Cottonmouths (Agkistrodon piscivorus), also called water moccasins, are venomous snakes (opens in new tab) 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 (opens in new tab). 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 (opens in new tab), as are copperheads (opens in new tab) and rattlesnakes (opens in new tab), according to Sara Viernum, a herpetologist based in Portland, Oregon. "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.
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 (opens in new tab). They have thick, muscular bodies covered in keeled, or 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 (opens in new tab) 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 (opens in new tab). 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.
Genus & species: Agkistrodon piscivorus
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. The University of Florida stated that trying to kill a snake greatly increases the risk of being bitten by one.
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 (opens in new tab), within striking distance," Viernum said. The striking patterns present on the juveniles fades with age.(opens in new tab)
Where do cottonmouths live?
Cottonmouths are native to the U.S. and range from southeastern Virginia to Florida, west to central Texas and north to southern Illinois and Indiana, according to the IUCN (opens in new tab). They primarily live in aquatic and wetland habitats, including swamps, marshes, drainage ditches, ponds, lakes and streams.
The Savannah River Ecology Laboratory noted that cottonmouths can be seen year-round during the day and at night, but they primarily hunt after dark, especially in the summer. They can be found basking in the sun during the day on rocks, logs and stumps, according to the Virginia Department of Wildlife Resources (opens in new tab).
There are three subspecies of cottonmouth recognized by the Integrated Taxonomic Information System (ITIS) (opens in new tab). These are Florida cottonmouths (Agkistrodon piscivorus conanti), found throughout Florida; western cottonmouths (Agkistrodon piscivorus leucostoma), found in the species’ western range, including Indiana, Illinois, Alabama, Oklahoma and Texas; and eastern cottonmouths (Agkistrodon piscivorus piscivorus), found in its eastern range, including 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. Florida cottonmouths typically have the most prominent bands and facial markings of the three subspecies, according to the U.S. Geological Survey (opens in new tab).
What do water moccasins eat?(opens in new tab)
Cottonmouths can 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 (opens in new tab) (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 in an attempt to lure females away from other male suitors. The males also fight each other when competing for females. Impregnated females have a gestation period of five months, on average. 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 (opens in new tab), cats, eagles and snapping turtles.
Experts don't really know how long cottonmouths can live. According to Texas Parks and Wildlife Department (opens in new tab) (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 spend hibernation 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 (opens in new tab).
Cottonmouths have a reputation for being dangerous, but in reality, they 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 (opens in new tab). 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.(opens in new tab)
Although bites are rare, cottonmouth venom is potent and can be deadly to humans. Anyone who suffers a cottonmouth bite should seek medical attention immediately. The Centers for Disease Control and Prevention (CDC) (opens in new tab) 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 (opens in new tab). 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 (opens in new tab) 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 (opens in new tab) 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.
Local cottonmouth populations can be threatened by wetland drainage for agriculture, development, and being disturbed or killed by humans. However, cottonmouths are categorized as a species of least concern on the IUCN Red List of Threatened Species (opens in new tab), 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. However, the IUCN last assessed the species in 2007, and the organization states that their assessment needs updating.
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 (opens in new tab).
- Online book: "Water Moccasin Snake Toxicity (opens in new tab)" (StatPearls Publishing, 2020)
- Illustrated book: "U.S. Guide to Venomous Snakes and Their Mimics (opens in new tab)" (Skyhorse, 2019). Available to buy at Amazon.
- The Virginia Herpetological Society (opens in new tab) website offers more information about cottonmouths.
This article was updated on June 14, 2021, by Live Science Staff Writer Patrick Pester.