Why Is It Called a Cottonmouth? The White Mouth Display

The gaping threat display

The cottonmouth (Agkistrodon piscivorus) is a heavy-bodied, often dark brown or almost black snake that lives in swamps, marshes and slow streams. Against such a dark body, the inside of its mouth looks startlingly pale, like a tuft of raw cotton.

When a cottonmouth feels threatened and cannot easily slip away, it often coils, pulls its neck and front body into an S-shape and tilts its head upward. Then it opens its mouth very wide, sometimes holding the pose for several seconds or longer. The display is often combined with a loud hiss and a rapidly vibrating tail. To a predator or a person, the message is clear: this animal is dangerous and should be left alone.

Many other common names come from the same habit. In parts of the South the snake has been called the gaper, and because it snaps its jaws shut when something touches its mouth, the name trap jaw has been used too.

Why show a white mouth?

A warning only works if it is easy to see. Cottonmouths live in shady, murky places and blend in well with mud, roots and leaf litter, so a sudden flash of bright white makes the snake impossible to overlook.

Showing off is also cheaper than fighting. Producing venom costs energy, and every strike at a large animal risks broken fangs or injury. If a gaping display persuades a raccoon, heron or human to back off, the snake avoids all of those costs. Field tests in South Carolina found that wild cottonmouths used threat displays far more often than they bit, which helps explain why their bad-tempered reputation is undeserved; see Are cottonmouths aggressive?.

The display does not mean the snake is about to attack. It is a defensive posture. If the threat moves away, the snake usually closes its mouth and moves off or stays still. Still, a cottonmouth can bite without warning if it is stepped on or grabbed, so the safest response to any snake is to keep your distance.

Where does the name water moccasin come from?

Water moccasin is the other common name for the cottonmouth, and many people in the South use it more often than cottonmouth. The water part is easy to explain: this is one of the few vipers that lives mostly in and around water, swims well and catches many fish and frogs.

The moccasin part is less clear. Moccasin is an Algonquian word for a soft leather shoe, and early English-speaking settlers used it as a name for several thick-bodied venomous snakes. The copperhead, for example, has been called the highland moccasin. Why the shoe word was applied to snakes is not known for certain, and no explanation is backed by good historical evidence.

Many scientists dislike the name water moccasin because people apply it to almost any snake they see in water. In much of the United States, the “moccasins” people report are harmless water snakes, a mix-up explained in cottonmouth vs water snake.

Other cottonmouth defenses

The white mouth is only one part of the cottonmouth’s defensive toolkit. Depending on the situation, it may also:

• Vibrate its tail. A cottonmouth often shakes the tip of its tail rapidly. In dry leaves this makes a buzzing sound that can resemble a rattlesnake, although the cottonmouth has no rattle.
• Release musk. Glands at the base of the tail produce a strong-smelling musk, which may be sprayed in thin jets if the snake is handled. Observers have compared the smell to a billy goat.
• Flatten its body. By spreading its ribs, the snake looks wider and more intimidating.
• Body bridge. Against snake-eating snakes such as kingsnakes, a cottonmouth may raise a loop of its body and swing it toward the attacker, keeping its head out of reach.
• Stay still or flee. Many cottonmouths rely first on camouflage, and about half of those tested in one study simply tried to escape.

Young cottonmouths have an extra trick that is not defensive at all. Their tail tips are bright yellow or greenish, and they wriggle them like a worm to lure frogs and lizards within range, a behavior called caudal luring.

Is the white mouth a good way to identify a cottonmouth?

A gaping white mouth is a strong sign that you are looking at a cottonmouth, but it is not a test anyone should try to provoke. Other features, such as the blocky head, the dark stripe through the eye and the way the snake floats high in the water, can be seen from a safe distance.

If you see a cottonmouth gaping at you, the right response is the one the snake is asking for: back away slowly and give it room. Cottonmouth bites are a medical emergency, although deaths are very rare. If anyone is bitten, call 911 (US) and seek emergency medical help immediately. For more on how this snake fits among other dangerous species, see venomous snakes in the US.