Thamnophis atratus atratus - Santa Cruz Gartersnake

Nonvenomous

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.

Size

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).

Appearance

A medium-sized slender snake with a head barely wider than the neck and keeled dorsal scales. Ground color is gray, brown or black. There is a wide yellowish to orange-yellow dorsal stripe, but with the side stripes absent or obscured.There may be small alternating dark spots on the sides. The throat is white or yellow, sometimes bright yellow. The underside is bluish or greenish sometimes with pink or yellow marks.

The following description is from Boundy, Jeff. Systematics of the Garter Snake Thamnophis atratus at the Southern End of Its Range. Proceedings of the California Academy of Sciences Volume 51, No. 6, p. 330. 1999.

"...midbody scale rows 19 (65%) or 17 (35%) ... vertebral stripe yellow to orange yellow and broad, averaging 4.0 (range 2.7 - 6.5) scale rows in the nuchal area ... lateral stripes absent; dorsum olive black, grading to dark olive at the ventrals; dorsal black spots obscure; iris dark brown; top of head dark olive brown to olive black, with a prominent parietal spot; supralabial suture marks narrow when present; demarcation between dorsal head color and dusky olive supralabials indistinct; chin cream, becoming deep yellow on the throat; venter abruptly becoming olive gray in the thoracic area, continuously darkening posteriorly; prominent yellow-orange midventral suffusion; dark markings absent from transverse ventral sutures..."

Behavior

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.

Adults have been found to forage actively, neonates are sit-and-wait foragers, and juveniles practice both forms of foraging.

Diet

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.

ourtship has been observed during March and April. Young are born live late summer to early fall.

Range

This subspecies is endemic to California.

According to Boundy, 1999, in his revision of T. atratus into three subspecies, T. a. atratus occurs in the "Santa Cruz Mountains and the southern San Francisco Peninsula, from the San Andreas rift lakes to the San Lorenzo River watershed and Uvas Canyon." Intergrades with T. a. zaxanthus occur in the southern Santa Cruz Mountains. (Authorities who do not recognize T. a. zaxanthus show the range of this subspecies also occuring in the East Bay south along the coast and inner south coast ranges into Santa Barbara County.)

Habitat

Creeks, streams, small lakes and ponds, in woodland, brush and forest and grassy ecotones. Seems to prefer shallow rocky creeks and streams. When found in muddy ponds there are usually rocky outcrops nearby.

Taxonomic Notes

This snake is known to hybridize with T. hammondii in Santa Barbara and San Luis Obispo Counties. 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 T. a. hydrophilus and T. a. atratus or T. a. zaxanthus. The snakes in this area were formerly classified as T. a. aquaticus (previously T. couchii aquaticus.)

T. atratus found in the east Bay and south along the inner coast ranges are now classified as T. a. zaxanthus by some taxonomists, including the taxonomy we follow here. Conservation Issues (Conservation Status) Not known to be threatened, but gartersnakes have been negatively impacted by competition with introduced bullfrogs in some areas.