Cottonmouth

Identification

The cottonmouth, also called the water moccasin, is a large, heavy-bodied pit viper with a broad, triangular head that is clearly wider than its neck⁴. Adults are dark olive, brown or nearly black, and their dark crossbands often fade until they are hard to see^1,4. Young snakes are boldly banded in brown on a lighter grey, tan or reddish background and have a bright yellow-green tail tip^4,10. The scales are keeled, giving a rough, matte look; like other cottonmouths it has vertical pupils and a heat-sensing pit between each nostril and eye^4,5.

Average adults are about 76–114 cm long, and the maximum recorded is 188 cm¹. Males grow larger than females⁴.

Distribution

The cottonmouth lives in the south-eastern United States, from south-eastern Virginia south through the Carolinas and Georgia, and west through Alabama, Mississippi and Louisiana to Texas, Oklahoma and Arkansas^1,4. It also ranges up the Mississippi valley to southern Missouri, southern Illinois, Indiana, western Kentucky and western Tennessee¹. In Texas it reaches the Rio Grande valley on the Mexican border¹.

A 2015 genetic study split the cottonmouths into two species². The cottonmouths of Florida and southern Georgia are now treated as the Florida cottonmouth (Agkistrodon conanti), and the two interbreed where their ranges meet, including in the Florida Panhandle^3,5. The Reptile Database follows this split, but notes that another genetic study did not support it¹.

Habitat

The cottonmouth is strongly tied to fresh water. It lives in cypress swamps, river floodplains, heavily vegetated wetlands and slow-moving streams⁴. It may also wander into nearby uplands, especially when dispersing, during droughts or while hunting at night⁴.

It often basks on banks, logs or low plants beside the water, but it seldom climbs high into trees⁴.

Behaviour

In hot weather the cottonmouth is mostly active at dusk and at night, but in cooler seasons it may be active by day⁴. It tends to rely on camouflage and warning displays rather than fleeing⁴. When threatened it coils, vibrates its tail and opens its mouth wide to show the white or pinkish lining that gives it its name⁴. This gape is a warning display, not a strike⁵.

Despite its reputation, it is not aggressive and does not chase people⁵. In field tests, 23 of 45 wild cottonmouths tried to escape when confronted, 28 of 36 used threat displays, and only 13 of 36 bit an artificial hand used in the tests⁶.

Diet

The cottonmouth hunts mainly by ambush but also forages actively on land and in water⁴. Its diet is broad and includes fish, frogs, salamanders, small turtles, snakes, birds and mammals⁴. Its scientific name, piscivorus, means “fish-eating”¹.

Unusually for a snake, it readily scavenges dead animals⁴. Young cottonmouths use their yellow-green tail tip as a lure to attract prey⁴.

Reproduction

The cottonmouth gives birth to live young^1,4. It starts breeding at two to three years old, and mating can take place in spring or autumn⁴. Females give birth in late summer or early autumn to litters of about 3–15 young, each 18–25 cm long, and they often do not breed every year⁴.

Some females stay near their newborns for a short time after birth⁴. Wild female cottonmouths have also been shown to produce young without mating, a process called facultative parthenogenesis⁷.

Venom and medical significance

The cottonmouth is dangerous to people. The World Health Organization lists it in Category 1, the highest medical importance, in the United States⁸. Its venom mainly destroys tissue and disrupts blood clotting, causing pain, swelling, clotting problems and sometimes local tissue death; nausea and low blood pressure can also occur⁴.

Deaths are rare when people get antivenom and supportive care promptly⁴. US treatment guidelines cover cottonmouth bites along with rattlesnake and copperhead bites, including when to give antivenom⁹. Anyone bitten should go to hospital immediately.

Conservation

The cottonmouth is listed as Least Concern on the IUCN Red List, based on an assessment published in 2007, before the 2015 split¹¹. It is widespread and adaptable, but local declines have been recorded because of wetland loss, road deaths and killing by people who fear it or mistake other snakes for it⁴.

Similar species

Harmless watersnakes (genus Nerodia) share the cottonmouth’s habitats, are often more common, and are frequently killed by people who mistake them for it⁵. Seen from directly above, a cottonmouth’s eyes cannot be seen; it also has vertical pupils and a heat pit, and lacks the thin dark vertical lines that watersnakes usually have on their lips, but head shape is not reliable because watersnakes often flatten their heads when alarmed⁵. Watersnakes usually flee into the water, and brown watersnakes often bask on branches high above it, while a cottonmouth seldom climbs high and often stands its ground and gapes^4,12. Young cottonmouths can look like copperheads, but their bands are full of dark spots and a broad dark stripe runs through the eye⁵.

These differences are not a safe basis for handling any snake. Keep your distance from every snake you cannot identify.

In captivity

The cottonmouth is a dangerous venomous snake and is not suitable as a pet. Most bites happen when people try to handle, kill or harass it⁴. If you meet one, step back and give it room to leave.