Reptiles
Cottonmouth
Agkistrodon piscivorus · also called cottonmouth, water moccasin
The cottonmouth, better known in much of its range as the water moccasin, is the heavy-bodied venomous snake of southeastern swamps and streams. Its name comes from what it does when it feels trapped: it throws its head back and opens its mouth wide, flashing a lining as white as cotton. It is the only semi-aquatic viper in the world — and one of the most misidentified snakes in North America.
Overview
Cottonmouths live across the southeastern United States, from Virginia to Florida and west to Texas, in and around swamps, marshes, streams, floodplain forest, and pond edges. Most are 65–90 cm long, with big individuals reaching 1.2–1.8 m. The body is thick, and the head is a broad, blocky wedge far wider than the neck, with vertical pupils and a heavy, scowling brow.
The white-mouth display
Cornered, a cottonmouth often stands its ground rather than fleeing. It coils, draws its head back, gapes to show the white mouth lining, vibrates its tail, and releases a pungent musk. It is a warning, not an attack — a snake given room to leave will usually take it.
Life along the water
It eats almost anything it can catch near water: fish, frogs, other snakes, small turtles, birds, small mammals, and carrion. Unlike harmless water snakes, it swims with its head up and body riding high. Young cottonmouths have a bright yellow tail tip they wiggle as a lure to draw in frogs and lizards, losing it as they darken with age. Females give live birth.
Mistaken identity
Non-venomous Nerodia water snakes share the same habitat and are routinely killed as “cottonmouths.” The harmless ones are slimmer, round-headed, round-pupilled, and tend to bolt into the water. Learning the difference saves a lot of snakes.
Status
The cottonmouth is Least Concern and common, though it declines where wetlands are drained and where snakes are killed on sight.
Cottonmouth: common questions
How dangerous is a cottonmouth bite?
It is a serious bite that needs hospital treatment, but it is rarely fatal to a healthy adult who gets prompt care. The venom is mainly tissue-destroying, and an untreated bite can cause severe pain, swelling, blistering, and lasting damage to muscle and skin around the wound, occasionally bad enough to threaten a limb. Deaths are very uncommon, with antivenom widely available across the snake's range. Most bites happen when people try to catch, kill, or handle the snake, so the single most effective way to avoid one is to leave any cottonmouth alone and give it room to retreat.
How can you tell a cottonmouth from a harmless water snake?
It is genuinely tricky, and countless harmless water snakes are killed because of the confusion. A cottonmouth is thick and heavy for its length, with a large blocky head clearly wider than its neck, a heavy brow that gives it an angry, scowling look, vertical cat-like pupils, and a single row of scales under the tail. Harmless Nerodia water snakes are more slender, have rounder heads that blend into the neck, round pupils, and often flee straight into the water when disturbed. Behaviour helps too — a cornered cottonmouth frequently stands its ground and gapes, showing the white mouth, rather than simply escaping.
Why is it called a cottonmouth or a water moccasin?
Cottonmouth comes from the defensive display. A threatened snake opens its mouth extremely wide and holds the pose, revealing a mouth lining that is a startling cottony white against the dark body, a clear visual warning to back off. Water moccasin is the older common name and refers simply to a venomous snake associated with water, moccasin being a long-standing regional word for a pit viper. Both names point at the same animal, and the snake also has a long list of local names such as swamp moccasin, black moccasin, and gapper.
Where and when are you likely to encounter a cottonmouth?
In the warmer parts of the southeastern United States, close to fresh water. Cottonmouths live along the margins of swamps, marshes, slow rivers, streams, drainage canals, ponds, and floodplain forest, and they also turn up in brackish coastal marsh and on barrier islands. They are active by day in cooler weather and largely at night in the heat of summer, and they often bask on banks, logs, and low branches over water, dropping in if disturbed. In cooler months they move to higher, drier ground and shelter in stump holes, crayfish burrows, and other retreats.