Common Garter Snake

Contents:
 
Links:
Taxon, Status, and Ranks Habitat Photos
General Description State Status Comments
Identification Tips Inventory & Research Needs Key Features
Phenology Threats & Mgmt Concerns
Range References Distribution Map

Taxon, Status, and Rank

Species Thamnophis sirtalis     (Linnaeus, 1766)
Family Colubridae
(Colubrids)
Status none
State Rank S5
Global Rank G5

Our three garter snake species are similar in general appearance and have coloration and patterns that vary regionally and by individual. All traits, especially scale counts, color and pattern, must be examined to correctly identify a garter snake to species. Although little appreciated, these are beautiful snakes with many stunning color variations.

General Description

This is a medium-sized dark snake with stripes. Typically three stripes are present; a thin dorsal stripe on the middle of the back and two thin lateral stripes on the 2nd and 3rd dorsal scale rows. The lateral or dorsal stripes may be absent in some individuals. Adults can reach 1372 mm (54 in.) total length but individuals in Washington are shorter. The pupil is round. The scales are keeled and there are usually 19 rows at mid-body, 7 scales lining the upper jaw (upper labials) and 10 scales lining the lower jaw (lower labial scales).

The Common Garter Snake has the greatest geographical distribution of any garter snake with a dorsal pattern that is the most variable of any of the garter snake species (Thamnophis sp.). The traits given below are specific to Washington and are based primarily on the authors’ observations and the description in Storm and Leonard 1995.

Storm and Leonard 1995 provide detailed descriptions for each subspecies found in Washington: T.s. concinnus occurs in the Pacific Coast and southern Puget Trough ecoregions; T. s. fitchi occurs in eastern Washington ecoregions; and T.s. pickeringii occurs in the Puget Trough lowlands, primarily the Puget Trough ecoregion and portions of surrounding ecoregions.The following is a general description of the various color forms most likely to be seen in eastern and western Washington.

Eastern Washington
In eastern Washington, Common Garter Snakes are black bodied with bright yellow dorsal and lateral stripes. Distinct red blotches are present on the sides of the body just above the lateral stripes. A red cheek blotch is usually present. The ventral coloration is yellow or buff. Individuals, in some areas, may have blue or turquoise lateral stripes instead of yellow and these snakes usually have blue ventral scales.

Western Washington
In western Washington, red blotches may or may not be present and the stripe coloration is much more variable.

Ground color is slate gray or black, sometimes with a bluish tinge. Stripe colors are yellow, green, turquoise or blue. All stripes may be the same color or the lateral stripes may differ from the dorsal stripe in color. The ventral coloration is usually yellow or cream under the chin with increasing black pigmentation toward the tail. Snakes with blue lateral stripes usually have blue ventral coloration.

Snakes without red blotches have white patches of skin between the scales on the sides of the body. This trait will not be obvious in most snakes without gently stretching the skin to separate the scales above the lateral stripes. This trait is usually more prominent near the head. Other variations include orange blotches instead of red; blotches only on the anterior portions of the snake; or red blotches that are hidden under dark scales. In very melanistic individuals, all the skin between the scales is black and no white or red blotches are present. See Photos Page.

Identification Tips

Differences in scale counts are important for distinguishing the garter snakes. Common Garter Snakes usually have 19 scales at mid-body, 7 upper labial scales and 10 lower labial scales. Occasionally, extra labial scales are present.

In Washington, only the Common Garter Snake has red blotches on the sides of the body above the lateral stripe. Snakes lacking red blotches usually have white blotches on the skin between the scales above the lateral stripes. No dark spotting is present on the dorsal surface.

In Western Washington, Northwestern Garter Snakes and Common Garter Snakes are the most difficult species to distinguish based on coloration. Dark bodied Northwestern Garter Snakes with green, turquoise, blue or yellow stripes are common. The scale counts differ; Northwestern Garter Snakes have 17 scales at mid-body (occasionally 19), 7 upper labial scales and 8 or 9 lower labials.

Striped Whipsnakes differ in having smooth scales, 15 dorsal scale rows, and a mid-dorsal area that is dark brown or black with multiple white or yellow stripes on the sides of the body. See Key Features Page.

Phenology

At low elevations, activity starts in March and continues into early November. Snakes may remain in the vicinity of the overwintering site for two or more weeks until mating is complete and weather conditions are appropriate for dispersal. At low elevations in western Washington, snakes may be surface active in the vicinity of the overwintering site whenever site conditions warm over about 13 °C (55°F).

The species breeds in spring after emergence. Males emerge before females and spend time on the surface near the overwintering sites. When a female emerges, the males will attempt to mate with her forming “mating balls” of many males around one female. After mating, snakes disperse to summer foraging areas. In the lower Puget Sound area, female garter snakes of all three species are commonly found clustered in open grassy areas. This behavior may be related to thermoregulation and gestation. The young are born in late summer and early fall depending on location. In the lowland Puget Sound area, neonates start to appear in late August and early September.

Range

Common Garter Snakes occur in all Washington Ecoregions. Occurrences are patchy in the central Columbia Basin. See Distribution Map.

For information on the complete range of this species, see NatureServe Explorer.

Habitat and Habits

In Washington, Common Garter Snakes are almost always found near water. Typical aquatic habitats include wetlands, bogs, ponds, lakes, springs, creeks, and rivers. They are good swimmers and can hunt both on and below the water surface. Juvenile Common Garter Snakes have been observed hunting newly metamorphosing Pacific Treefrogs in shallow, seasonal, water bodies. Typical terrestrial habitats include sunny areas near water such as meadows, oak patches, forest openings, and shrubby areas. In the spring and fall snakes may be found away from water as they move to and from terrestrial overwintering sites.

Common Garter Snakes defend themselves by releasing the contents of their cloaca and musk glands then smearing this pungent foul smelling mixture over themselves and their attacker. Some will also regurgitate the content of their stomach and most will bite.

Common Garter Snakes are one of the few vertebrates that can successfully prey on the extremely toxic Rough-skinned Newt. Recent research indicates that these snakes harbor significant amounts of active toxin in their tissues after consuming a newt. The amount of toxin in the liver varies depending on the number of newts eaten, but the amount in many cases is enough to severely incapacitate or kill avian predators and negatively affect mammalian predators.

State Status Comments

This is one of our most common snake species. No declines have been reported in Washington. Declines in amphibian populations could potentially result in Common Garter Snake declines. Reported declines in other states and Canadian provinces have been due to habitat loss and overcollecting for the pet and scientific trade.

Inventory and Research Needs

Observations can be submitted to the Washington Department of Fish and Wildife WSDM database by contacting Lori Salzer by E-mail salzeljs@dfw.wa.gov.

Threats and Management Concerns

Over hunting or collecting, wanton killing and destruction of overwintering sites can result in local declines. Road mortality is also a threat in areas where snakes cross roads to access overwintering or foraging habitat.

References

Hallock and Leonard (1997); Rossman et al. (1996); Storm and Leonard (1995); Williams et al. (2004)

Personal communication: B. Leonard


Hallock, L.A. and McAllister, K.R. 2009. Common Garter Snake. Washington Herp Atlas. http://www1.dnr.wa.gov/nhp/refdesk/herp/

Last updated: May 2009


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