Thamnophis sirtalis

Geographic Range

Common garter snakes are native to the Nearctic region only. They occur throughout much of North America, although they are largely absent from the arid southwestern United States. Common garter snakes are found throughout eastern North America from Florida to coastal Quebec, west to British Columbia, south into southern California east of the Sierras, and throughout the less arid areas of the southwest. Isolated populations occur on mountain ranges in New Mexico and northern Mexico (New Mexico garter snakes, Thamnophis sirtalis dorsalis ). They are found as far south as the Gulf of Mexico in the eastern United States.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Common garter snakes are very widespread, highly adaptable and can survive extreme environmental conditions. Common garter snakes are found in a wide variety of habitats, including meadows, marshes, woodlands, and hillsides. They tend to prefer moist, grassy environments. They are often found near water, such as near the edges of ponds, lakes, ditches, and streams, and are common in suburban and urban areas with plenty of cover (debris, boards, vegetation, logs, or rocks).

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland
• forest forest

• Wetlands
• marsh marsh
• swamp swamp

• Other Habitat Features
• suburban suburban
• agricultural agricultural
• riparian riparian

Physical Description

Common garter snakes are highly variable in color pattern. They typically have three light stripes that run along the length of their body on a black, brown, gray, or olive background. The stripes can be white, yellow, blue, greenish, or brown. One stripe runs down the center of the snake's back, the other two stripes run alongside this central stripe. Sometimes the stripes are absent or poorly defined. Some garter snakes have alternating rows of dark spots that run along the stripes, making the stripes look more like checkerboard patterns of light, rather than lines. Common garter snakes have a head that is wider than the neck and is uniformly dark. Their tongues are red, tipped in black, and their scales are keeled (with a raised ridge along the length of the scale). The chin, throat and belly resemble the stripes in coloration, ranging from white to yellow, greenish, blue, or brown.

Common garter snakes grow to be 46 to 137 cm in total length. Males are generally smaller than females and have longer tails. Young common garter snakes are born at 12.5 to 23 cm long and are similar in appearance to the adults. There are many dozens of recognized regional populations of common garter snakes that have distinct color patterns. In some areas there are populations that have a high percentage of entirely black garter snakes. Common garter snakes are similar in appearance to their close relatives, ribbon snakes ( Thamnophis sauritus ) and Butler's garter snakes ( Thamnophis butleri ).

• Other Physical Features
• heterothermic heterothermic
• polymorphic polymorphic

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

The young grow quickly and become mature in their second or third year, when they reach about 55 cm in length. Growth continues throughout the lifespan of these snakes.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

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.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

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 two to three months. Most females in the northern parts of their range give birth to from 4 to 80 young between late July and October. Most litters range from 10 to 40 young and litter size depends on the size of the female, with larger females giving birth to larger litters. Upon birth, baby garter snakes are independent and must find food on their own.

Common garter snakes become sexually mature at 1.5 years (males) or two years (females).

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• sexual sexual
• fertilization fertilization
• ovoviviparous ovoviviparous
• sperm-storing sperm-storing

Female common garter snakes nurture their young in their bodies until they are born. The mother gives birth to live young, she doesn't lay eggs. Newly born snakes tend to stay around their mother for several hours or days but she provides no parental care or protection after they are born.

• Parental Investment
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

The average lifespan of wild common garter snakes is approximately two years. Most common garter snakes probably die in their first year of life. Common garter snakes reach sexual maturity, and maximum size, at 3 to 4 years of age. The lifespan of common garter snakes kept in captivity is longer, between 6 and 10 years. One captive common garter snake lived to be 20 years old, but few wild snakes live this long.

Behavior

Common garter snakes are active mainly during the day and are active through a wider range of temperatures than most snakes. They hibernate from late October through March or early April, but can be found basking on rocks during mild winter days. Common garter snakes hibernate in natural cavities or burrows, such as rodent burrows, crayfish burrows, under rock piles, or in stumps. Some populations may have to travel fairly long distances to get to their hibernation sites from their summer feeding and breeding areas. Common garter snakes are mainly solitary but congregate in large numbers at good places to hibernate over the winter. They hibernate together to ensure that they maintain a minimum body temperature for survival. Lying together and forming tight coils, garter snakes can prevent heat loss and keep their bodies warmer.

Like other cold blooded animals, common garter snakes use thermoregulation to control their body temperature. They bask in the sun during the morning hours to maintain a preferred body temperature between 28° and 32° 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 body temperature from falling too low, many garter snakes sleep together to maintain a warm environment, such as they do when they hibernate.

• Key Behaviors
• terricolous
• diurnal diurnal
• migratory migratory
• hibernation hibernation
• solitary solitary

Communication and Perception

Common garter snakes communicate with each other primarily through touch and smell, especially for breeding. Outside of the breeding season they do not interact much with other snakes. They use their forked tongues to collect chemicals from the air and insert these forks into a special organ in the roof of their mouth, which interprets these chemical signals, called pheromones. Pheromones can be used as a tracking device for 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. After they are born, baby snakes follow the same pheromone trails to feed and locate other common garter snakes. Snakes are also sensitive to vibrations and have reasonably good vision.

