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Common garter snakes are highly variable in color pattern. — Picture taken by steve hillebrand

General: The Common garter snake, Thamnophis sirtalis, is one of the most common species of snake in their native habitats across much of North America. This non-venomous, cold-blooded snake is highly adaptable, thriving in urban and suburban environments and gardens, much to the dismay of the gardeners.

Description: Common garter snakes are an abundant, cold-blooded species exhibiting high levels of variation in coloration and appearance. Dozens of subspecies that have been identified by distinct color variations; however, variation in color has been found within populations, indicating that coloration alone may not be enough to identify a subspecies1.

Common garter snakes grow throughout the duration of their lives and are, on average, 18” to 26” in length but may grow as long as 49”2. Males and females are similar in coloration but males are smaller than females with longer tails relative to their size. The head, compared to the neck and body, is wide. Scales of the Common garter snake are keeled, meaning there is an elevated ridge running the length of each scale3. Body coloration may range from black, red, brown, gray to green.

Populations living in Florida and southern Georgia generally have bluish coloration on their bodies4. The underside, chin and throat (generally similar in color to the stripes on the body) may be green, blue-gray, green, or white, occasionally with black spots. There are typically three stripes running the length of the body on scales 2 and 3 (ranging from yellow, brown, blue, orange, gray, green, or white in color,) the middle stripe running down the center of the back. These stripes may be solid or may become undefined or entirely absent. In some specimens, two rows may join to form a thick bar or may become checkered or spotted in appearance5. Some populations of Common garter snakes exhibit red or orange coloration between the scales. This coloration may extend to the tips of the scales. The top of the head is generally dark with a pair of spots that are light or almost white in color. The eyes are large, relative to the size of the head. Upper lip scales are marked by dark vertical bars and the tongue is red and tipped in black.

Common garter snakes live an average of 2 years in the wild and 6 to 10 years in captivity. One Common garter snake living in captivity survived to 20 years of age6.

Since Common garter snakes are cold blooded, they must use thermoregulation to maintain an appropriate body temperature of 28° to 32° C7. To do this, snakes bask in sunlight during the morning and as body temperatures fall during the day, snakes become less active. Despite Common garter snakes being a solitary species, they are known to gather at night to sleep in groups to avoid becoming too cold. Common garter snakes can be active throughout the year and may be seen basking atop snow on warm winter days. This particular species is active across a greater temperature range than many other species of snakes due to a greater tolerance of cold, enabling one of the shortest hibernation periods of all snake species in nearctic habitats. Common garter snakes hibernate in large numbers during the winter, from October to early April, in natural cavities (such as crayfish or mammal burrows or ant mounds,) under debris such as rocks or tree stumps, in dams, or on hillsides. Sleeping in tightly coiled groups allows snakes to maintain body temperatures. Hibernation sites are different from summer grounds (used for feeding and breeding,) and snakes have been known to travel great distances from one site to the other.
Common garter snakes use touch and smell to communicate with one another, generally for reproductive purposes. Before mating, males secrete pheromones to attract females coming out of hibernation. Snakes use their tongues to sample chemicals from the air (including these pheromones,) using a specialized organ located on the roof of the mouth to decipher these chemicals. Common garter snakes are also able to sense vibrations and have developed vision8.

Habitat: Common garter snakes inhabit a variety of moist environments, such as woodlands, marshes and meadows. They prefer maintaining close proximity to water and cover under grass or brush, opting for pond or lake edges, streams, or ditches. In the case that water is not readily available, Common garter snakes have been known to travel long distances to locate a new source.

A highly adaptable species, Common garter snakes can survive extreme conditions9. Their adaptability allows them to thrive in urban and suburban settings.

