Common Garter Snake (Thamnophis sirtalis). 



Description: The Common garter snake consists of two subspecies in western Montana, both 

 ranging from 16-42" 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 71 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 1 982a). Young garter snakes have approximately the 

 same coloration as the adults. 



Habitat and Habits: 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 confused 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. Garter 

 snakes are capable of biting, but their teeth are rarely long enough to break the skin. The 

 Common garter was found between 2350-4800 ft. elevation in this survey. 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: Both subspecies are probably present throughout the KNF, but the red-sided subspecies 

 was much more common in the present survey. Garter snake populations appear to be stable 

 based on casual observations and data from this survey. This species appears to be more 

 common than the Western terrestrial garter snake. 



Montana Natural Heritage Program Rank: G5 S5. 



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