Common Garter Snake (Thamnophis sirtalis): Facts and Range

Appearance and color forms

A typical common garter snake has three pale stripes running the length of a darker body: one down the middle of the back and one low on each side. The stripes are usually yellow, but can be white, greenish, bluish or brown, and the background color ranges from black and dark brown to olive and gray. Many individuals also have a checkerboard of dark spots between the stripes, which can make the stripes look broken.

The scales are keeled, meaning each one has a small ridge down the middle, which gives the snake a slightly rough, matte look. The head is a little wider than the neck, and the tongue is red with a black tip.

Because the species covers such a huge area, it varies enormously. About a dozen subspecies are recognized, and several are famous for their colors:

• Eastern garter snake (T. s. sirtalis): the classic form of the eastern United States and Canada, with yellowish stripes on a dark background. Some populations around the Great Lakes include many all-black (melanistic) snakes.
• Red-sided garter snake (T. s. parietalis): the northern Great Plains and Prairie Provinces, with red bars or blotches along the sides. This is the subspecies of the Narcisse snake dens in Manitoba.
• San Francisco garter snake (T. s. tetrataenia): often called one of the most beautiful snakes in North America, with a turquoise belly and back stripe, black and red side stripes and a red head.
• Red-spotted garter snake (T. s. concinnus) and valley garter snake (T. s. fitchi): western forms from Oregon, Washington, California and the interior West.

Common garter snakes can be confused with ribbon snakes and other garter snake species. Ribbon snakes are thinner, with a longer tail and a small pale spot in front of each eye.

How big are common garter snakes?

Most adults measure 46–90 cm (18–35 in) in total length, including the tail. The average is often given as about 55 cm (22 in), and the record is about 137 cm (54 in). Females are larger and heavier than males, while males have relatively longer tails. A typical adult weighs around 150 g (5 oz) or less.

Newborn garter snakes are only 12.5–23 cm (5–9 in) long. Like other snakes, garter snakes keep growing slowly throughout their lives, so the largest individuals are usually old females.

Habitat and range

The common garter snake has one of the largest ranges of any North American reptile. It is found from southern Florida and the Gulf Coast north through the eastern United States to Quebec and the Maritime Provinces, west across the Great Plains and southern Canada to British Columbia, and south along the Pacific coast into California. It reaches farther north than any other snake in the Americas. It is largely missing from the dry deserts of the Southwest, with only scattered populations in parts of New Mexico and northern Mexico.

Within this range it is a generalist. Common garter snakes live in damp meadows, marshes, the edges of ponds, lakes, ditches and streams, prairies, woodland clearings, farmland and hillsides from sea level into the mountains. They do well near people as long as there is cover, such as logs, boards, rock piles, tall grass and garden beds, which is why they are so often seen in suburbs and city parks.

Diet and hunting

Common garter snakes are carnivores that hunt by sight and smell during the day. Their menu changes with age, place and season:

• Earthworms, a staple for young snakes and many adults
• Frogs, toads and tadpoles, including toads with toxic skin secretions
• Salamanders and newts
• Small fish, caught in shallow water
• Leeches, slugs and insects
• Occasionally small rodents, nestling birds and other snakes

Garter snakes do not constrict. They grab prey with many small teeth and simply swallow it alive, often starting with the head. Their saliva contains mild secretions that may help subdue small prey but are not dangerous to people; see Are garter snakes venomous? for the details.

Some populations in the Pacific Northwest and California have a remarkable resistance to tetrodotoxin, the deadly nerve poison of rough-skinned newts. These snakes can eat newts that would kill most other predators, a case of predator-prey coevolution explained in How do garter snakes eat toxic newts?.

Behavior and winter dens

Common garter snakes are active by day and stay active at lower temperatures than most snakes, which helps explain how far north they live. They bask in the morning to warm up and usually keep their body temperature around 28–32 °C (82–90 °F) when they can. In hot weather they shift activity to mornings and late afternoons.

In mild southern areas they can be active almost all year. Farther north they spend the cold months underground in a state of winter dormancy called brumation. They gather in communal dens, or hibernacula, such as rock crevices, rodent and crayfish burrows, stumps, wells and old foundations, and some snakes travel several kilometers between summer feeding areas and their dens. The largest known dens are at Narcisse in Manitoba, where tens of thousands of red-sided garter snakes overwinter in limestone sinkholes. More on how snakes survive the cold is in Do snakes hibernate?.

Outside the breeding season and the den, garter snakes are mostly solitary. They communicate through scent: skin pheromones tell snakes the sex of another snake, and they can follow each other’s scent trails with their flicking tongues.

Reproduction and life cycle

Mating starts in spring as soon as the snakes leave their dens. Males emerge first and wait at the den for the females. When a female appears, many males may court her at once, forming a writhing “mating ball,” which is most spectacular at the big northern dens; see Why do garter snakes form mating balls?. Females can also mate in autumn and store sperm over the winter.

Garter snakes are live-bearers. Instead of laying eggs, the female keeps the developing young inside her body for about two to three months and gives birth between late July and October. Litters usually contain 10–40 young, with a recorded range of about 4–80, and bigger females have bigger litters. The newborns are independent from the start, and the mother gives them no care.

Males can mature at about 1.5 years and females at about 2 years, when they are around 55 cm (22 in) long.

Lifespan

Life is short for most wild garter snakes. Many die in their first year, and the average lifespan in the wild is estimated at about 2 years, although some individuals live 4 years or more. In captivity they commonly live 6–10 years, and one captive snake reached about 20 years.

Predators and defenses

Because they are small and abundant, garter snakes are eaten by a long list of predators: hawks, herons, crows, gulls, raccoons, foxes, skunks, opossums, large fish, American bullfrogs and other snakes such as racers and kingsnakes. Shrews and even large spiders can take newborns.

Their defenses rely more on bluff than danger. A threatened garter snake usually freezes, trusting its camouflage, and then tries to slip away into vegetation or water. If grabbed, it thrashes, bites and smears a foul-smelling musk from glands at the base of its tail. Some flatten their bodies to look bigger, and some hide the head while waving the tail.

Conservation status

The IUCN Red List assesses the common garter snake as Least Concern because of its huge range, large population and tolerance of farmland and suburbs. Local threats include draining of wetlands, traffic on roads near dens, pesticide use and, in the past, mass collecting at northern dens for the pet and biological supply trades.

The exception is the San Francisco garter snake, confined to a small area of San Mateo County and northern Santa Cruz County in California. It has been listed as endangered by the US government since 1967, largely because of wetland loss and the decline of its main prey, the California red-legged frog. At Narcisse in Manitoba, snow fences and tunnels under the highway were built to cut road deaths after a sharp population drop around 1999, and the dens have since largely recovered.

Myths and facts

• Myth: garter snakes are completely nonvenomous. Mostly true for people, but not quite for prey: garter snakes have mild oral secretions from a gland in the upper jaw. They are still harmless to humans.
• Myth: the “garden snake” is its own species. There is no species by that name; it is a folk version of “garter snake” and usually refers to the common garter snake or one of its close relatives.
• Fact: garter snakes can be poisonous. Snakes that eat toxic newts can keep the toxin in their liver for weeks, so they can be dangerous to eat as well as mildly venomous.

Relationship with people

For many children in North America, the garter snake is the first wild snake they ever see up close. It is the official state reptile of Massachusetts, and the mass emergence at Narcisse draws visitors from around the world each spring. Garter snakes also help gardeners and farmers indirectly by eating slugs and other invertebrates. The best way to enjoy them is from a short distance: watch them bask, let them move on, and leave them in place.