Cottonmouths (Agkistrodon piscivorus), also called water moccasins, are venomous snakes found in the southeastern United States. They're called cottonmouths because of the white coloration on the inside of their mouths, which they display when threatened.

Cottonmouths are semiaquatic, so they're comfortable both swimming in water (hence their other common name of water moccasin) and basking on land. They are the only venomous snake in the U.S. that spends a lot of time in the water, Live Science previously reported. Other local names for cottonmouths include black moccasins, gapers, mangrove rattlers, snap jaws, stub-tail snakes, swamp lions, trap jaws, water mambas and water pilots.

Cottonmouths are pit vipers, as are copperheads and rattlesnakes, Sara Viernum, a herpetologist based in Portland, Oregon, told Live Science. "Like all pit vipers, [cottonmouths] have heat-sensing facial pits between their eyes and nostrils," Viernum said. These specialized pits are able to detect minute differences in temperature so that the snake can accurately strike the source of heat, which is often potential prey. Cottonmouths rarely bite humans, and usually only do so when provoked.

Sara Viernum

Herpetologist

Sara Viernum is a senior wildlife biologist, and for the last several years has provided professional consulting services to the power transmission and distribution, renewable energy, transportation, mining, and oil and gas sectors. Viernum is a certified wildlife biologist with The Wildlife Society. Viernum has a master’s in biology and herpetology from Jacksonville State University, and a bachelor’s degree in wildlife biology from Murray State University.

How to identify a water moccasin

Cottonmouths are relatively large, ranging from 2 to 4 feet long (61 to 122 centimeters), according to the Savannah River Ecology Laboratory. They have thick, muscular bodies covered in ridged scales and blocky heads with large jowls. Their pupils are vertical, similar to cat pupils, and they have dark stripes next to each nostril. Their coloration varies from dark brown or black to olive, banded brown or yellow.

Cottonmouths are often confused with nonvenomous water snake species from the genus Nerodia. Cottonmouths and Nerodia species have similar coloring and patterns and are all usually found near water. Even though water snakes are nonvenomous, they can still bite and are often killed by humans out of fear that they are cottonmouths.

There are a few ways you can tell a nonvenomous water snake from a venomous water moccasin, or cottonmouth, according to the University of Florida. Water snakes are slender compared with cottonmouths, which are thicker and heavier. Water snakes also have longer, thinner tails, and their heads are a similar width to their necks, whereas a cottonmouth's head is thick, blocky and noticeably wider than the snake's neck. Water snake pupils are round, not vertical and cat-like like the pupils of cottonmouths. Water snakes also lack the facial pits that are characteristic of pit vipers, such as cottonmouths.

Cottonmouth taxonomy

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Squamata

Family: Viperidae

Genus & species: Agkistrodon piscivorus

Source: ITIS

When threatened, nonvenomous water snakes, such as northern water snakes (Nerodia sipedon) and southern water snakes (Nerodia fasciata), often try to appear bigger than they are by flattening their bodies and heads. This flattening makes them look more like cottonmouths. A water snake's flattened head will look more triangular in shape, but not blocky and thick, like a cottonmouth's head. A water snake's head will also still be a similar width to the neck, even when flattened. Trying to kill a snake greatly increases the risk of being bitten by one, according to the University of Florida.

Juvenile cottonmouths have more distinctive bands across their bodies and are lighter brown compared with adult cottonmouths. Juveniles also have bright-yellow tail tips that they use as lure to attract prey. "They undulate the tail tip slowly back and forth to lure prey, such as frogs, within striking distance," Viernum said. The striking patterns present on the juveniles fades with age.

Young cottonmouth snakes have striking patterns that fade with age. (Image credit: kristianbell via Getty Images)

Cottonmouth vs. Water Moccasin

Are they the same thing?

Cottonmouth and water moccasin are interchangeable terms for the venomous water snake.

Where do cottonmouths live?

Cottonmouths range from southeastern Virginia to Florida, west to central Texas and north to southern Illinois and Indiana, according to the International Union for Conservation of Nature. They primarily live in aquatic and wetland habitats, including swamps, marshes, drainage ditches, ponds, lakes and streams.

Cottonmouths can be seen year-round during the day and at night, but they primarily hunt after dark, especially in the summer, according to the Savannah River Ecology Laboratory. They can be found basking in the sun during the day on rocks, logs and stumps, according to the Virginia Department of Wildlife Resources.

There are three subspecies of cottonmouth recognized by the Integrated Taxonomic Information System (ITIS). These are Florida cottonmouths (Agkistrodon piscivorus conanti), found throughout Florida; western cottonmouths (Agkistrodon piscivorus leucostoma), found in Indiana, Illinois, Alabama, Oklahoma and Texas; and eastern cottonmouths (Agkistrodon piscivorus piscivorus), found in Georgia, South Carolina, North Carolina and southeastern Virginia.

