| San Francisco garter snake | |
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Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Suborder: | Serpentes |
| Family: | Colubridae |
| Genus: | Thamnophis |
| Species: | |
| Subspecies: | T. s. tetrataenia |
| Trinomial name | |
| Thamnophis sirtalis tetrataenia (Cope, 1875) | |
| Synonyms | |
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TheSan Francisco garter snake (Thamnophis sirtalis tetrataenia) is a slender multi-coloredsubspecies of thecommon garter snake. Designated as anendangered subspecies since the year 1967,[1] it isendemic toSan Mateo County and the extreme northern part of coastalSanta Cruz County inCalifornia.
Some researchers estimate that there are only 1,000 to 2,000 adult snakes of the subspeciesT. s. tetrataenia remaining. However, the full extent of the snakes' habitat has not been fully documented, and many snakes may utilize creeks and other waterways that are currently unexplored. This garter snake prefers wet and marshy areas, and because of its elusive nature, it is difficult to see or capture.
TheSan Francisco garter snake, a subspecies of the common garter snake, is found in scattered wetland areas on theSan Francisco Peninsula from approximately the northern boundary of San Mateo County south along the eastern and western bases of theSanta Cruz Mountains, at least to the UpperCrystal Springs Reservoir, and along thePacific coast south to Año Nuevo Point, and thence to Waddell Creek in Santa Cruz County.[2] It is difficult to obtain reliable distribution information and population statistics for the San Francisco garter snake, because of the elusive nature of this reptile 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. TheU.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 ofwetlands in San Mateo County over the last sixty years. However, in many areas where it still occurs, it is not rare, but is actually quite common and can be viewed with good success once its behavior is understood.
Adult San Francisco garter snakes can grow to a total length of 18–55 inches (46–140 cm).[3] They have keeled dorsal scales of blue-green, bordered by stripes of black, red (sometimes orange), and blue-green. Their head is barely wider than the neck, and is red. Its eyes are large compared to other species of garter snakes, giving the snake good eyesight to be primarily active during the day. Studies have shown that snout length is indicative of reproductive behaviors, as great snout length is correlated with ovarian follicle growth, which increases sexual activity of the female subspecies.
Garter snakes are practically harmless to humans. Their bites are fatal to their prey, but only cause a mild irritation for humans. With mildly toxic venom in their saliva, garter snakes possess no fangs.
The preferredhabitat of the San Francisco garter snake is a densely vegetated pond near an open hillside where it can sun, feed, and find cover inrodentburrows; however, markedly less suitable habitat can be successfully used. This subspecies can also be found in forests with dense canopies. Temporaryponds and other seasonalfreshwater bodies are also appropriate. This subspecies avoidsbrackish marsh areas because its preferredprey, theCalifornia red-legged frog (Rana draytonii), cannot survive insaline water. Emergent and bankside vegetation such as cattails (Typha spp.), bulrushes (Scirpus /Schoenoplectus spp.), and spike rushes (Juncus spp. andEleocharis spp.) apparently are preferred and used for cover.[4] The zone betweenstream andpond habitats andgrasslands or bank sides is characteristically utilized for basking, while nearby dense vegetation or water often provide escape cover. The subspecies occasionally uses floatingalgal or rush mats, when available. The biggest population is found in a protected tract owned bySan Francisco International Airport.[5]
San Francisco garter snakes forage extensively inaquatic habitats. Adult snakes feed primarily onCalifornia red-legged frogs (Rana draytonii), which are federally listed as threatened. They may also feed onjuvenileAmerican 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 uponPacific treefrogs (Pseudacris 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 thetoxicCalifornia newt (Taricha torosa) without incurring sickness or death.
For a brief period from 1996 to 2000 there was confusion over the differentiation of the San Francisco garter snake from two other subspecies from California, theCalifornia red-sided garter snake (T. s. infernalis) and thered-spotted garter snake (T. s. concinnus). Barry petitioned[6] theInternational 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 historicaltaxonomic arrangement of subspecies within this evolutionary lineage. Accordingly, the subspeciestetrataenia was reaffirmed for the San Francisco garter snake and the racesconcinnus andinfernalis retain their historical definition.
The San Francisco garter snake cohabits ecosystems that also host a subspecies of two other garter snake species: thecoastal garter snake (Thamnophis elegans terrestris), a subspecies of thewestern terrestrial garter snake (T. elegans), and theSanta Cruz garter snake (Thamnophis atratus atratus) a subspecies of theaquatic garter snake (T. atratus). These three subspecies are known to prey upon the same foods; however, their preferences are slightly different. Herpetologist Sean Barry notes that the three subspecies divide up the food resources as follows:
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[7] analyzed sequences inmitochondrial DNA to determine relationships within the subspecies of 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 common garter snake subspecies 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 color traits that are diagnostic forT. s. tetrataenia are the result of local selection, rather than long-term isolation from the other subspecies ofT. sirtalis in central California. On the other hand, the article places the three nearest populations ofT. s. infernalis toT. s. tetrataenia inSonoma County,Contra Costa County, andSanta Clara County into a separate group that exhibits an "elevated rate ofmolecular evolution". The authors suggest that sequencingnuclear 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.
Many of the factors that led to the listing of the San Francisco garter snake in 1967 continue to affect the subspecies. These environmental elements include loss of habitat from agricultural, commercial and urban development, as well as collection by reptile fanciers and breeders.[8] Collection of these endangered animals by private citizens remains illegal.[9]
In 2023 the San Francisco garter snake was featured on aUnited States Postal ServiceForever stamp as part of theEndangered Species set, based on a photograph fromJoel Sartore'sPhoto Ark. The stamp was dedicated at a ceremony at theNational Grasslands Visitor Center inWall, South Dakota.[10]
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