Common Garter Snake {Thamnophis sirtalis) 



Description: The Common Garter Snake consists of two color phases in western Montana, and 

 ranges from 18-52" in length. Both phases have three yellow longitudinal stripes: one 

 located dorsally and one on each side. Between the yellow stripes is a black stripe broken 

 with red spots in one color phase but lacking red in the other. The form lacking red spots is 

 not known from eastern Montana, but may be present there and should be watched for. 

 Ventral coloration varies from yellow to bluish, and some individuals of the red-sided color 

 phase have small black spots on the edge of the ventral scales. The dorsal scales are keeled, 

 and normally there are 7 upper labial scales. 

 Young: The coloration of young snakes is similar to that of the adults; young are live-bom. 



Similar species: The Western Terrestrial Garter Snake has black spots overlapping the dorsal 

 yellow stripe; the background color between stripes tends to be brownish. The Plains Garter 

 Snake has the side yellow stripe on the 3rd and 4th scale rows above the belly scales and the 

 dorsal stripe is often orange or red. 



Habitat and Habits: Garter snakes are found in all forest habitats but are more common at lower 

 elevations around marsh-bog-pond situations, where they prey on young fish, frogs, toads, 

 mice and invertebrates. They are sometimes confused with water snakes because of their 

 frequent aquatic exploits; in reality there are no "true" water snakes in Montana. Typical of 

 most garter snakes, they emit a noxious secretion when handled and can be aggressive when 

 disturbed. Garter snakes eat a variety of vertebrates and invertebrates, with the Common 

 Garter Snake concentrating more on amphibians than the Western Terrestrial Garter Snake. 

 The Common Garter Snake is a live-bearer, giving birth to 12-18 young during the summer 

 in Colorado (Hammerson 1982a). 



Surveying: Timed-sight surveys may be conducted around wetlands and riparian feeding areas 

 or at denning areas where higher concentrations of garter snakes occur; clear mornings are 

 the best survey times. Much distributional information may come from recording incidental 

 sightings. More intensive research may be done using funnel traps in combination with drift 

 fences. More intensive research and survey projects may use mark-recapture or 

 radiotelemetry techniques. 



Status: Common Garter Snakes are not known from the Sioux-CNF. There is an historic record 

 of them from about 20 miles south of Chalk Butte (Appendix 4). All records of garter snakes 

 should be documented until the distribution of the three species is better understood; of 

 particular interest would be documentation of denning sites. 



Montana Natural Heritage Program Rank: G5 S5. 



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