Identifying the different subspecies is difficult. Their markings vary considerably, and the subspecies can interbreed where their ranges overlap.

What do water moccasins eat?

A water moccasin eats a bullfrog. The snake's diet consists of fish, small mammals, birds and amphibians; reptiles such as lizards, baby alligators and turtles; and other snakes, including smaller water moccasins. (Image credit: Paul S. Wolf/Shutterstock.com)Cottonmouths hunt prey in water or on land. They eat fish, small mammals, birds, amphibians and reptiles — including other snakes and even smaller water moccasins, according to the University of Michigan's Animal Diversity Web (ADW). Cottonmouths kill with a single, venomous bite, then wrap around their prey until it stops moving before swallowing their food whole.

Cottonmouths mate in spring, usually from April to May. During the mating process, males slither around, waving their tails to lure females away from other male suitors. The males also fight each other when competing for females. Females have a gestation period of five months. Cottonmouths are ovoviviparous, which means that eggs incubate inside the mother's body. Females give birth to live young every two to three years, in litters of about 10 to 20 offspring.

Baby cottonmouths are born brightly colored and go off on their own as soon as they're born. Most baby cottonmouths don't make it to adulthood because they are eaten by other animals, such as raccoons, cats, eagles and snapping turtles.

Experts don't really know how long cottonmouths can live. According to Texas Parks and Wildlife Department (TPWD), cottonmouths live less than 10 years in the wild. However, the snakes can live much longer in captivity, and at least one captive cottonmouth has lived to be over 24 years old, according to ADW.

Cottonmouths may hibernate over winter in the colder, northern parts of their range. They hibernate in burrows made by other animals, including crayfish and tortoises, or under some other form of cover, such as rotting stumps, according to the IUCN.

Are water moccasins aggressive?

Cottonmouths rarely bite humans unless they are picked up or stepped on. They may stand their ground against potential predators, including humans, by using defensive behaviors.

"When a cottonmouth feels threatened, it will coil its body and open its mouth wide to expose the white coloration of the inside of its mouth," Viernum said. The flash of white contrasts with the snake's dark body colors to create a startling display. "Exposing the white of the mouth serves as a warning signal to potential predators."

Cottonmouths may also make themselves stink to deter predators by spraying a foul-smelling musk from glands in the base of their tail, according to the Florida Museum of Natural History. Cottonmouths can also shake their tails a bit like a rattlesnake and can make a vibrating sound by doing so, but they don't have an actual rattle, like rattlesnakes do.

A cottonmouth looking up from the grass. (Image credit: Shelby L Earle/Shutterstock.com)

Cottonmouth bite

Although bites are rare, cottonmouth venom can be deadly to humans. Anyone who suffers a cottonmouth bite should seek medical attention immediately. The Centers for Disease Control and Prevention (CDC) stated that for venomous bites, the sooner antivenom can be administered, the sooner irreversible damage from the venom can be stopped. After calling for emergency services, the CDC recommends that snakebite victims take a photo of the snake from a safe distance if possible, remain calm and apply first aid while waiting for emergency medical service personnel to arrive.

Humans bitten by pit vipers, such as cottonmouths, will almost always feel an immediate burning pain where they've been bitten, and these bite wounds usually begin to swell within five minutes, according to TPWD. Skin discoloration around the wound is also common.

Cottonmouth venom is mainly composed of hemotoxins that break down blood cells, preventing the blood from clotting or coagulating, according to Viernum. The hemotoxins lead to "hemorrhaging throughout the circulatory system wherever the venom has spread," she said. Being bitten and injected with cottonmouth venom can lead to "temporary and/or permanent tissue and muscle damage; loss of an extremity, depending on the location of the bite; internal bleeding; and extreme pain around the injection area," Viernum added.

The University of Florida stated that 7,000 to 8,000 people are bitten by venomous snakes in the U.S. each year, but only about five to six people die from their bites. Cottonmouths have accounted for less than 1% of all snakebite deaths in the U.S., according to TPWD.

Conservation status

Cottonmouths are categorized as a species of least concern on the IUCN Red List of Threatened Species, which means that across nearly all of its range, the species is at low risk of extinction. Cottonmouths have a wide distribution, and the IUCN presumes that the cottonmouth population is large and relatively stable.

Many cottonmouths live in protected state and national parks, and the species is also protected by state law in some places. In Missouri, for example, all snakes are protected from being killed, including cottonmouths, according to the Missouri Department of Conservation.

Additional resources

Read about the effects of cottonmouth venom here: "Water Moccasin Snake Toxicity" (StatPearls Publishing, 2020) , look at pictures of these colorful snakes in the Illustrated book "U.S. Guide to Venomous Snakes and Their Mimics" (Skyhorse, 2019), or get more information about cottonmouths from the Virginia Herpetological Society website offers more information about cottonmouths.

Updated on Live Science Aug. 16, 2023.