A red-sided gartersnake swims around in a small cattle pond on a sunny spring afternoon in Alameda County. I wanted to get a closer look, so I walked over to the snake's side of the pond, but then it swam to the other side, again and again, until I got tired of going round in circles.
A juvenile CA red-sided gartersnake in Marin County.
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 larged 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
Ground color is dark olive to black.
The dorsal stripe is wide and well-defined, and yellowish to bluish in color.
Light stripes along the lower sides are not very distinct, often blending in with the color of the belly.
There are red bars alternating with the ground color along the sides above the lateral stripes.
The head is red or orangish.
The underside is bluish gray, sometimes with dark markings.
Snakes with robin's-egg-blue stripes and blue underneath are found north of the Bay Area into Sonoma County at least as far north as Sebastapol and Santa Rosa. Snakes from Sea Ranch in Sonoma county lack the blue, so the transition takes place somewhere between Sebastapol and Sea Ranch.
Some snakes in the southern part of the Bay Area may have bluish coloring with a yellow dorsal stripe.
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 frogs and newts and their larvae, 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 - forests, mixed woodlands, grassland, chaparral, farmlands, often near ponds, marshes, or streams.
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 infernalis - California Red-sided Gartersnake, is endemic to California, ranging from Humboldt County south, along the North Coast ranges (excluding much of the San Francisco peninsula) and east of the San Francisco Bay, then south along the central and south coasts to San Diego County.
(The eastern edge of the range of this subspecies as shown on my distribution maps is approximate and subject to change. For example: after I learned that an example of this subspecies was found on the east side of San Luis Reservoir in Merced County, I expanded the range east to cover that area. It's also possible there is a wide area of intergradation where both subspecies can be found. Researchers are no longer concerned with subspecies, so there is little information for me to go by.)
Common Gartersnakes from the Santa Clara River area in Ventura County south, may prove to be a new species (Stebbins 2003). I have followed the California Dept. of Fish and Wildlife's designation of these south coast snakes by indicating them separatly on the map, but grouping then with T. s. infernalis until more research proves otherwise. (See notes on taxonomy below about the South Coast Gartersnake.)
I have been confused about which subspecies is present in this area, T. s. infernalis or T. s. fitchi. Field guides show either one or the other in the area, but they don't show it as a range of intergradation where both might be found. I have heard from a reliable source that a respected herpetologist working in the area told him that the Common Gartersnakes within 5-10 miles of the coast in the area are T. s. infernalis, but this picture of a snake found 3 miles from the coast in San Luis Obispo County has no red on the head, making it T. s. fitchi, although it has large bright red spots on the sides similar to T. s. infernalis.
- Rossman et al. in The Garter Snakes - Evolution and Ecology 1996 * show T. s. fitchi as the subspecies present along the central coast from south of Monterey Bay to Santa Barbara County.
- In a 2002 study of western T. sirtalis, Janzen et al show T. s. fitchi as present in the area.
- Robert Stebbins, in his 1985 and 2003 western herp field guides, shows T. s. infernalis as the subspecies present along the entire south coast with two areas in question in Ventura and Orange/San Diego counties. (His 2012 field guide does not show subspecies.)
My range map here used to show T. s. infernalis in that area, following Stebbins, and then as an intergrade range, but I have decided now to show only T. s. fitchi in the area for these reasons:
- A herpetologist who has surveyed the area told me that snakes in the area he has found have all keyed out to be T. s. fitchi - from the Salinas Valley south to Vandenburgh in Santa Barbara County.
- I have seen several specimens from the area that were clearly T. s. fitchi, including one from right on the coast at San Simeon;
- All of the pictures of common gartersnakes in the area I have seen that can be identified look like T. s. fitchi, but none look like T. s. infernalis, including pictures I've looked at on iNaturalist and in the H.E.R.P. database. (Some are labled T. s. infernalis, but that is not evident from the pictures);
I don't have the resources to check all of the museum specimens from the area, but until I see evidence that T. s. infernalis also inhabits the area (T. s. fitchi clearly is there) I'll show only T. s. fitchi. If you have seen pictures or other evidence of a snake that is clearly T. s. infernalis inhabiting the area, please let me know.
Rossman et al (1996) show the elevation record for the species (not specifically this subspecies) at 8,333 feet (2540 m.). Stebbins (2003) shows it as
8,000 ft. (2,438 m).
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.)
Common Gartersnakes in Southern California have been treated as a unique taxon by the State of California that is different from T. s. infernalis, the California Red-sided Gartersnake. These snakes are called Thamnophis sirtalis ssp. - South Coast Gartersnake. This subspecies has not been formally described, so its recognition is not common. These are are a few mentions I have found:
The State of California has designated Common Gartersnake populations along the coastal plain from Ventura County to San Diego County as a Species of Special Concern. These snakes are listed as a separate subspecies by the California Dept. of Fish and Wildlife's Special Animals List - T. sirtalis ssp. - south coast gartersnake (Coastal plain from Ventura Co. to San Diego Co., from sea level to about 850 m).