During the breeding season, common garter snakes use 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.

• Communication Channels
• tactile tactile
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• infrared/heat infrared/heat
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Common garter snakes typically eat earthworms , amphibians , leeches , slugs , snails , insects , crayfish , small fish and other snakes . They seem immune to the toxic skin secretions of toads and can eat them without harm. Occasionally small mammals , lizards , or baby birds are eaten as well. Common garter snakes find their prey using their excellent sense of smell and their vision. They use several different hunting methods, such as peering, craning, and ambushing to capture their prey. The different techniques describe the way the snakes move while they hunt. They immobilize their prey using their sharp teeth and quick reflexes. The saliva of common garter snakes may be slightly toxic to some of their small prey, making it easier to handle them while they are being eaten. Like other snakes, they swallow their food whole.

• Primary Diet
• carnivore carnivore
  • eats terrestrial vertebrates

• Animal Foods
• birds
• mammals
• amphibians
• reptiles
• fish
• insects
• terrestrial non-insect arthropods
• mollusks
• terrestrial worms
• aquatic crustaceans

Predation

Common garter snakes are eaten by a wide variety of predators, which varies throughout their range. Large fish , bullfrogs , snapping turtles , milk snakes , American crows , hawks , great blue herons , raccoons , foxes , squirrels , and shrews are some of the animals that prey on common garter snakes. They rely on stealth and camouflage for protection, and will flee into water to escape predators on land. Their stripes make them difficult to see properly and capture in grassy areas. If unable to flee they coil to make themselves appear larger, and may strike and bite. If grabbed, these snakes writhe and release a foul-smelling secretion; they will also urinate on their attacker.

• Anti-predator Adaptations
• cryptic cryptic

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.

Common garter snakes are infected by a parasitic nematode that lives in the tissues of their tail. Snakes with this condition often have shortened or stubby tails. The nematodes live part of their lifecycle in small aquatic crustaceans and in amphibian larvae. The snakes are infected when they eat the amphibian larvae.

Economic Importance for Humans: Positive

Common garter snakes act to control populations of insect and mollusk pests. They are a common and welcome sight to many gardeners. They also tame easily and are sometimes kept as pets.

• Positive Impacts
• pet trade pet trade
• controls pest population

Economic Importance for Humans: Negative

On very rare occasions people have been known to have allergic reactions to common garter snake saliva after handling one and being bitten. These are extremely rare, though, and their reputation as harmless snakes is well deserved.

• Negative Impacts
• injures humans
  • bites or stings

Conservation Status

Common garter snakes are some of the most common and abundant snakes throughout the eastern United States, at least partly because they do well in urban and suburban areas. Despite the fact that they are harmless snakes, they are often persecuted by humans. Pesticide use in some areas has significantly reduced common garter snake populations. Habitat destruction and over-collection for the commercial pet trade have also led to a decline in the number of garter snakes in the wild. Water pollution is 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 produce smaller numbers of young each year. It is important to continue to monitor populations of 'common' species as declines in their populations can tell us a great deal about environmental health.

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.

Other Comments

Some of the subspecies of common garter snakes ( 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.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Allison Poor (editor), University of Michigan-Ann Arbor.

Ryan Zimmerman (author), Western Maryland College, Randall L. Morrison (editor), Western Maryland College.

References

Bartlett, R., P. Bartlett. 2001. Reptiles Keeper's Guide: Garter and Ribbon Snakes . New York: Barron's Educational Series, Inc.

Conant, R., J. Collins. 1998. Peterson Field Guide to Reptiles and Amphibians of Eastern and Central North America . Boston: Houghton Mifflin.

Harding, J. 1997. Amphibians and Reptiles of the Great Lakes Region . Ann Arbor, Michigan: University of Michigan Press.

LeMaster, M., I. Moore, R. Mason. 2001. Conspecific trailing behavior of red-sided garter snakes, *Thamnophis sirtalis parietalis*, in the natural environment. Animal Behavior , 61: 827-833.

Reynolds, M., F. Gould. date unknown. "Thamnophis" (On-line). Accessed April 3, 2002 at http://www.thamnophis.com/artic22.htm .

Shine, R., P. Harlow, M. LeMaster, I. Moore, R. Mason. 2000. The transvestite serpent: Why do male garter snakes court (some) other males?. Animal Behavior , 59: 349-359.

Stebbins, R. 1985. Peterson Field Guide to Western Reptiles and Amphibians . Boston: Houghton Mifflin.

U.S. Fish and Wildlife Service, 2001. "*Thamnophis sirtalis tetrataenia*" (On-line). Accessed April 24, 2002 at http://ecos.fws.gov/ .

Wechsler, D. 2001. Garter Snakes . New York: The Rosen Publishing Group, Inc.