Thamnophis sirtalis concinnus - Red-spotted Gartersnake
The Garter snake is a Colubrid snake genus (Thamnophis) common across North America, ranging from Alaska and Canada to Central America. It is the single most widely distributed genus of reptile in North America. The garter snake is also the Massachusetts state reptile.
There is no real consensus on the classification of species of Thamnophis. Disagreement among taxonomists and sources, such as field guides, over whether two types of snakes are separate species or subspecies of the same species is common. They are also closely related to the snakes of the genus Nerodia, and some species have been moved back and forth between genera.
Garter snakes spread throughout North America. The common garter snake (Thamnophis sirtalis) is the only species of snake to be found in Alaska, and is one of the northernmost species of snake in the world, possibly second only to the Crossed Viper, Vipera berus. The genus is so far ranging due to its unparticular diet and adaptability to different biomes and landforms, with varying proximity to water. However, in the western part of North America, these snakes are more aquatic than in the eastern portion. Northern populations hibernate in larger groups than southern ones.
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 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 head garter snake .
A Garter snake eating a frog
Garter snakes, like all snakes, are carnivorous. Their diet consists of almost any creature that they are capable of overpowering: slugs, earthworms, leeches, lizards, amphibians, birds, fish, toads and rodents. "Pinkie" mice (frozen baby mice) are a good source of food for the snakes because they provide many of the vitamins needed. Earthworms, however, do not provide everything needed. When living near the 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 upon live animals, they will sometimes eat eggs.
Common Garter Snake
The posterior tooth of a garter snake.
A young Garter snake
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, sometimes male garter snakes produce both male and female pheromones. During mating season, this fact 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 so allowing them to be 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 emerge into 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.
A garter snake.
Garter snakes go into brumation before they mate. They stop eating for about two weeks beforehand to clear their stomach of any food that would rot there otherwise. Garter snakes begin mating as soon as they emerge from brumation. During 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-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 mother's body. Garter snakes are ovoviviparous meaning they give birth to live young. However, this is different than being truly viviparous, which is seen in mammals. Gestation is two to three months in most species. As few as 3 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 to be born in a single litter is 98.
Garters were long thought to be nonvenomous, but recent discoveries have revealed that they do in fact produce a mild neurotoxic venom. Garter snakes cannot kill humans with the small amounts of venom they produce, which is comparatively mild, and they also lack an effective means of delivering it. They do have enlarged teeth in the back of their mouth, but their gums are significantly larger. Whereas most venomous snakes have anterior or forward venom glands, the Duvernoy's gland of garters are posterior (to the rear) of the snake's eyes. The mild venom is spread into wounds through a chewing action.