Not Dangerous (Non-poisonous) - This snake does not have venom that is dangerous to most humans.
Gartersnakes have toxins in their saliva which can be deadly to their prey and their bite might produce an unpleasant reaction in humans, but they are not considered dangerous to humans.
Adults of this species measure 18 - 55 inches in length (46 - 140 cm), but the average size is under 36 inches (91 cm).
A medium-sized snake with a head barely wider than the neck and keeled dorsal scales.
The eyes are relatively large compared with other gartersnake species.
Some average scale counts: 7, occasionally 8, rarely 6 or 9, upper labial scales, often with black wedges.
10 lower labial scales.
The rear pair of chin shields are longer than the front.
Average of 19 scales at mid-body.
Color and Pattern
A wide blue-green or greenish yellow dorsal stripe is bordered with black stripes.
Below the black stripe is a continuous red stripe, bordered below by another black stripe.
Below that is a bluish or greenish yellow lateral stripe.
There may also be a thin line of black below the lateral stripe on the edge of the belly.
The underside is bluish-green.
Occasionally, the red stripes may be marked with black, similar to T. s. infernalis.
The head is red.
Primarily active during daylight.
A good swimmer.
The species T. sirtalis is capable of activity at lower temperatures than other species of North American snake.
Often escapes into water when threatened. When first handled, typical of gartersnakes, this snake often releases cloacal contents and musk, and strikes.
Diet and Feeding
Eats a wide variety of prey, including amphibians and their larvae (the endangered California Red-legged Frog - Rana draytonii, is a main food source), fish, birds, and their eggs, small mammals, reptiles, earthworms, slugs, and leeches.
This species is able to eat adult Pacific Newts (genus Taricha) which are deadly poisonous to most predators.
An arms race between T. sirtalis and the tetrodoxin poison contained in Taricha has been documented, with newt toxicity varying by location and snake resistance to the toxin also varying by location.
(Edmund D. Brodie III. Patterns, Process, and the Parable of the Coffeepot Incident: Arms Races Between Newts and Snakes from Landscapes to Molecules. From In the Light of Evolution: Essays from the Laboratory and Field edited by Jonathan Losos (Roberts and Company Publishers). 2010.)
There is evidence * that when Common Gartersnakes (Thamnophis sirtalis) eat Rough-skinned Newts (Taricha granulosa) they retain the deadly neurotoxin found in the skin of the newts called tetrodotoxin for several weeks, making the snakes poisonous (not venomous) to predators (such as birds or mammals) that eat the snakes. Since California Newts (Taricha torosa) also contain tetrodotoxin in their skin, and since gartersnake species other than T. sirtalis also eat newts, it is not unreasonable to conclude that any gartersnake that eats either species of newt is poisonous to predators.
Mating occurs in the spring (and possibly the fall ) and young are born live, spring to fall.
Utilizes a wide variety of habitats, preferring grasslands or wetlands near ponds, marshes and sloughs. May overwinter in upland areas away from water.
The species Thamnophis sirtalis - Common Gartersnake, has the largest distribution of any gartersnake, ranging from the east coast to the west coast and north into Canada, farther north than any other species of snake in North America.
This subspecies, Thamnophis sirtalis tetrataenia - San Francisco Gartersnake, is endemic to California, found only on the San Francisco peninsula from near the southern San Francisco County line south to Rancho del Oso State Park in Santa Cruz County.
Rossman et al (1996) show the elevation record for the species at 8,333 feet (2540 m.). Stebbins (2003) shows it as 8,000 ft. (2,438 m).
This subspecies occurs at much lower elevations along the coast and associated mountains.
Notes on Taxonomy
SSAR Herpetological Circular No. 43, 2017 shows this note regarding the species Thamnophis sirtalis: "Analyses of mitochondrial and nuclear data suggest that this species may be composed of multiple independently evolving lineages often not concordant with the subspecific taxonomy (F. Burbrink, pers. comm.)."
