Cottonmouth (Agkistrodon piscivorus): Water Moccasin Facts

What does a cottonmouth look like?

The cottonmouth is a heavy-bodied snake with a broad, blocky head that is clearly wider than its neck. Like all pit vipers, it has vertical, cat-like pupils and a heat-sensing pit on each side of the face, between the eye and the nostril. A broad dark stripe runs back from each eye, giving many cottonmouths a masked look, although this stripe is fainter in western populations. Its scales are keeled, with a small ridge down the middle, so the skin looks dull rather than shiny.

Color changes a lot with age. Young cottonmouths are boldly patterned, with 10–17 dark brown crossbands on a tan, brown or reddish-brown background, and the tip of the tail is bright yellow or greenish. As the snake grows, the background darkens and the bands fade. Many old adults are almost uniform olive-brown, dark gray or black, with only the head markings still visible. The belly is pale to dark and often blotched.

Because young cottonmouths are banded and coppery, they are often mistaken for copperheads, their closest relatives. Dark adults are more often confused with harmless water snakes, as explained in cottonmouth vs water snake.

How big do cottonmouths get?

The cottonmouth is the largest member of its genus. Most adults measure about 65–90 cm (26–35 in) in total length, and males are usually longer and heavier than females. Snakes over 1.2 m (4 ft) are uncommon, and the largest reliably measured specimen, from the Great Dismal Swamp region of Virginia and North Carolina, was about 188 cm (74 in) long.

Cottonmouths are thick and muscular for their length, so a 1 m (3.3 ft) snake looks much bigger than a slender snake of the same length. Stories of “giant moccasins” well over 2 m (6.5 ft) are not backed by measured specimens.

Northern and Florida cottonmouths: one species or two?

For decades, scientists recognized one cottonmouth species with three subspecies: the eastern, western and Florida cottonmouths. A genetic study by Frank Burbrink and Timothy Guiher, published in 2015, found that the eastern and western forms are not distinct, while the snakes of peninsular Florida form a separate lineage.

Most herpetologists now recognize two species. The northern cottonmouth (Agkistrodon piscivorus) covers most of the range, and the Florida cottonmouth (Agkistrodon conanti) lives in peninsular Florida and the southernmost part of Georgia. The two meet and interbreed in a zone across southern Georgia and northern Florida. They look and behave very much alike, and most of the facts in this profile apply to both. The IUCN Red List assessment, made in 2007 before the split, still covers them together.

Where do cottonmouths live?

The northern cottonmouth lives in the southeastern and south-central United States. Its range runs from the Great Dismal Swamp in southeastern Virginia south through the Carolinas, Georgia and Alabama, and west through Mississippi, Louisiana and Arkansas to eastern Oklahoma and central Texas. Along the Mississippi and Ohio valleys it reaches north to southeastern Missouri, western Kentucky, southern Illinois and southern Indiana. It also lives on many barrier islands along the Atlantic and Gulf coasts.

Cottonmouths are the most aquatic of the Agkistrodon snakes. They live in swamps, marshes, bayous, sluggish streams, ponds, lake edges, ditches, canals and rice fields, and they enter brackish marshes and sometimes swim in salt water between islands. They usually avoid fast, deep, cold water. They are not tied to water every minute, though: large adults have been found more than 1.6 km (1 mile) from the nearest pond or creek, in pine woods, palmetto thickets and coastal dunes.

In the north of the range, cottonmouths spend the winter dormant in stumps, root holes, burrows and rock crevices, often on slightly higher ground near wetlands. In the far south they may stay active through most of the winter on warm days.

What do cottonmouths eat?

Cottonmouths are carnivores with one of the broadest diets of any North American snake. Their scientific name, piscivorus, means “fish-eater”, and fish and frogs make up most of their food. They also eat salamanders, other snakes (including smaller cottonmouths), lizards, small turtles, baby alligators, birds, eggs, shrews, mice and rats, and young snakes sometimes eat insects. Common prey includes catfish, bass, sunfish and southern leopard frogs.

They are opportunists. Cottonmouths gorge on fish and tadpoles trapped in shrinking pools as wetlands dry out in late summer, and they are among the few snakes regularly seen eating carrion, such as dead fish thrown from docks. In Florida, cottonmouths have even been recorded eating young invasive Burmese pythons.

