Thamnophis radix
Plains Garter Snake

Plains garter snakes are found throughout the North American plains region, from the Oklahoma panhandle, northernmost Texas, and northeastern New Mexico north to southern Alberta, Saskatchewan, and Manitoba, and east through Minnesota, Wisconsin, and Indiana. (Ernst and Ernst, 2003)

Plains garter snakes are found in meadows, prairies, and other grasslands near sources of water, such as ponds, streams, marshes, and sloughs. They may also be found in swampy areas or along rivers. They may be found in suburban or urban vacant lots. Habitats they occupy may be influenced by the presence of a congener; where they co-occur with common garter snakes (Thamnophis sirtalis), they may be found in more dry habitats than common garter snakes. (Ernst and Ernst, 2003)

Plains garter snakes are long, striped garter snakes, usually from 40 to 70 cm long, but occasionally up to 109.5 cm. They have a dorsal and two lateral, yellow or orange stripes on a background scale color of dark brown to dark greenish. Lateral stripes are on scale rows 3 and 4. The sides may have some red pigmentation. Scales are keeled and measure 19 to 21 rows at the mid-body. There is a row of black spots between the lateral stripes and the ventral scales. They have an undivided anal plate. Males are slightly larger, with more ventral and subcaudal scales and slightly longer tails. Male tails are about 20.5 to 27.8% of total body length, whereas females have tails that are 17.6 to 27.5% of their body length. Males also have tubercles on their chin shields. There are no described subspecies. (Ernst and Ernst, 2003)

Plains garter snakes grow at a rate of approximately 1.1 cm per week during their first year. Growth rates slow in subsequent years. (Ernst and Ernst, 2003)

Males track females via pheromone trails and compete for mating opportunities with receptive females. Males crawl alongside females and push on her with their noses while their bodies undulate. They touch the female's back with their tongues and attempt to copulate. If the female is receptive, she will raise her tail and allow copulation. A seminal plug may be inserted to deter copulation with another male. Both males and females can mate with multiple individuals. (Ernst and Ernst, 2003)

Mating takes place after these snakes emerge from hibernation, in April or May. Females are sexually mature in their 2nd or 3rd year. Plains garter snakes give birth to live young from June through September, after a gestation period of 83 to 102 days. There are from 5 to 60 young in a litter, but usually 10 to 20. Litters may be larger in northern parts of the range, litter size varies with nutritional status and size of the female. Young are born at sizes from 11.9 to 24.1 cm and 0.93 to 2.48 g. (Ernst and Ernst, 2003)

Females gestate and give birth to live young, investing significant nutritional resources. After the young are born, there is no further parental involvement. (Ernst and Ernst, 2003)

A captive plains garter snake was recorded living to almost 8 1/2 years. (Ernst and Ernst, 2003)

Plains garter snakes are active from March to November, depending on the region. They may be active for shorter periods in northern portions of their range. They hibernate in burrows or rock crevices, although they may emerge on warm, winter days. They often hibernate in rodent burrows or ant mounts, but have also been found in crayfish burrows, under sidewalks, in other man made crevices, and even underwater. Activity patterns depend on air temperatures. They are active during the day at warm temperatures, usually between 21 and 29 degrees Celsius. Once the air temperature goes above 31 degrees Celsius, these garter snakes switch their active period to night. At much lower temperatures, prairie garter snakes become inactive. Where they co-occur with common garter snakes (Thamnophis sirtalis), plains garter snakes generally have higher body temperatures under the same conditions. Daily activity patterns are also influenced by breeding condition, males spend more time active during mating season and females spend more time active when they are pregnant. These snakes are solitary outside of brief interactions with mates and grouping during hibernation. There is some evidence that thy can recognize individuals and form dominance relationships. (Ernst and Ernst, 2003)

Plains garter snakes stay within relatively small home ranges for long periods of time, moving only up to 76 meters over periods of over a year. They are not reported to be territorial. Individuals released at distances from their area of capture were able to return to their home range over relatively long distances. Population densities have been estimated at between 69 to 123 per hectare. (Ernst and Ernst, 2003)

Plains garter snakes use their sense of smell extensively. They find prey, mates, and hibernacula by following chemical trails. They also use vision and vibrations to detect threats and navigate. Some evidence suggests they may navigate using polarized light. Males use touch in courtship rituals. (Ernst and Ernst, 2003)

Plains garter snakes eat a wide variety of animal prey, overlapping significantly with the prey preferences of common garter snakes (Thamnophis sirtalis). They have been recorded preying on frogs and toads, salamanders, fish, birds, small rodents, leeches, earthworms, and grasshoppers. Amphibians eaten include northern cricket frogs (Acris crepitans), American toads (Bufo americanus), great plains toads (Bufo cognatus), tree frogs (Hyla species), striped chorus frogs (Pseudacris triseriata), plains leopard frogs (Rana blairi, northern leopard frogs (Rana pipiens), and various salamanders. They have been recorded eating mosquitofish (Gambusia affinis), southern redbelly dace (Phoxinus erythrogaster), bluntnose minnows (Pimephales notatus), bank swallows (Riparia riparia), and eastern meadowlarks (Sturnella magna). Plains garter snakes find prey by following an olfactory trail, then grabbing prey once they catch up with them. (Ernst and Ernst, 2003)

Plains garter snakes may be preyed on by birds of prey, such as red-shouldered hawks (Buteo lineatus), Swainson's hawks (Buteo swainsoni), kestrels (Falco sparverius), and northern harriers (Circus cyaneus). Other predators include foxes (Vulpes), coyotes (Canis latrans), striped skunks (Mephitis mephitis), mink (Neovison vison), domestic cats (Felis catus), and milk snakes (Lampropeltis triangulum). Humans also incidentally and intentionally kill prairie garter snakes. These garter snakes will bite, emit a foul smelling musk, or defecate to discourage predators. Their lateral stripes make them difficult to see in their grassy habitats and as they move. Plains garter snakes also have a series of antipredator displays that they will use, including hiding their heads, striking with the mouth closed or open, coiling or balling up their bodies, extending the body flat on the substrate, and waving the tail. They might also take refuge in water. Responses to threats vary with age. (Ernst and Ernst, 2003)

Plains garter snakes are important predators of amphibians, earthworms, leeches, and other animals in their prairie habitats. (Ernst and Ernst, 2003)

Plains garter snakes are important members of the native prairie habitats they are found in.

There are no adverse effects of plains garter snakes on humans. These are nonvenomous snakes that are shy and retiring, in general, although they will bite if threatened. (Ernst and Ernst, 2003)

Plains garter snakes are not considered threatened, although regional populations may be vulnerable. They are considered endangered in Ohio and a species of concern in Arkansas. (Ernst and Ernst, 2003)

Genetic evidence suggests that T. radix is most closely related to Thamnophis butleri and Thamnophis brachystoma, among Thamnophis species. (Ernst and Ernst, 2003)

Thamnophis radix fossils are known from the Pliocene of Nebraska. (Ernst and Ernst, 2003)