Common garter snake

The common garter snake ("Thamnophis sirtalis") is a species of natricine snake, which is indigenous to North America and found widely across the continent. Most common garter snakes have a pattern of yellow stripes on a black, brown or green background, and their average total length (including tail) is about , with a maximum total length of about . The average body mass is . Common garter snakes are the state reptile of Massachusetts.
Description.
Common garter snakes are thin snakes. Few grow over about long, and most stay smaller. Most have longitudinal stripes in many different colors. Common garter snakes come in a wide range of colors, including green, blue, yellow, gold, red, orange, brown, and black.
Life history.
The common garter snake is a diurnal snake. In summer, it is most active in the morning and late afternoon; in cooler seasons or climates, it restricts its activity to the warm afternoons.
In warmer southern areas, the snake is active year-round; otherwise, it sleeps in common dens, sometimes in great numbers. On warm winter afternoons, some snakes have been observed emerging from their hibernacula to bask in the sun.
Venom.
The saliva of a common garter snake may be toxic to amphibians and other small animals. Garter snakes have a mild venom in their saliva. For humans, a bite is not dangerous, though it may cause slight itching, burning, and/or swelling. Common garter snakes may also secrete a foul-smelling fluid from postanal glands when handled or harmed.
Common garter snakes are resistant to naturally found poisons such as that of the American toad and rough-skinned newt, the latter of which can kill a human if ingested. They have the ability to absorb the toxin from the newts into their bodies, making them poisonous, which can deter potential predators.
The common garter snake uses toxicity for both offense and defense. On the offensive side, the snake's venom can be toxic to some of its smaller prey, such as mice and other rodents. On the defensive side, the snake uses its resistance to toxicity to provide an important antipredator capability. A study on the evolutionary development of resistance of tetrodotoxin compared two populations of "Thamnophis" and then tested inside a population of "T. sirtalis". Those that were exposed to and lived in the same environment as the newts ("Taricha granulosa") that produce tetrodotoxin when eaten were more immune to the toxin (see figure).
While resistance to tetrodotoxin is beneficial in acquiring newt prey, costs are associated with it as well. Consuming the toxin can lead to reduced speed and sometimes no movement for extended periods of time, along with impaired thermoregulation. The antipredator display that this species uses demonstrates the idea of an "arms race" between different species and their antipredator displays. Along the entire geographical interaction of "T. granulosa" and "T. sirtalis", patches occur that correspond to strong coevolution, as well as weak or absent coevolution. Populations of "T. sirtalis" that do not live in areas that contain "T. granulosa" contain the lowest levels of tetrodotoxin resistance, while those that do live in the same area have the highest levels of tetrodotoxin resistance. In populations where tetrodotoxin is absent in "T. granulosa", resistance in "T. sirtalis" is selected against because the mutation causes lower average population fitness. This helps maintain polymorphism within garter snake populations.
Subspecies.
Current scientific classification recognizes 13 subspecies (ordered by date):
A trinomial authority in parentheses indicates that the subspecies was originally described in a genus other than "Thamnophis".
Etymology.
The subspecific name "fitchi" is in honor of the American herpetologist Henry Sheldon Fitch.
The subspecific name "lowei" is in honor of the American herpetologist Charles Herbert Lowe.
The subspecific name "pickeringii" is in honor of the American naturalist Charles E. Pickering.
Reproduction.
Generally, populations include far more males than females, so during mating season, they form "mating balls", in which one or two females are completely swamped by ten or more males. Sometimes a male snake mates with a female before hibernation, and the female stores the sperm internally until spring, when she allows her eggs to be fertilized. If she mates again in the spring, the fall sperm degenerate and the spring sperm fertilize her eggs. The females may give birth ovoviviparously to 12 to 40 young from July through October.
