General Description: The Terrestrial Gartersnake is a slender, medium-sized snake. Adults range from 20–30 inches (50.8–76.2 cm) in total length. The coloration of Terrestrial Gartersnakes varies across their range, but in South Dakota, individuals have a thin tan or pale yellow middorsal stripe extending down the entire length of the body. The background coloration is typically tan or olive and an additional lateral stripe exists on each side of the body that occurs on scale rows two and three. Patterning and coloration between the middorsal and lateral stripes can be variable, but it is generally a series of alternating black blotches. These black blotches extend onto the middorsal stripe, partially obscuring it. All individuals have keeled scales, an undivided anal scale, and black pigmented ventral (belly) scales. Similar to other gartersnakes, females reach larger lengths and weights than males and there are no external characters that can be used to differentiate males from females. Typically, males have a longer tail length relative to the total length, while females have a smaller tail length that quickly tapers to a point. Terrestrial Gartersnakes can easily be differentiated from Plains Gartersnakes (Thamnophis radix) by having lateral stripes on scale rows two and three (Plains Gartersnakes have lateral strips on scale rows three and four), and from Common Gartersnakes (Thamnophis sirtalis) by the presence of black pigmentation on the ventral scales (Common Gartersnakes lack pigmentation on ventral scales).
Behavior: Terrestrial Gartersnakes are typically active from March to November, but this can vary regionally. This is an active, primarily diurnal species of snake, though, during the summer may shift activity to become more nocturnal to avoid activity during the heat of the day. Typically beginning in September these snakes will begin to move to overwintering hibernacula. Hibernacula are often rock crevices, mammal burrows, artificial (man-made) cover such as under concrete or wood piles. On warm days from November to March, individuals can be found emerging from hibernacula to bask in the sun, though they will not move far from the entrance. Prey consists mostly of amphibians, fish, earthworms, or other soft-bodied invertebrates, however, they are also known to consume small mammals and chicks of ground nesting birds. Unlike most other colubrid snakes, Plains Gartersnakes do not constrict prey, but instead, subdue and rapidly consume prey. Upon capture, Terrestrial Gartersnakes will release a foul-smelling musk.
Reproduction: Males of this species typically emerge in advance of females from hibernation around mid-April and will wait near the hibernacula for females to emerge. Once a female emerges, multiple males will attempt to court her. Eventually, a male is successful in breeding with this female who then disperses away from the hibernacula. Males will continue to stay at the hibernacula attempting to mate with females for several weeks. Females give live birth to an average of 20 young in the late summer. Sexual maturity is reached typically in two years.
Habitat: Terrestrial Gartersnakes can typically found in close proximity to aquatic habitats such as streams, reservoirs, and wetlands, though have been found in great distance from water sources.
Species Range: Terrestrial Gartersnakes are only found from the Rocky Mountains west to the California coast and north into southern Canada. Populations in the Black Hills of South Dakota represent the easternmost distribution of this species.
South Dakota Range: The Terrestrial Gartersnake is primarily found in the Black Hills. Recently, populations have been discovered in the northwest corner of the state (Harding County) though voucher specimens are absent from this region.
South Dakota Status: This species is not listed by South Dakota Game, Fish and Parks.