Within their native habitats, Common garter snakes fall prey to many predators, including snapping turtles, foxes, squirrels, great blue herons, hawks, larger snakes, skunks, raccoons, opossum, American crows, bullfrogs and large fish. To avoid predators, Common garter snakes may hide in grass stripes (using their strips as camouflage,) or flee to water, but when attacked, they tend to coil to appear larger, strike and bite10. When caught, Common garter snakes release an unpleasant secretion from glands located at the base of the tail.

Threats: Common garter snakes are considered a low risk species; however, certain threats exist that are detrimental to population numbers. Urbanization has lead to a loss of the Common garter snake’s native habitats. Pesticides and chemicals also damage habitats. Common garter snakes have also experienced a decrease in food supplies, due to disease and competition by invasive species. Snakes living in developed areas often fall prey to dogs, cats, automobiles or lawnmowers and face the risk of being captured for domestication.

Location: Common garter snakes reside in the nearctic region, and may be found in a variety of habitats across much of North America, including woodlands, marshes, meadows, and hillsides. Populations may be found as far south as Florida’s western coast on the Gulf of Mexico, north to British Columbia, and west to California. The Common garter snake is generally absent from arid environments like the American Southwest, although several isolated populations exist in New Mexico and northern areas of Mexico11.

Diet: Common garter snakes are low-level predators, feeding mainly on earthworms and amphibians. Their diets also include insects, frogs, toads, salamanders, snails, leeches, slugs, crayfish, fish, tadpoles, snakes, baby birds and bird eggs, small lizards, and small mammals (such as mice.) These snakes are skilled swimmers, swimming slowly and locating prey with their highly developed senses of smell and sight. They capture prey using a variety of hunting methods, including craning, peering, and ambushing12. The saliva of the Common garter snake is slightly toxic, partially immobilizing their small prey, making it easier for the snake to swallow their meal whole. The quick reflexes and sharp teeth of the Common garter snake make them a well-honed predator in their habitats.

Toads and some other amphibians emit toxic chemicals to ward off predators; however, the Common garter snake is one of the few species able to consume these animals, unaffected by the chemicals.

Reproduction: The Common garter snake is a viviparous species, meaning that it bears live young. Mating occurs in the spring, immediately after snakes emerge from hibernation dens. The males exit first, surrounding the females as they emerge, secreting pheromones that will attract them. The females select their mate and as mating begins, snakes coil together, forming a breeding ball. After mating, females return to their summer habitats to feed and locate appropriate birthing areas. The males stay near the hibernation den to try and mate again with remaining females. Some females may chose not to mate in a particular season since they have the ability to store sperm from previous mating13.

Gestation lasts for 8 to 12 weeks, as young snakes are incubated in the female’s lower abdomen. Females typically give birth to litters containing 10 to 80 young snakes that are 5” to 9” in length14. Young snakes, similar in appearance to adults, are born between late July and October and are immediately independent, responsible for finding their own food and survival. The young tend to remain in close proximity to their mothers for up to a few days despite the mother offering no care15. Common garter snakes grow quickly and reach sexual maturity in 1.5 to 2 years (males generally reaching sexual maturity first.)

Notes of Interest

Despite often being feared, Common garter snakes should be a welcomed part of any garden as they aid in controlling insect and pest populations.

Common garter snakes may be tamed with appropriate handling, resulting in their popularity as pets.

The saliva of the Common garter snake may cause an allergic reaction in individuals who have handled a snake or been bitten16.

The San Francisco garter snake (Thamnophis sirtalis tetrataenia,) a subspecies of the Common garter snake, is an endangered species and in 1967 was placed on the U.S. and California Endangered Species lists.

Footnotes

http://srelherp.uga.edu/snakes/thasir.htm
http://animaldiversity.ummz.umich.edu/accounts/Thamnophis_sirtalis/
http://www.inhs.illinois.edu/animals_plants/herps/species/th_sirtali.html
http://www.arkive.org/common-garter-snake/thamnophis-sirtalis/
http://www.fcps.edu/islandcreekes/ecology/eastern_garter_snake.htm

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Garter snake – Thamnophis sirtalis, Facts

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