water moccasin or cottonmouth, highly venomous snake, Ancistrodon piscivorus, of the swamps and bayous of the S United States. Like the closely related copperhead, it is a pit viper and has a heat-sensitive organ for detecting warm-blooded prey. The young are born live. The young snake is a pale reddish brown with transverse dark brown bands edged with white; as it ages the colors dull to a blotched olive or brown and then to an unmarked olive or blackish in old specimens. The maximum length is 6 ft (2 m), the average from 3 to 4 ft (90–120 cm). A good climber, the water moccasin often relaxes on branches overhanging the water. If startled it erects its head and shows the white interior of its mouth—hence the name cottonmouth. It eats both warm-blooded and cold-blooded animals. It is aggressive in the wild state but may become quite tame in captivity. It is classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Crotalidae.