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Thamnophis radixPlains Garter Snake

By Tanya Dewey

Geographic Range

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)

Habitat

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)

Physical Description

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)

  • Sexual Dimorphism
  • male larger
  • sexes shaped differently
  • Range length
    109.5 (high) cm
    43.11 (high) in

Development

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)

Reproduction

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)

  • Breeding interval
    Plains garter snakes breed once yearly.
  • Breeding season
    Plains garter snakes breed in April or May.
  • Range number of offspring
    5 to 60
  • Average number of offspring
    10-20
  • Range gestation period
    83 to 102 days
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years

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)

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

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

  • Range lifespan
    Status: captivity
    8.5 (high) years

Behavior

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 they can recognize individuals and form dominance relationships. (Ernst and Ernst, 2003)

Home Range

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)

Communication and Perception

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)

Food Habits

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 (Anaxyrus americanus), great plains toads (Anaxyrus cognatus), tree frogs (Hyla species), striped chorus frogs (Pseudacris triseriata), plains leopard frogs (Lithobates blairi), northern leopard frogs (Lithobates pipiens), and various salamanders. They have been recorded eating mosquitofish (Gambusia affinis), southern redbelly dace (Chrosomus 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)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • fish
  • insects
  • terrestrial worms

Predation

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)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

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

Economic Importance for Humans: Positive

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

Economic Importance for Humans: Negative

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)

Conservation Status

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)

Other Comments

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)

Contributors

George Hammond (editor), Animal Diversity Web Staff.

Tanya Dewey (author), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

piscivore

an animal that mainly eats fish

polarized light

light waves that are oriented in particular direction. For example, light reflected off of water has waves vibrating horizontally. Some animals, such as bees, can detect which way light is polarized and use that information. People cannot, unless they use special equipment.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Ernst, C., E. Ernst. 2003. Snakes of the United States and Canada. Washington, D.C.: Smithsonian Books.

Harding, J. 2000. Amphibians and Reptiles of the Great Lakes Region. Ann Arbor, Michigan: University of Michigan Press.

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