Common Garter Snake {Thamnophis sirtalis). 



Description: The Common Garter Snake consists of two subspecies in western Montana, both 

 ranging from 40-105 cm in snout- vent length. Both subspecies 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 T. s. parietalis but not in T. s. fitchii. Ventral 

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

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

 normally there are 7 upper labial scales. The Common Garter Snake is a live-bearer giving 

 birth to 12-18 young during the summer (Hammerson 1982a). Young garter snakes have 

 approximately the same coloration as the adults. 



Habitat and Habits: Common Garter Snakes are found in all forest habitats but more so at lower 

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

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

 frequent aquatic exploits, but there are no water snakes in Montana. Typical of most garter 

 snakes, they emit a noxious secretion when handled and can be aggressive when disturbed. 

 Common Garter Snakes are capable of biting, but their teeth are rarely long enough to break 

 the skin. Common Garter Snakes were found between 3500-4400' elevation during the 1995 

 survey, but probably occur at higher elevations than this. Four individuals were found 

 together along Sweathouse Creek (4350') on 17 June; three were 30-35 cm and one was 70 

 cm in total length. Other individuals measured in 1995 were 45-55 cm in total length. 

 Individuals have been encountered in the BNF area from 19 March to 26 October. In 

 Wyoming, the Common Garter Snake is replaced at higher elevations by the Western 

 Terrestrial Garter Snake (Baxter and Stone 1985). Garter snakes eat a variety of vertebrates 

 and invertebrates, with the Common Garter Snake concentrating more on amphibians than 

 the Western Terrestrial Garter Snake. 



Surveying: Approximate numbers around marsh-bog habitats can be estimated based on a mark- 

 recapture approach, or sight surveys, if done on a regular basis and under warm, sunny 

 conditions. 



Status: The Common Garter Snake appears to be less common than the Western Terrestrial 

 Garter Snake in the BNF area; it has been reported 13 times from only the Stevensville 

 District and Darby District areas, where this species was again found in 1995. This species is 

 probably more abimdant and widespread than the current reports indicate, as five of the 13 

 records were made during 1995. Several reports from the Lake Como area indicate that there 

 has been a viable population there for several decades (records from around the lake span 

 1932-1995). This species should be looked for at higher elevations in the motmtains, and in 

 the West Fork and Sula districts; encounters should be documented (to distinguish it from 

 the Western Terrestrial Garter Snake), reported to the Heritage Program, and kept on file. 



Montana Natural Heritage Program Rank: G5 S5. 



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