In the early part of sex, when snakes are coming out of hibernation, the males generally emerge first to be ready when the females wake up. Some males assume the role of a female and lead other males away from the burrow, luring them with a fake female pheromone. After such a male has led rivals away, he "turns" back into a male and races back to the den, just as the females emerge. He is then the first to mate with all the females he can catch. This method also serves to help warm males by tricking other males into surrounding and heating up the male, and is particularly useful to subspecies in colder climates (such as those inhabited by "T. s. parietalis"); this type of mimicry is primarily found in that subspecies. These deceptive males have been found to mate with females significantly more often than males that do not exhibit this mimicry. 
Habitat.
The habitat of the common garter snake ranges from forests, fields, and prairies to streams, wetlands, meadows, marshes, and ponds, and it is often found near water. Depending on the subspecies, the common garter snake can be found as far south the southernmost tip of Florida in the United States and as far north as the southernmost tip of the Northwest Territories in Canada. It is found at altitudes from sea level to mountains.
Diet.
The diet of "T. sirtalis" consists mainly of amphibians and earthworms, but also fish, small birds, and rodents. Common garter snakes are effective at catching fast-moving creatures such as fish and tadpoles.
As prey.
Animals that prey on the common garter snake include large fish (such as bass and catfish), American bullfrogs, common snapping turtles, larger snakes, hawks, raccoons, foxes, wild turkeys, and domestic cats and dogs.
Conservation.
Water contamination, urban expansion, and residential and industrial development are all threats to the common garter snake. The San Francisco garter snake ("T. s. tetrataenia"), which is extremely scarce and occurs only in the vicinity of ponds and reservoirs in San Mateo County, California, has been listed as an endangered species by the U.S. Fish and Wildlife Service since 1967.
Antipredatory displays.
Garter snakes exhibit many different behaviors to ward off predators. Garter snakes exhibit a greater variety of body postures than other snakes. Under selection by predation, these snakes have developed postural responses that are highly variable and heritable. These are highly variable even within a single population. Different postures indicate whether the snake is preparing to flee, fight, or protect itself. Different biological factors such as body temperature and sex also influence whether the snake exhibits certain antipredatory behaviors.
The warmer the temperature of a garter snake, the more likely the snake is to flee a predator; a snake with a cooler body temperature is more likely to remain stationary or attack. Male garter snakes are also more likely to flee. Garter snakes that exhibit more aggressive antipredatory displays tend to also be fast and have high stamina. However, the reason for this correlation is unknown.
The first response of the snake to a predator is often a bluff. When the snake was teased with a finger under laboratory conditions, the snake reacted aggressively, but once touched, it became passive. This may be because the snake is disinclined to attack an organism it sees as larger than itself. Garter snakes do not exhibit mimicry or aposematic coloration; relying on cryptic coloration for protection, they will freeze until they know they are spotted, then attempt a stealthy departure.
The decision of a juvenile garter snake to attack a predator can be affected by whether the snake has just eaten or not. Snakes that have just eaten are more likely to strike a predator or stimulus than snakes that do not have a full stomach. Snakes that have just eaten a large animal are less mobile.
Another factor that controls the antipredatory response of the garter snake is where on its body the snake is attacked. Many birds and mammals prefer to attack the head of the snake. Garter snakes are more likely to hide their heads and move their tails back and forth when being attacked close to the head. Snakes that are attacked in the middles of their bodies are more likely to flee or exhibit open-mouthed warning reactions.
Age may be another factor that contributes to antipredatory responses. As garter snakes mature, the length of time for which they can engage in physical activity at 25 °C increases. Juvenile snakes can only be physically active for 3–5 minutes. Adult snakes can be physically active for up to 25 minutes. This is mostly due to aerobic energy production; pulmonary aeration increases up to three times in adult garter snakes when compared to juveniles. The quick fatigue of the juveniles limits the habitats they can live in, as well as their food sources. It also affects the antipredator response of both juvenile and adult garter snakes; without sufficient energy production, the snake cannot effect an antipredatory response.