Reptiles
Garter Snake
Thamnophis · also called gartersnake, garden snake
The garter snake is the snake most North Americans have actually met — small, striped, harmless, hunting slugs in the flower bed. It is the most widely distributed snake on the continent, it survives eating newts that would kill almost anything else, and in the far north it spends winter piled up with thousands of others underground.
Overview
Garter snakes are the genus Thamnophis, around 35 species, found only in North America from Canada to Costa Rica. They are small to medium snakes, usually with pale stripes running the length of a darker body, and they are carnivores of damp habitats, eating worms, slugs, frogs, fish, and salamanders, which they swallow alive — they are not constrictors. The common garter snake ranges further north than any other American snake.
Barely venomous
Garter snakes make a mild venom in a rear gland and chew it into prey with grooved back teeth. On a person it does nothing worse than slight itching or swelling. A cornered garter snake almost never bites — it releases foul musk instead.
An arms race with newts
Where garter snakes overlap with toxic Pacific newts, some snake populations have evolved resistance to tetrodotoxin and can eat newts that would kill a raccoon. The newts have become more toxic in response — one of the clearest cases of coevolution known.
Dens and live birth
Northern garter snakes overwinter in huge communal dens; the Narcisse dens in Manitoba host tens of thousands of snakes and the largest gathering of snakes on Earth, with mating balls of dozens of males around each female. Garter snakes give birth to live young, 10 to 40 or more per litter, independent from birth.
Status
Garter snakes as a group are Not Evaluated. Most are common and thrive in suburbs and farmland, but the San Francisco garter snake is Endangered, and the northern Mexican and narrow-headed garter snakes are declining from lost wetlands and introduced bullfrogs and crayfish.
Garter Snake: common questions
Are garter snakes dangerous or venomous?
Not in any way that matters to a person. Garter snakes do produce a mild venom, made in a gland at the back of the jaw and worked into prey by chewing with small grooved rear teeth, and it helps them subdue frogs and worms. A garter snake that manages to chew on a human finger might cause slight local swelling, redness, or itching in a sensitive person, similar to a mild allergic reaction, but there is no risk of serious harm and no need for antivenom. Garter snakes are also very reluctant to bite, preferring to flee or to release foul musk. They are among the safest snakes a person can encounter.
How do garter snakes eat poisonous newts and survive?
Through a coevolutionary arms race. Pacific newts, in the genus Taricha, contain tetrodotoxin, the same lethal nerve poison found in pufferfish, in enough quantity to kill most predators that swallow one. In areas where common garter snakes have preyed on these newts for a long time, the snakes have evolved changes to the sodium channels in their nerves and muscles that make them largely immune, so they can eat a newt that would kill a bird or a raccoon. The newts have responded by becoming even more toxic, and the snakes more resistant, in a back-and-forth that has produced newts far more poisonous than any predator now requires.
Why do thousands of garter snakes gather in one place?
For winter survival and then for mating. In cold regions there are few underground spaces that stay above freezing all winter, so garter snakes travel, sometimes several kilometres, to a suitable communal den, or hibernaculum, and pack in by the hundreds or thousands. When they emerge in spring, males come out first and wait, and as each female appears she is mobbed by dozens of males forming a mating ball. The Narcisse Snake Dens in Manitoba, Canada, host the largest known gathering of snakes anywhere on Earth and are a spring tourist attraction.
What is the difference between a garter snake and a garden snake?
They are the same animal. Garden snake, and sometimes garter snake pronounced as gardener snake, is just a common regional variant of the name, used because these small striped snakes are so often found in gardens, hunting slugs and worms. The correct name is garter snake, thought to come from the resemblance of the pale side stripes to old-fashioned sock garters. In Britain, garden snake is more often used loosely for the unrelated grass snake, which fills a similar niche there, so the name can cause confusion between continents.