The food of cottonmouths consists mainly of small vertebrates and occasionally invertebrates that are found near water. Fish, amphibians, and reptiles make up nearly 70 per cent of the diet. Carrion is also eaten and cannibalism occurs occasionally. Food is obtained by lying in ambush or by active searching. The young are known to lure their prey within striking range by waving their yellow tails in a manner suggestive of writhing grubs. The method of obtaining prey differs according to the kind of prey. Generally, cottonmouths retain their hold on fish or frogs but release mice and larger prey after delivering a bite.
The major causes of mortality of cottonmouths are obscure. Predators are known to include alligators, indigo snakes, king-snakes, largemouth bass, and blue herons; there are probably numerous others. Heavy parasitic infestations were found among the snakes examined. Snake mites, Ophionyssus natricus, became increasingly abundant on almost all captive snakes in April and May of 1963. Lung flukes (Ochetosoma sp.) were in 16 of 20 captive snakes, and many preserved specimens contained nematodes (Kalicephalus sp.) in the stomach and/or tapeworms (Ophiotaenia sp.) in the intestine. Although parasitic infestation causes discomfort and may lower resistance to other detrimental factors, it is difficult to attribute death to the effect of any particular kind of parasite. Miscellaneous causes of death of some captive snakes also were discussed.
The maximal body temperatures tolerated by four cottonmouths were between 38° and 40° C., but a temperature of 38° was lethal to a fifth individual. Cottonmouths have been found on occasion when other snakes were inactive because of low temperatures, but minimal temperatures tolerated by this species are not known. The annual cycle of activity is dependent upon temperature and thus varies from north to south. Cottonmouths generally migrate inland in autumn, usually to dry forested hillsides, where they den along with other species of snakes. After a few warm days in spring they migrate back to the water's edge. The diel activity cycle likewise depends upon temperatures but is influenced by other factors as well. In spring and autumn, the snakes are active mostly on warm, sunny days, whereas in summer they are active mostly at night. In order to maintain adequate internal temperatures, much time is spent basking mostly in a characteristic flat, resting coil either beside a body of water or above water on limbs of dead trees. In this position the snake is ready either for a short strike or a hasty getaway.
Juveniles appear particularly aggressive and strike repeatedly when approached, a behavioral pattern definitely favoring survival. Adults vary in disposition, usually appearing sluggish and lazy, but they are capable of striking rapidly when disturbed. The typical threat display consists of lying in a coiled position with the mouth opened widely, exposing the white interior, and with the tail vibrating rapidly. The striking posture resembles the resting coil except that the anterior part of the body is raised off the ground and the mouth is sometimes opened. Musk is often ejected in a fine spray from glands in the tail as a further defensive action.
"Head bobbing," more properly described as spastic contractions of the body, was observed in captives when food was introduced into a cage containing several individuals or when one of the snakes was returned to the cage after being handled. Reports in the literature also have connected these jerking movements with courtship. The response appears to be elicited whenever a nervous state is recognized in another individual and may serve to protect the jerking individual from aggressive advances of the former.
The relatively heavy appearance of the body, sluggish habits, and cryptic coloration are correlated with the development of venom and fangs. The poison apparatus has developed primarily as a means of causing rapid death in prey and secondarily, perhaps, to begin the digestion of small animals that are the usual prey, but it is also important as a defensive device. The venom contains at least eight constituents that aid in its action on prey. Toxicity of the venom is difficult to determine because of numerous variables, but cottonmouth venom is generally believed to be less potent than that of most rattlesnakes and more potent than that of the copperhead. Snakes in general are more resistant to snake venoms than other vertebrates of similar size, but there is no immunity even to their own venom.
About ten per cent of the approximately 5000 bites of poisonous snakes per year in the United States are attributable to cottonmouths, and about seven per cent of the approximately 14 deaths per year are caused by cottonmouths.