A cottonmouth usually hunts by ambush or by slowly searching the water’s edge. Fish are often cornered in shallow water, against the bank or under logs. Small prey may be held in the jaws until the venom works, while larger, dangerous prey is struck and released, then tracked down by scent. Young cottonmouths use their yellow tail tip as a lure, wriggling it like a worm to draw frogs and lizards close. They can strike and bite in water as well as on land; see Can cottonmouths bite underwater?.

Behavior and the famous threat display

Cottonmouths can be active by day or night. On cool mornings and in spring and fall they bask in the sun on logs, banks and branches, while in hot summer weather they shelter in shade by day and hunt after dark. They are mostly solitary and usually stay within a home range that averages around 1 hectare (2.5 acres) and almost always includes water.

When a cottonmouth swims, it floats high on the surface, with much of its body buoyant and its head often held up at an angle. Most harmless water snakes swim with the body lower in the water.

When threatened, a cottonmouth often stays put rather than racing away. It may coil, vibrate its tail, hiss and throw its head back with the mouth gaping wide to show the cotton-white lining, the display behind its common name. It can also flatten its body and release a strong-smelling musk from glands at the base of its tail. These are warnings, not attacks, and they are explained in Why is it called a cottonmouth?.

Reproduction and life cycle

Cottonmouths mate mainly in spring, though mating has been seen in many of the warmer months. Rival males settle disputes with ritual combat, raising and twisting their front bodies against each other in a test of strength rather than biting.

Like other pit vipers in North America, cottonmouths give birth to live young. The eggs develop inside the mother for several months, and the babies are born in late summer or early fall, mostly in August and September. Litters usually contain six to eight young, with a recorded range of 1 to 16. Females typically breed every other year, because they need time to rebuild their fat reserves. Mothers have been seen staying near their newborn litters for a week or two, until the young shed their skin for the first time.

Newborn cottonmouths measure 22–35 cm (9–14 in), already have working fangs and venom, and get no feeding care. With plenty of food, females can mature in about three years. Captive females have also produced young by facultative parthenogenesis, or “virgin birth”, without mating with a male.

How long do cottonmouths live?

Lifespan in the wild is not well studied, and many young snakes are eaten in their first year. On average, only a small fraction of each litter is thought to reach adulthood. In captivity, cottonmouths have lived to about 24 years, the longest recorded lifespan for the species.

Predators and threats

Young cottonmouths have many enemies: herons, egrets, hawks, eagles, owls, raccoons, largemouth bass, gar, snapping turtles and snake-eating snakes such as kingsnakes. Against snake predators, cottonmouths sometimes use body bridging, raising a loop of the body and swinging it toward the attacker. Adults are taken mainly by American alligators, large birds of prey and people.

People are the biggest threat. Cottonmouths and the harmless water snakes mistaken for them are often killed on sight, run over on roads near wetlands and lose habitat when swamps and marshes are drained or built over.

Conservation status

The cottonmouth is listed as Least Concern on the IUCN Red List, based on an assessment in 2007, with a stable population trend. It is widespread and still common in many southern wetlands. At the northern edge of its range it is rare: it is listed as endangered by the state of Indiana, where it is known only from a few wetlands in the far south of the state.

Are cottonmouths aggressive?

Cottonmouths have a fearsome reputation, but field research does not support the idea that they are aggressive. In a well-known study published in 2002, J. Whitfield Gibbons and Michael Dorcas tested wild cottonmouths in South Carolina. About half tried to escape, most used threat displays, none bit when a person simply stood beside them, and few bit even when stepped on with a protective boot. More detail is in Are cottonmouths aggressive?.

The venom is still dangerous. It mainly destroys tissue and disrupts blood clotting, causing severe pain and swelling, and bites can leave lasting damage. Deaths are very rare in the United States, where antivenom is widely available. Most bites happen when people try to handle, kill or step on a snake they did not see. The best approach is to watch cottonmouths from a distance, look where you step and sit near water, and call 911 immediately if anyone is bitten. You can compare it with other species in venomous snakes in the US.