Gray: North: Aquatic Gartersnakes formerly recognized as
the subspecies Thamnophis atratus aquaticus, now recognized as an area of intergradation
between the three recognized subspecies.
South: T. a. atratus x T. a. zaxanthus intergrades.
North of the San Francisco Bay, there is a very large intergrade range between T. a. hydrophilus, T. a. atratus, and T. a. zaxanthus. The snakes in this area were formerly classified as T. a. aquaticus (previously T. couchii aquaticus.)
This subspecies is no longer recognized. More pictures and information about these intergrades can be seen here.
We were just about to leave after lunch when I saw this guy slip into the water out of the corner of my eye. I started changing lenses knowing he would come up nearby as the pool was only 4 feet across. You can imagine my surprise to see him come up with this very angry trout. I took a number of shots as they fought. They were in still water about 4 inches down. They even went over a 2 foot waterfall but the snake never let go. Eventually he got his back end up on a rock, slowly dragged the fish out and eventually began swallowing it."
Habitat, Mendocino County. These small pools of water along the edge of a wide riverbank in summer, contained Foothill Yellow-legged Frogs and tadpoles, Bullfrogs, and Oregon Gartersnakes.
Habitat, Mendocino County
More pictures of this animal and its natural habitat are available on our Northwest Herps page.
An Oregon Gartersnake basks on a rock in a
River in Mendocino County, and swims away.
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 are 18 - 40 inches long (46 - 102 cm). Most snakes encountered are generally 18 - 28 inches long (46 - 71 cm).
Neonates are 7 - 10 inches ( 18 - 25 cm).
A medium-sized slender snake with a head barely wider than the neck and keeled dorsal scales.
Some average scale counts: Average of 8 upper labial scales, 6 and 7 not enlarged. 11 lower labial scales.
Rear pair of chin shields is longer than the front.
The internasals are longer than they are wide and pointed in front.
Average of 19 or 21 scales at mid-body.
Color and Pattern
Ground color is gray, olive-gray, or brownish.
This snake may have a light stripe on the back and a light stripe along the lower part of each side.
The dorsal stripe and the side stripes may be absent or obscured, not contrasting sharply with the ground color, leaving a checkered appearance instead of striped. There are usually alternating dark spots on the sides.
The throat is light in color.
The underside is light and unmarked with a pinkish or purplish tint toward the tail.
A highly-aquatic snake, able to remain underwater, but also found away from water.
Active during the day, and after dark during very hot weather.
Can be active most of the year when conditions allow, but primarily found spring through fall.
When threatened, this snake will often escape into water, hiding on the bottom. If it is frightened when picked up, it will often strike repeatedly and release feces from the cloaca and expel musk from anal glands.
Diet and Feeding
Probably eats mainly amphibians and their larvae, including frogs, tadpoles, and aquatic salamander larvae (newts and giant salamanders, Taricha and Dicamptodon ), but small fish are also eaten. Captives have also taken small rodents. Leeches may also be consumed - I saw a recently-captured T. a. zaxanthus regurgitate two leeches.
Adults tend to forage actively. Neonates are sit-and-wait foragers. Juveniles practice both types of foraging.
This species has been observed eating adult Pacific Newts (genus Taricha) which are deadly poisonous to most predators.
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.
Courtship has been observed during March and April.
Young are born live late summer to early fall.
Creeks, streams, rivers, small lakes and ponds, in woodland, brush and forest. Seems to prefer shallow rocky creeks and streams.
This subspecies, Thamnophis atratus hydrophilis - Oregon Gartersnake, ranges from northern Sonoma County north along the coast to Douglas County, Oregon, and east throughout the north coast ranges and to the lower Pit River area. It is absent from much of the coast around Humboldt County.
The species Thamnophis atratus - Aquatic Gartersnake, ranges from Santa Barbara County north through the coast ranges into southwest Oregon.
Notes on Taxonomy
This snake is known to hybridize with T. couchii in Shasta County. For a long time T. atratus was considered a subspecies of T. couchii. In 1987 it was classified as a distinct species.
North of the San Francisco Bay, there is a very large intergrade range between the Oregon Gartersnake and T. a. atratus or T. a. zaxanthus. The snakes in this area were formerly classified as T. a. aquaticus (previously T. couchii aquaticus.)
Thamnophis - Greek - thamnos - shrub or bush, and ophis - snake, serpent
atratus - Latin - clothed in black, mourning - refers to the dark dorsal color hydrophilus - Greek - hydor - water, and philus - loving - refers to the snakes aquatic proclivities
Stebbins, Robert C., and McGinnis, Samuel M. Field Guide to Amphibians and Reptiles of California: Revised Edition (California Natural History Guides) University of California Press, 2012.
Stebbins, Robert C. California Amphibians and Reptiles. The University of California Press, 1972.
Samuel M. McGinnis and Robert C. Stebbins. Peterson Field Guide to Western Reptiles & Amphibians. 4th Edition. Houghton Mifflin Harcourt Publishing Company, 2018.
Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. 3rd Edition. Houghton Mifflin Company, 2003.
Behler, John L., and F. Wayne King. The Audubon Society Field Guide to North American Reptiles and Amphibians. Alfred A. Knopf, 1992.
Powell, Robert., Joseph T. Collins, and Errol D. Hooper Jr. A Key to Amphibians and Reptiles of the Continental United States and Canada. The University Press of Kansas, 1998.
Bartlett, R. D. & Patricia P. Bartlett. Guide and Reference to the Snakes of Western North America (North of Mexico) and Hawaii. University Press of Florida, 2009.
Bartlett, R. D. & Alan Tennant. Snakes of North America - Western Region. Gulf Publishing Co., 2000.
Brown, Philip R. A Field Guide to Snakes of California. Gulf Publishing Co., 1997.
Ernst, Carl H., Evelyn M. Ernst, & Robert M. Corker. Snakes of the United States and Canada. Smithsonian Institution Press, 2003.
Wright, Albert Hazen & Anna Allen Wright. Handbook of Snakes of the United States and Canada. Cornell University Press, 1957.
Rossman, Douglas A., Neil B, Ford, & Richard A. Siegel. The Garter Snakes - Evolution and Ecology. University of Oklahoma press, 1996.
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.
This snake is not included on the Special Animals List, which indicates that there are no significant conservation concerns for it in California.