Thamnophis atratus zaxanthus - Diablo Range 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

Thamnophis atratus is 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 distinct yellow or orange stripe on the back and a light stripe along the lower part of each side on the 2nd and 3rd scale rows. There may be small alternating dark spots on the sides, most noticable on juveniles. 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. 328. 1999.

"...midbody scale rows 19 (85%) or 17 (15%)... vertebral stripe relatively broad, averaging 3.2 (range 1.5 - 5.0) scale rows on the nape... vertebral stripe yellow to orange-yellow; lateral stripe conspicuous, pale green; dorsum dark gray to black; iris gray or brown; top of head dark, olive black, with a prominent parietal spot; supralabial suture marks narrow when present; demarcation between dorsal head color and pale supralabials distinct; chin cream, becoming bright yellow on the throat, grading to pale green in the thoracic region; ventral color darkening slightly posteriorly; midventral suffusion yellow to orange; dark markings absent from transverse ventral sutures; eye moderate in size..."

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) but small fish are also eaten, and possibly small rodents. I have also seen this snake regurgitate two leeches.

Reproduction

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

Range

This subspecies, Thamnophis atratus zaxanthus - Diablo Range Gartersnake, is endemic to California. According to Boundy, 1999, in his original description of this snake, Thamnophis atratus zaxanthus occurs in the "inner Coast Range from Napa and Solano to Santa Barbara counties and the Santa Lucia Range." This includes the East Bay south along the southern edge of the Santa Cruz Mountains where this subspecies interegrages with T. a. atratus.

The species Thamnophis atratus - Aquatic Gartersnake, ranges from Santa Barbara County north through the coast ranges into southwest Oregon, including most of the state north of Santa Barbara County and west of the interior valleys.

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

For a long time T. atratus was considered a subspecies of T. couchii. In 1987 it was classified as a distinct species. In 1999 Boundy revised the species to include T. a. zaxanthus.

North of the San Francisco Bay, there is a very large intergrade range between the 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.)

Conservation Issues (Conservation Status)

Not known to be threatened, but gartersnakes have been negatively impacted by competition with introduced bullfrogs in some areas.