Thamnophis sirtalis
(Common Garter Snake)

General Description:
Common Garter Snakes can be distinguished from Western Terristrial Garter Snakes because they usually have 7 upper labial scales, while the later usually have 8 upper labial scales.  The 6th and 7th upper labial scales are not enlarged in Common Garter Snakes as they are with Terrestrial Garter Snakes.   Common Garter Snakes have dorsal scales that are keeled and their pupils are round.   The dorsal coloration of Common Garter Snakes differs from Western Terrestrial Garter Snakes, because the ground color is generally darker (black) and there are usually red checks and markings present laterally and on the rear labial scales.  However, there are populations that lack any red coloration.  Common Garter Snakes generally have three light stripes like their congeners, but the dark spots, if present, are usually obscured by the dark ground color, and the dorsal stripe has an even margin.  The ventral coloration of Common Garter Snakes is a gray with dark markings.

Common Garter Snakes are medium-sized snakes that can attain sizes of up to 132 cm (52 in.) in length (Storm and Leonard 1995).

Common Garter Snakes usually mate after leaving their winter hibernation site (fall breeding may occur) (Charles R. Peterson pers. com. 1998).   Multiple males will vie for the attention of reproductive females, and can form mating "snake balls".  As a result of this reproductive behavior, several different males may fertilize eggs within the same brood (Storm and Leonard 1995).  Young are born between July and September, and brood sizes  generally number between 10 and 18 (Storm and Leonard 1995).  However, litters of up to 85 young are possible in some portions of their range (Behler and King 1979).  Young Common Garter Snakes resemble adults.

Habitat:
Common Garter Snakes are usually found in habitats associated with water, such as streams, rivers, lakes, ponds and marshes.  They can also be found in open meadows and coniferous forests (Nussbaum et al. 1983).

Idaho Distribution:
In Idaho, the Common Garter Snake is less common than the Western Terrestrial Garter Snake, but is generally distributed in a similar pattern.

Diet:
Preys chiefly on earthworms, frogs, toads, salamanders, and fishes, less regularly on slugs, leeches, small mammals and birds, and rarely on insects, spiders, and small snakes.

Ecology:
Nocturnal/diurnal; nocturnal activity often occurs during hot weather.  Hibernate/aestivates.   Hibernates underground, in or under surface cover at times with other snakes species.  Active from about March or April through October in northern range and at higher elevations; active season is longer in southern range, to year-round in Florida.   Thousands of individuals may aggregate at hibernacula in northern range.   Population density estimate in different areas vary from about 10-100/ha.   Home range size has been variously reported as 0.8-14 ha.  May migrate several km from hibernacula to foraging areas.  Individuals will exude musk and fecal material from anus to repel predators.

Reproduction:
Female gives birth to up to 85 young (13-26 on average, commonly 8-12), usually in July or August (earlier in southern range, to early October in north.)

Conservation:

Status:	Unprotected nongame species
Global Rank:	G5
State Rank:	S5