In its 2018 Sportfishing Regulations, the California Department of Fish and Wildlife restricts the take of south coast common gartersnakes in L.A., Orange, Riverside, San Diego, and Ventura counties.
In a report published in 1994, Jennings and Hayes list the "South Coast Garter Snake, Thamnophis sirtalis sp." as a species of special concern and describe its distribution, habitat, life history, and status:
"This California endemic is known only from scattered localities along the southern California coastal plain; apparently from the Santa Clara River Valley (Ventura County: SDSNH 4376; UCSB uncat.; S. Sweet, pers. comm.), south to the vicinity of San Pasqual (San Diego County: Klauber 1929; Figure 49). Verified sightings and museum specimens indicate that this taxon historically occurred from near sea level (Ballona Creek and Playa del Rey Marsh, Los Angeles County: Von Bloeker 1942) to ca. 832 m (Lake Henshaw, San Diego County: R. Fisher, pers. comm.)
The South Coast garter snake appears restricted to marsh and upland habitats near permanent water that have good strips of riparian vegetation (Grinnell and Grinnell 1907; S. Sweet, pers. comm.), probably because such sites provide the right combination of prey and refuge sites. Historical records of this taxon also exist for meadow-like habitats adjacent to marshlands (Van Bloeker 1942). Data are lacking on the microhabitats required for bearing young.
Status: Endangered; of the 24 known historic localities for this taxon, 18 (75%) no longer support snakes. Extensive urbanization and flood control projects have destroyed most sites; some more isolated locations, such as in the Santa Monica Mountains, appear to have lost snakes following heavy floods or extended droughts (DeLisle et al. 1986). Habitat loss through agriculture, urbanization, and flood control projects, as well as the presence of many introduced aquatic predators threatens the six remaining localities where this snake still exists. This taxon can be notorious difficult to find in some areas (Klauber 1929; S. Sweet, pers. comm.)."
(Jennings, Mark R.
and Marc P. Hayes. Amphibian and Reptile Species of Special Concern in California. California Department of Fish and Game, published November 1, 1994.)
In a 2016 publication on herp species of special concern in California, Thompson et al discuss the South Coast Gartersnake, showing it as:
Garter Snake, Southern Populations, Thamnophis sirtalis (Linnaeus 1758). They remark that southern coastal populations of T. sirtalis have not been formally described as a distinct taxon, mentioning some ongoing studies on them, and discuss the snake's life history, taxonomy, the nature and degree of threat to it, and more.
(Robert C. Thomson, Amber N. Wright, and H. Bradley Shaffer. California Amphibian and Reptile Species of Special Concern. University of California Press, 2016.)
The northern population of T. sirtalis is not considered threatened.
South Coast Gartersnake
The California Department of Fish and Game lists the South Coast Gartersnake as a California Species of Special Concern. Jennings and Hayes in a 1994 publication state that 75 percent of the known historic localities for this snake no longer support snakes due to habitat loss from urbanization and flood control projects, floods, extended droughts, and introduced aquatic predators.
In California Amphibian and Reptile Species of Special Concern (2016) the authors remark that southern coastal populations of T. sirtalis have a "...very small range in a heavily-impacted part of the state. In addition, these populations have been extirpated from most of their historical range, which justifies a Priority 1 Species of Special Concern designation." The authors attribute the population decline of the taxon to habitat loss and fragmentation and natural events such as floods and droughts. They mention possible threats from introduced aquatic predators, introduced water snakes, and vegetation shifts and the availability of intermittent and ephemeral water bodies utilized by the species due to climate change.
North American Gartersnakes
Thamnophis sirtalis - (Linnaeus, 1758) - Syst. Nat., 10th ed., Vol. 1, p. 222 Thamnophis sirtalis infernalis - (Blainville, 1835) - Nouv. Ann. Mus. Hist. Nat. Paris, Vol. 4, p. 291, pl. 26, fig. 3
The following conservation status listings for this animal are taken from the November 2020 California "Special Animals List" and the November 2020 "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.
The species T. sirtalis is not included on the Special Animals List, which indicates that there are no significant conservation concerns for it in California.
T. sirtalis populations along the coastal plain from Ventura County to San Diego County are listed as a separate subspecies by the Special Animals List - T. sirtalis pop. 1 - south coast gartersnake (Coastal plain from Ventura Co. to San Diego Co., from sea level to about 850 m) with this note:
CDFW "Species of Special Concern" treats this population as a distinct taxon, though it is more commonly treated as a subpopulation of Thamnophis sirtalis infernalis, the California red-sided gartersnake.
The California state rankings and listings shown below apply only to the South Goast Garter Snake - T. sirtalis pop. 1.
NatureServe Global Ranking
The species is: Secure—Common; widespread and abundant.
South Coast Gartersnake:
This subspecies is: Critically Imperiled - Imperiled
NatureServe State Ranking
South Coast Gartersnake:
Critically Imperiled - Imperiled
U.S. Endangered Species Act (ESA)
California Endangered Species Act (CESA)
California Department of Fish and Wildlife
South Coast Gartersnake:
Species of Special Concern