Garter snakes (genus Thamnophis) are the familiar striped "garden snakes" of North America. All of them bear live young rather than laying eggs, and most are fast–moving snakes that are active during the day. Because garter snakes tend to be found near water or in moist habitats, they are less common in arid Nevada than in many other states. Nonetheless, three species occur in the state, and a fourth apparently was found in Nevada in historic times.
Two of the Nevada species—the western terrestrial garter snake (Thamnophis elegans) and the common garter snake (Thamnophis sirtalis)—are generalists in the sense that they are found in a variety of habitats and prey on many different kinds of aquatic and terrestrial animals, including fishes, amphibians, earthworms and rodents. The western terrestrial garter snake is by far the most widespread garter snake in Nevada, occurring throughout most of the northern half of the state, with an isolated population in Ash Meadows. In the Sierra Nevada, these snakes are black with distinct light stripes, while east of the Sierra, they are typically dull brown or grey with less distinct stripes. Although they are usually found near water, western terrestrial garter snakes are less tied to aquatic habitats than are the other Nevada garter snakes, which may explain why the species is especially widespread in the state.
Unlike other garter snakes, western terrestrial garter snakes actually constrict small mammals and, in that sense, are the miniature equivalents to a king snake or python. However, unlike such typical constricting snakes, western terrestrial garter snakes are extremely haphazard constrictors; they often form a chaotic jumble of coils that has to be reformed as the prey repeatedly slips out of the snake's grasp. This disorganized behavior may represent an incipient, relatively inefficient stage in the evolution of constriction.
The common garter snake has the largest overall geographic range of any garter snake species, but, unlike the western terrestrial garter snake, it is absent from much of the arid western U.S. In Nevada, it occurs only in the northwest, from just south of Lake Tahoe to the Oregon border. Individuals of this species found in Nevada are striking in appearance, being black or dark brown with distinct light stripes and deep red spots between the stripes.
The third Nevada garter snake species, the Sierra garter snake (Thamnophis couchii), occurs around Lake Tahoe and along the Truckee, Carson and Walker Rivers and their tributaries, with unverified sightings as far east as the Clan Alpine Range. These are fairly drab snakes, usually brown or grey with indistinct stripes. Sierra garter snakes are more aquatic than western terrestrial or common garter snakes, and are almost always found in or near clear streams and rivers, where they feed on fishes and amphibians. While western terrestrial and common garter snakes often feed in water by swimming haphazardly with their mouths open, capturing prey that they bump into by chance, Sierra garter snakes use vision to orient toward and strike at prey underwater. In clear water where prey are not extremely abundant, the directed attacks of Sierra garter snakes are a more effective way of catching prey than the random strategies of the other garter snakes.
A fourth garter snake species, the Mexican garter snake (Thamnophis eques), is known in Nevada from a single specimen collected in 1911 along the Colorado River, near the southern tip of the state. This species probably has been extirpated from the area because of damming and other disturbances of the river.
Although garter snakes are not considered venomous, they have a gland above the upper jaw on either side (corresponding to the venom gland of vipers and other venomous snakes) that produces potentially toxic secretions. In general, bites from garter snakes are harmless because these snakes lack fangs and thus cannot efficiently inject the gland's secretions. However, prolonged bites by western terrestrial and common garter snakes have caused swelling and localized bleeding in people, presumably because unusually large amounts of the secretions seeped into the victims. The "venom" of garter snakes may help them overcome prey, but this has not yet been studied.