Garter Snake (Thamnophis sirtalis)
Order: Squamata, Family: Colubridae
This is a common and abundant species throughout North America. It is the most common of the many species of garter snakes. It is the only snake species in Alaska, and ranges further north there and in Canada than any other North American reptile. In the east it occurs all the way south to Florida and Texas, but is absent from the arid southwest.
I. GEOGRAPHIC RANGE
- This is a common and abundant species throughout North America. It is the most common of the many species of garter snakes. It is the only snake species in Alaska, and ranges further north there and in Canada than any other North American reptile. In the east it occurs all the way south to Florida and Texas, but is absent from the arid southwest.
II. PHYSICAL DESCRIPTION
- Like all Garter Snakes, these snakes have lateral stripes or rows of spots (usually white, yellow, can be greenish, bluish, orange, or red), against a darker background (black, dark green, greenish-brown, olive). There is wide variation within the species. T. Sirtalis can be distinguished from other Thamnophis species by the location of the lateral strip. In sirtalis it is always limited to the second and third rows of scales up from the ventral scales. There is usually a double-row of black spots between the lateral stripes.
- Distinguishing between the many species of Thamnophis and their subspecies is tricky, and reference to a detailed field guide is necessary.
- Common garter snakes are 12-23 cm long at birth, and adults range in length from 46 cm to 130 cm. Females are usually larger than males.
- Some of the subspecies of Thamnophis sirtalis are: Thamnophis sirtalis semifasciatus (Chicago Garter Snake), Thamnophis sirtalis parietalis (Red-sided Garter Snake), Thamnophis sirtalis dorsalis (New Mexico Garter Snake), Thamnophis sirtalis annectens (Texas Garter Snake), Thamnophis sirtalis similes (Blue-stripe Garter Snake), Thamnophis sirtalis pallidulus (Maritime Garter Snake), Thamnophis sirtalis fitchi (Valley Garter Snake), Thamnophis sirtalis infernalis (California red-sided Garter Snake), and Thamnophis sirtalis tetrataenia (San Francisco Garter Snake). All of these subspecies are similar, but vary in details of coloration and geographic distribution.
- Length: 46 to 130 cm
III. LIFEPSAN/LONGEVITY
- The lifespan of a Common Garter Snake is difficult to determine. Several studies have been conducted to determine the average life of a Common Garter Snake in the wild. Unfortunately the studies do not agree on the life span, but the average has been determined to be approximately two years.
- However the life span of a Common Garter Snake kept in captivity appears to be longer, between 6-10 years, with the oldest known captive Common Garter Snake living 14 years.
- Development — Newborn garter snakes are quite similar to adults, and don't change much as they age and grow. They become sexually mature at 1.5 years (males) or two years (females).
- Predation — Garter snakes are eaten by many medium-sized predators. They rely on stealth and camouflage for protection, and will flee into water to escape predators on land. If unable to flee they coil to make themselves appear larger, and may strike and bite. If grabbed, garter snakes defecate and release noxious musky chemicals to discourage predators.
- Garter snakes main predators are hawks, crows, and raccoons. Other snakes, such as king and coral snakes, will also hunt them. Juvenile Garter Snakes must be particularly careful because they are not only hunted by the predators of adult snakes, but by shrews and frogs as well.
Predators:
- coral snake
- crows
- hawks
- king snake
- raccons
- shrews
- Ecosystem Roles — Garter snakes are low-level predators, feeding on many small animals and in turn being eaten by other predators higher in the food web. These snakes are one of the few kinds of animals that can eat toads, newts, and other amphibians with strong chemical defenses.
IV. FOOD HABITS
- The diet of the Common Garter Snake varies as much as its habitat. The snake typically eats earthworms, small frogs and toads, and fish, but sometimes eat small mammals, lizards, or baby birds. Thamnophis sirtalis uses several different hunting methods, such as peering, craning, and ambushing to capture its prey. The different techniques describe the way the snake moves while it hunts. It immobilizes its prey using its sharp teeth, and quick reflexes. Like other snakes, Thamnophis sirtalis swallows its food whole.
Foods eaten:
- fish
- frogs(incl. tadpoles)
- toads (incl. tadpoles
- earthworms
- small mammals
- lizards
- baby birds
V. REPRODUCTION
Common garter snakes are ovoviviparous (bearing live young). The young are incubated in the lower abdomen, about half way down from the snake's body. Gestation is usually 2-3 months, and upon birth, baby Garter Snakes are independent and must find food on their own.
The clutch or litter size ranges from three to fifty, but the average size is just fifteen.
Breeding season: Mating occurs in the spring, timing varies with local climate
Number Produced: 3 to 50
Gestation Period: 2 to 3 months
Sexual Maturity: 1.5 to 2 years
These snakes begin mating in the spring as soon as they emerge from hibernation. The males leave the den first and wait for the females to exit. Once the females leave the den the males surround them. The males give off pheromones that attract the females. After the female has chosen her mate and mated, she returns to her summer habitat to feed and to find a proper birth place. However, the males stay to re-mate with other available females. The females have the ability to store the male's sperm until it is needed and thus a female may not mate if she does not find a proper partner.
The female Garter Snake leaves the young to defend themselves.
VI. BEHAVIOR
- Common Garter Snakes are extremely sociable and use have complex systems of chemical communication. Male Common Garter Snakes use skin lipids as pheromonal cues for sex recognition because female and male skin pheromones are extremely different. However, some males are occasionally born with both female and male skin pheromones.
- During mating season these males with female pheromones are courted by other males. The confusion often allows the males with female pheromones to mate first because the other males are courting the wrong sex. Shine et al. (2000) hypothesized that the behavior could provided a mating advantage to the genetically altered males.
- Pheromones are not only used for social interactions, but they can also be used as a tracking device for adult and juvenile garter snakes. Using their acute sense of smell, common garter snakes can locate other snakes or trails left behind by other snakes through the pheromones given off by their skin. During the fall months, they travel a great distance to locate a suitable place for hibernation. Most of them hibernate together to ensure that they maintain a minimum body temperature for survival. Lying together and forming tight coils, Garter Snake can prevent less heat loss and keep there bodies warmer. After they are born, baby snakes follow the same pheromone trails to feed and locate other Common Garter Snakes.
- Like other cold blooded animals, the Common Garter Snake uses thermoregulation to control its body temperature. It will bask in the sun during the morning hours to maintain a preferred body between 28 degrees — 32 degrees C throughout the day. During the evening hours their body temperature falls rapidly depending on the type of shelter they have chosen for the evening. To prevent their bodies from falling too low, many garter snakes will sleep together to maintain a warm environment, such as they do when they hibernate.
VII. HABITAT
- The Common Garter Snake is very widespread, highly adaptable and can survive extreme environmental conditions. It survives in suburban and urban locations as well as in woodlands. Its most common habitats include meadows, marshes, woodlands, hillsides, and along streams and in drainage ditches. In arid regions garter snakes stay close to water, and will not be found far from a stream or pond.
VIII. ECONOMIC IMPORTANCE FOR HUMANS
- Positive — These harmless snakes are important parts of many ecosystems in North America.
- Conservation — Habitat destruction and over-collection by for the commercial pet trade have led to a decline in the number of garter snakes in the wild. Water pollution is also a problem for this species, because so much of its food is aquatic. Northern populations are more vulnerable than southern ones, because they hibernate in larger groups (which are easily harvested) and produces smaller numbers of young each year. One subspecies, the San Francisco garter snake, Thamnophis sirtalis tetrataenia, is considered endangered, and placed on the US and California Endangered Species list in 1967. Other subspecies may be protected by state laws.
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