There are seven upper lip scales. The side stripe is
   confined to the second and third tier scales. Distinguishing this
   species from other garter snakes are red markings between its dorsal
   and side stripes, yielding the appearance of a wide continuous stripe.
   If captured it thrashes about wildly, may bite, and smears its captor
   with excrement and odorous fluids from the anal scent glands.
   Wetland habitat of the San Francisco garter snake in Millbrae,
   California
   Enlarge
   Wetland habitat of the San Francisco garter snake in Millbrae,
   California

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 relation 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.

   Biologist Don Roberson has noted the following analysis regarding food
   competition among several garter snakes. 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, analysis of DNA structures add further
   complication. 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 colour 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.

   Don Roberson points out that In the San Francisco Bay Area, there is a
   group of three populations of T. s. infernalis in Sonoma County, Contra
   Costa County, and Santa Clara County formed a divergent diagnosable
   group that exhibited an "elevated rate of molecular evolution".
   Roberson notes that these three populations are a mere 25 to 125
   kilometers from tetrataenia in San Mateo County and yet genetic
   distinctness is readily apparent." Roberson also points out 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.

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