Garter snake, sometimes called garden snake or gardener snake, is the common name given to the nearly harmless, small to medium-sized snakes belonging to the genus Thamnophis. Endemic to North America, they can be found from the Subarctic plains of Canada to Central America. The garter snake is the state reptile of Massachusetts.

With no real consensus on the classification of species of Thamnophis, disagreement between taxonomists and sources, such as field guides, over whether two types of snakes are separate species or subspecies of the same species is common. Garter snakes are closely related to the genus Nerodia (water snakes), with some species having been moved back and forth between genera.

Habitat

Garter snakes are present throughout most of North America. They have a wide distribution due to their varied diets and adaptability to different habitats, with varying proximity to water; however, in the western part of North America, these snakes are more aquatic than in the eastern portion. Garter snakes populate a variety of habitats, including forests, woodlands, fields, grasslands, and lawns. They almost exclusively inhabit areas with some form of water, often an adjacent wetland, stream, or pond. This reflects the fact that amphibians are a large part of their diet.

Behavior

Garter snakes have complex systems of pheromonal communication. They can find other snakes by following their pheromone-scented trails. Male and female skin pheromones are so different as to be immediately distinguishable. However, male garter snakes sometimes produce both male and female pheromones. During the mating season, this ability fools other males into attempting to mate with them. This causes the transfer of heat to them in kleptothermy, which is an advantage immediately after hibernation, allowing them to become more active. Male snakes giving off both male and female pheromones have been shown to garner more copulations than normal males in the mating balls that form at the den when females enter the mating melee.

If disturbed, a garter snake may coil and strike, but typically it will hide its head and flail its tail. These snakes will also discharge a malodorous, musky-scented secretion from a gland near the cloaca. They often use these techniques to escape when ensnared by a predator. They will also slither into the water to escape a predator on land. Hawks, crows, raccoons, crayfish, and other snake species (such as the coral snake and king snake) will eat garter snakes, with even shrews and frogs eating the juveniles.

Being heterothermic, like all reptiles, garter snakes bask in the sun to regulate their body temperature. During hibernation, garter snakes typically occupy large, communal sites called hibernacula. These snakes will migrate large distances to brumate.

Diet Garter snakes, like all snakes, are carnivorous. Their diet consists of almost any creature they are capable of overpowering: slugs, earthworms, leeches, lizards, amphibians (including frog eggs), ants, crickets, minnows, and rodents. When living near water, they will eat other aquatic animals. The ribbon snake (Thamnophis sauritus) in particular favors frogs (including tadpoles), readily eating them despite their strong chemical defenses. Food is swallowed whole. Garter snakes often adapt to eating whatever they can find, and whenever, because food can be scarce or abundant. Although they feed mostly on live animals, they will sometimes eat eggs.

Venom

Garter snakes were long thought to be nonvenomous, but recent discoveries have revealed they do, in fact, produce a mild neurotoxic venom. Garter snakes cannot kill humans with the small amounts of comparatively mild venom they produce, and they also lack an effective means of delivering it. They do have enlarged teeth in the back of their mouths, but their gums are significantly larger. The Duvernoy's gland of garters are posterior of the snake's eyes. The mild venom is spread into wounds through a chewing action.

Reproduction

Garter snakes go into brumation before they mate. They stop eating for about two weeks beforehand to clear their stomachs of any food that would rot there otherwise. Garter snakes begin mating as soon as they emerge from brumation. During the mating season, the males mate with several females. In chillier parts of their range, male common garter snakes awaken from brumation first, giving themselves enough time to prepare to mate with females when they finally appear. Males come out of their dens and, as soon as the females begin coming out, surround them. Female garter snakes produce a sex-specific pheromone that attracts male snakes in droves, sometimes leading to intense male to male competition and the formation of mating balls of up to 25 males per female. After copulation, a female leaves the den/mating area to find food and a place to give birth. Female garter snakes are able to store the male's sperm for years before fertilization. The young are incubated in the lower abdomen, at about the midpoint of the length of the female's body. Garter snakes are ovoviviparous, meaning they give birth to live young. However, this is different from being truly viviparous, which is seen in mammals. Gestation is two to three months in most species. As few as three or as many as 80 snakes are born in a single litter. The young are independent upon birth. On record, the greatest number of garter snakes reported to be born in a single litter is 98.

Conservation status

Despite the decline in their population from collection as pets (especially in the more northerly regions in which large groups are collected at hibernation),pollution of aquatic areas, and the introduction of bullfrogs as potential predators, garter snakes are still some of the most commonly found reptiles in much of their ranges. The San Francisco garter snake (Thamnophis sirtalis tetrataenia), however, is an endangered subspecies and has been on the endangered list since 1969. Predation by crawfish has also been responsible for the decline of the narrow-headed garter snake (Thamnophis rufipunctatus).