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The Red-sided Gartersnake is a subspecies of the Common Gartersnake—the most northerly distributed reptile in North America. It ranges from northern Canada into Montana, North Dakota, and Minnesota, south through Colorado, Kansas, and Missouri, and into Texas and Oklahoma. In Wyoming it is found near permanent water at the lower elevations of the prairie life zone in the northern and eastern counties of the state. It inhabits the margins of pond, marsh, slough, and streamside habitats associated with grassland, wetland, and riparian communities. The Valley Gartersnake (T. s. fitchi) is a closely related subspecies occurring in the western counties; similarities due to interbreeding are thought to occur within the state.
These snakes are adapted to cooler climates and are often the first species of reptile to emerge from hibernacula in the spring. They are active from April into October. Dens in Manitoba, Canada have accumulations of 5,000-8,000 individuals, but in Wyoming densities less than 100 are the norm. They are diurnal predators feasting primarily on fish, frogs and toads, supplemented with invertebrates and small rodents. When encountered in the field they quickly flee to cover, but may occasionally take a defensive stance by coiling and striking. When handled they usually twist and thrash about and may attempt to bite—in addition to emitting foul anal secretions—before becoming docile.
It is a medium sized snake with a maximum length of 48 inches, but generally in the range of 24-30 inches; females are much larger than males. The scales are keeled and the anal plate is entire. The background is dark olive or brown with 3 longitudinal stripes: the dorsal midline is pale yellow or cream and the lateral stripes are a duller yellow and located on the 2nd and 3rd rows from the ventral plates. The area between the dorsal and lateral stripes has two alternating rows of black spots set against a red background that form a checkerboard pattern. The belly is gray or pale yellow, sometimes with a pale bluish-green tint.
The presence of black spots on the anterior margin of the ventral scales help to identify this subspecies from the Valley Gartersnake (please see T. s. fitchi for additional information). The Wandering Gartersnake also has lateral stripes on the 2nd and 3rd row of scales, but it clearly lacks the red background above the lateral stripes. The Plains Gartersnake has lateral stripes on the 3rd and 4th row of scales and has distinct black bars on the posterior margins of the labial scales along the upper jaw.
Mating usually occurs in early spring immediately after emerging from the den. Copulation is also reported to occur later in the summer or fall and viable sperm is retained within the female overwinter with fertilization and birth occurring the following year. Multiple males pursue the pheromone scent trails left by females, and successful males leave a gelatinous copulatory plug in the female’s cloaca to prevent further insemination from other males. The plug is discharged after several days. Young are born alive in August and early September with litter sizes in the range of 12-24. Sexual maturity is reached in the 2nd or 3rd year.