In 1995, Doug Rossman and Jeff Boundy re-named the Thamnophis sirtalis found on the San Francisco Peninsula T. s. infernalis, (removing the name T. s. tetrataenia, but recognizing that the snakes were still subspecifically distinct), and lumped the coastal T. sirtalis with T. s. concinnus. This taxonomy is shown on the range map in the 1996 book, The Garter Snakes - Evolution and Ecology 1. In 1998, Sean Barry and Mark Jennings petitioned the International Commission on Zoological Nomenclature (ICZN) to restore the previous nomenclature 2. With no opposition from Boundy or Rossman, the ICZN agreed to restore the name T. s. tetrataenia to Common Gartersnakes on the San Francisco peninsula 3. Nevertheless, some authors either missed the restoration of this nomenclature or chose to ignore it, and their work still reflects Rossman and Boundy's nomenclature.
(Thanks to Sean Barry for this clarification in a personal communication.)
Listed as endangered by the state and by the federal government. The habitat of this snake has declined severely due to urban development and agricultural land use and altering of the waterways needed by this snake. These habitat changes have also reduced populations of one of this snake's main food sources, the California Red-legged Frog - Rana draytonii. Some authorities believe that the remaining fragmented populations of this snake could be further threatened by overcollecting for the pet trade. San Francisco Gartersnakes are popular pets in Europe, where it is possible that there are more of these snakes than there are in the wild in California.
North American Gartersnakes
San Francisco Gartersnake
Thamnophis sirtalis - (Linnaeus, 1758) - Syst. Nat., 10th ed., Vol. 1, p. 222 Thamnophis sirtalis tetrataenia - (Cope, 1875) - in Yarrow, in Wheeler's Rep. Surv. W. 100th Mer., Vol. 5, Zool., p. 546
Thamnophis - Greek - thamnos - shrub or bush, and ophis - snake, serpent
sirtalis - sirtalis, like a garter - probably refers to the to striped pattern tetrataenia - Greek - tetra - four, and Latin - taenia - stripes or bands, refers to the dorsal color areas
1 Rossman, Douglas A., Neil B, Ford, & Richard A. Siegel. The Garter Snakes - Evolution and Ecology. University of Oklahoma press, 1996.
2 Barry, Sean J., and Mark R. Jennings. 1998. Coluber infernalis Blainville 1835 and Eutaenia sirtalis tetrataenia Cope in Yarrow, 1875 (currently Thamnophis sirtalis infernalis and T. s. tetrataenia; Reptilia: Squamata): proposed conservation of the subspecific names by the designation of a neotype for T. s. infernalis. Bulletin of Zoological Nomenclature 55: 224-228.
3 CZN 2000. Opinion 1961. Coluber infernalis Blainville 1835 and Eutaenia sirtalis tetrataenia Cope in Yarrow, 1875 (currently Thamnophis sirtalis infernalis and T. s. tetrataenia; Reptilia: Squamata): conservation of the subspecific names by the designation of a neotype for T. s. infernalis. Bulletin of Zoological Nomenclature 57: 191-192.
Joseph Grinnell and Charles Lewis Camp. A Distributional List of the Amphibians and Reptiles of California. University of California Publications in Zoology Vol. 17, No. 10, pp. 127-208. July 11, 1917.
The following conservation status listings for this animal are taken from the October 2021 California "Special Animals List" and the October 2021 "State and Federally Listed Endangered and Threatened Animals of California" list, both of which are produced by multiple agencies and available here: https://www.wildlife.ca.gov/Data/CNDDB/Plants-and-Animals. You can check the link to see if there are more recent lists.
If no status is listed here, the animal is not included on either list. This most likely indicates that there are no serious conservation concerns for the animal. To find out more about an animal's status you can go to the NatureServe and IUCN websites to check their rankings.
NatureServe Global Ranking
The species is: Secure—Common; widespread and abundant.
This subspecies is: Imperiled - At high risk of extinctiion due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors.