Montana Field Guide

Common Gartersnake - Thamnophis sirtalis

Adult common garter snakes range from 16 to 42 inches in length. This snake has two color variations in Montana. The first has three yellow longitudinal stripes (one dorsal and two lateral) and a black stripe broken by red spots between the yellow stripes. The lateral stripes are located on the second and third scale rows above the belly scales. The second color variation has the same striping pattern but lacks the red dots. In both variations, the background color between stripes is black to dark olive. The belly color ranges from yellow to bluish, and some individuals of the red-sided color variation have small black spots on the edge of the belly scales. The dorsal scales are keeled, and normally there are seven scales on the upper lip (FWP). Coloration varies geographically. There are 19 dorsal scale rows at mid-body and lateral stripes on the 2nd and 3rd scale rows (also on row 4 in subspecies annectens). There are 7 upper labials, 1 preocular, and 3 postoculars. The scales are keeled, and the anal undivided. The total length of adults is usually 41 to 66 centimeters (up to 131 centimeters). Common garter snakes are around 12 to 23 centimeters at birth (Conant and Collins 1991, Smith and Brodie 1982).

The western terrestrial garter snake has black spots overlapping the dorsal yellow stripe, and the background color between stripes tends to be brownish. The plains garter snake has its lateral yellow stripes on the third and fourth scale rows above the belly scales, and the dorsal stripe is often orange or red (FWP). It differs from other sympatric garter snakes by the following combination of characteristics: lateral stripe confined to the 2nd and 3rd scale rows (except in subspecies annectens), seven upper labials, tail less than 27% of total length, and 19 scale rows at mid-body.

Montana Range

The common garter snake is nonmigratory, although it has been observed traveling up to 17.7 kilometers between hibernacula and summer range (Gregory and Stewart 1975).

Garter snakes are found in nearly all habitats, but most commonly at lower elevations around water. Females give birth to 6 to 18 live young during summer. They eat a variety of vertebrates and invertebrates (FWP). They prefer moist habitats and are found most often along the borders of streams, ponds and lakes (Franz 1971, Brunson and Demaree 1951, Anderson 1977). They may travel long distances (4 to 17 kilometers) from hibernacula to forage in preferred habitat (Gregory and Stewart 1975).

Common garter snakes prey extensively on amphibians, especially during metamorphosis (Gregory 1984, Fitch 1965, Gregory and Stewart 1975). The western Montana diet varied little with the season and consisted of (% by number) Anura 46 (mostly Bufo boreas 23, Abystoma macrodactylum 13), Hirudo 41, and Oligochaeta 7. Slugs, birds and small mammals are also taken (Anderson 1977).

Thamnophis sirtalis parietalis is noted to specialize in feeding in aquatic systems (Anderson 1977).

Common garter snakes may mate in fall but most mate soon after emergence (late April to early June). Air temperature may trigger spring mating; mating is most intense at temperatures more than 15 degrees C. (Aleksink and Gregory 1974). Parturition is in late July to August in Kansas.