The northern cottonmouth averages 30 to 42 inches in length. It has a heat-sensitive pit on each side of the head between the eye and the nostril. The head is ﬂattened and much wider than the neck. The pupil of each eye is vertically elliptical. The scales are weakly keeled (ridged). This snake may be plain black or brown above with little evidence of a pattern. Some have dim crossbands across the back.
The northern cottonmouth may be found in the southern tip of Illinois. This snake lives in sloughs and swamps. The cottonmouth is semiaquatic and is mainly active at night. When disturbed, it will either stay where it is and vibrate its tail or slowly crawl away. It may move its head up and back with the mouth open showing the white interior, hence its name of “cottonmouth.” This snake hibernates in dens in rock crevices with other species of snakes. Mating occurs mainly in the spring but may occur at any time the snake is active. The female gives birth to a brood of from one to 15 young in August or September. The tip of the young cottonmouth’s tail is yellow. The northern cottonmouth injects prey with venom then waits for it to succumb before eating it. This snake eats ﬁshes, mice, small birds, lizards, snakes, amphibians and insects.