Although usually active during the day, the common garter snake may restrict its hunting activity to the early morning and evening on hot days, and it may also be partially nocturnal at times (2) (3) (8). An excellent swimmer (5), the common garter snake frequently hunts by swimming slowly along the margins of pools, often sweeping its open mouth from side to side and seizing any prey it encounters (2) (3). It may also track prey on land by scent and by sight (3) (6), or catch earthworms by locating their casts and thrusting its snout into the tunnel below (2). The saliva of the common garter snake is slightly toxic, possibly helping to immobilise prey. However, its bite is generally harmless to humans (3) (6).
The common garter snake takes a diverse range of prey, including fish, amphibians and their tadpoles, earthworms, leeches, and other aquatic and terrestrial invertebrates, such as insects, slugs and crayfish. It also hunts small mammals and birds, and occasionally eats carrion (2) (3) (4) (5) (6). Earthworms and amphibians often make up the majority of its diet (3) (4) (6).
Predators of the common garter snake include large fish, turtles, birds, mammals, bullfrogs, domestic animals and even other snakes (6). This species’ first line of defence is to escape into water or move away, but if caught it will try to smear its captor with excrement and a foul-smelling discharge from glands at the base of its tail (3) (5) (6).
More tolerant of the cold than any other garter snake (2) (5), the common garter snake is one of the last snake species to enter hibernation in cooler regions, and one of the first to emerge again in spring (2) (3). In northern parts of its range, the common garter snake may be active from March to November (2) (3) (6), but in more southerly locations it may leave the winter den to bask during warmer weather (3) (6), or may be active year-round (2).
The winter hibernation sites of this species include rocky hillsides, mammal or crayfish burrows, stream banks, decaying logs, tree stumps, ant mounds, and man-made sites such as dams (3) (6). In some areas, particularly in the north, large numbers of common garter snakes may share the same site, sometimes with other snake species (3). The mass emergence of common garter snakes in spring can then be followed by large mating aggregations, in which numerous males and females engage in frenzied courtship and mating behaviour (2) (3). In other areas, the common garter snake may not hibernate in such large numbers. Some mating also occurs in the autumn, with the sperm remaining viable in the female until the following spring (2) (3).
After mating, common garter snakes disperse to their summer feeding grounds, which can sometimes be some distance away from the hibernation site (2) (3) (6). The female common garter snake usually gives birth to around 7 to 20 live young, although litter size can sometimes reach 70 or more (3) (4). The young are born between May and November (2) (5), and measure between 12.5 and 23 centimetres at birth (3) (4) (6).
The common garter snake reaches maturity at two to four years old, with males maturing at a smaller size than females (2) (6). Near the northern limits of the common garter snake’s range, females rarely give birth in two successive years (9).