Common Garter Snake {Thamnophis sirtalis) 



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

 ranging from 1 8-52" in length. Both phases have tliree yellow longitudinal stripes: one 

 located dorsally and one on each side on the 2nd and 3rd scale rows above the belly scales. 

 Between the yellow stripes is a black (or dark green) background, broken with red spots in 

 one color phase but lacking red in the other. 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-born. 



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

 yellow stripe; the background color between stripes tends to be paler dusky green, gray or 

 browTi. 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, but 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. The Common Garter Snake was reported at two locations in 1995, both about 

 5600 feet, on the HNF by A. Harper and S. Hendrickson; historically they have also been 

 found at low elevations in the intermountain valleys. 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 surx'eys 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 tecliniques. 



Status: Common Garter Snakes were found in the Rocky Mountains and Elkhom Mountains on 

 the HNF. There is also a historic record of them from the valley between the two locations. 

 Given the small number of records from throughout the area, all records should be 

 documented until the distribution is better understood; of particular interest would be 

 documentation of denning sites. Sightings of sirtalis from the Big Belt Mountains and Rocky 

 Mountains north of McDonald Pass should be documented to confirm their presence in those 

 areas. Only the red-sided color phase has observed in this area, however the color phase 

 lacking red spots should be watched for. The Common Garter Snake is much less abundant 

 than the Western Terrestrial Garter Snake in this area, as it is currently in northwestern 

 Montana. 



Montana Natural Heritage Program Rank: G5 S4. 



18 



