Reptiles
Common Garter Snake
Thamnophis sirtalis · also called common garter snake, garden snake, gardener snake
The common garter snake is the snake most North Americans have actually seen — a slim, striped, harmless-to-people reptile of gardens, ditches, and pond edges. It is also the continent’s most widespread snake, ranging further north than any other, and it has a few genuinely remarkable tricks: mass winter dens, deceptive mating tactics, and an immunity to one of nature’s worst poisons.
Overview
Common garter snakes live across almost the whole of the United States and well into Canada, reaching further north than any other American snake. They favour damp habitats — meadows, marshes, streamsides, woodland edges, and suburban gardens — and are active by day, foraging on the move for worms, slugs, amphibians, and small fish. They are small, usually well under a metre, with a pale back stripe and a side stripe on each flank.
Barely venomous
Garter snakes have a mild venom produced at the back of the upper jaw and worked in by chewing. It helps them overcome frogs and toads; to a person a bite is trivial, causing at most minor swelling or itching. A cornered garter snake is far more likely to flee or smear its attacker with foul musk than to bite.
Surviving winter together
In cold regions, garter snakes hibernate communally. Across a wide area they funnel into traditional dens — rock fissures, sinkholes, burrows — below the frost line, and in parts of Manitoba tens of thousands overwinter in a single site, among the densest snake gatherings on Earth. They emerge together in spring.
Mating balls and mimics
At the den in spring, a female emerging from hibernation is swarmed by dozens of males in a “mating ball.” Some males mimic female pheromones, drawing rival males toward themselves and away from real females. Females store sperm and later give live birth to 10–40 young.
Immune to newt poison
In the Pacific Northwest, garter snakes are the main predator of the rough-skinned newt and its lethal tetrodotoxin. Resistant snake populations carry nerve-channel mutations that neutralise the toxin, and the two species are locked in an escalating arms race of toxicity and resistance.
Status
The common garter snake is Least Concern and abundant. One isolated subspecies, the San Francisco garter snake, is Endangered through wetland loss.
Common Garter Snake: common questions
Are garter snakes venomous or dangerous?
Garter snakes do produce a mild venom, made in a gland at the back of the upper jaw and delivered by chewing rather than by hollow fangs, and it helps them subdue frogs, toads, and other soft prey. To a human it is essentially harmless — a bite from a garter snake may cause slight swelling, redness, or itching in some people, similar to a bee sting reaction, but there is no serious risk. Garter snakes are also reluctant to bite, preferring to flee, hide, or release a foul-smelling musk from glands near the tail if grabbed. They are among the safest snakes to encounter and are often a person's first "pet snake."
How does a garter snake survive a Canadian winter?
By denning communally, deep underground. As autumn cools, garter snakes across a wide area converge on traditional hibernation sites — a deep rock fissure, an old mammal burrow, a stony hillside — that stay above freezing below the frost line. In parts of Manitoba, tens of thousands of red-sided garter snakes pack into limestone sinkholes together, one of the largest concentrations of snakes anywhere on Earth. They spend up to eight months there in near-dormancy, then emerge en masse in spring to mate before dispersing to summer feeding grounds.
What is a garter snake "mating ball"?
When garter snakes leave the winter den in spring, males emerge first and wait. As each female comes out, she is quickly surrounded by dozens or even a hundred males, all trying to mate, forming a squirming clump called a mating ball. There is intense competition — some males release chemicals that mimic a female's pheromones, causing other males to court them instead and pull the crowd away from an actual female, giving the mimic a clear run. Females store sperm and give birth to live young — typically 10 to 40 — in late summer.
How can a garter snake eat a poisonous newt?
In the Pacific Northwest, the common garter snake is the main predator of the rough-skinned newt, whose skin is loaded with tetrodotoxin, one of the deadliest natural toxins. Certain garter snake populations have evolved mutations in the sodium channels of their nerves and muscles that make those channels far less sensitive to tetrodotoxin, so the snake can swallow and digest a newt that would paralyse and kill almost any other animal. This has driven a coevolutionary arms race — where resistant snakes are common, newts have evolved ever-higher toxin levels, and the snakes ever-greater resistance.