Colorado's Snakes

by Lauren J. Livo

Evaluate the criteria below to identify a species.

1a.	Rattle present at end of tail.	go to 2
1b.	No rattle present.	go to 3
2a.	Numerous small scales on top of head, maximum total length about 4'.	Crotalus viridis Western (prairie) rattlesnake
2b.	Head covered by large scales, maximum total length about 2'.	Sistrurus catenatus Massasauga
3a.	Stripes running the length of the body.	go to 4
3b.	Body plain, banded, or blotched, but without longitudinal stripes.	go to 9
4a.	Scales smooth; longitudinal black and white stripes.	Masticophis taeniatus Striped whipsnake
4b.	Scales keeled.	go to 5
5a.	Distinct black bars at edge of scales on upper lips.	go to 6
5b.	Not as above.	go to 7
6a.	Pale stripe on side of body on 3rd and 4th scale rows above belly scales.	Thamnophis radix Plains garter snake
6b.	Pair of glossy black blotches behind head; pale stripe on side of body on 2nd and 3rd scale rows above belly scales.	Thamnophis cyrtopsis Blackneck garter snake
7a.	Red color on sides.	Thamnophis sirtalis Common garter snake
7b.	No red color on sides.	go to 8
8a.	Belly with conspicuous double row of half-moon markings.	Tropidoclonion lineatum Lined snake
8b.	Belly variable, but without half-moon markings.	Thamnophis elegans Western terrestrial garter snake
9a.	Body with distinctly keeled scales.	go to 10
9b.	Scales on body smooth or very weakly keeled.	go to 13
10a.	Nose distinctly upturned.	go to 11
10b.	Not as above.	go to 12
11a.	Well-ordered, circuluar spots running head to tail with a single dark horizontal stripe connecting the eyes and a light stripe immediately above it.	Heterodon nasicus Western hognose snake
11b.	Spots blotchy and less pronounced, or absent entirely. Head darker with dark markings extending several inches down back.