Common Garter Snake {Thamtwphis 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 confiised 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). This species was seen only once in 1998, on 23 April 

 (Appendix 2). 



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 ftinnel traps in combination with drift 

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

 radiotelemetry techniques. 



Status: Common Garter Snakes are most common in western Montana, and infrequently reported 

 from the plains of eastern Montana. There are only 5 records from the eastern half of the 

 state since 1995. The single record for 1998 was made at Armells Creek in Rosebud County 

 (Appendix 2). Some previous reports of this species may actually be misidentified Plains 

 Garter Snakes. The Common Garter Snake appears to have a very patchy distribution in 

 Wyoming (Baxter and Stone 1985), and this might be the case in eastern Montana as well. All 

 records of garter snakes should be thoroughly 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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