The cottonmouth, also known as the water moccasin, is the only semi-aquatic viper found in North America. Present in swamps, marshes, and bodies of water throughout the southeastern United States, accurate identification is necessary for public safety. As a pit viper, this species possesses venom capable of inflicting a painful and potentially dangerous bite. Understanding its visual and behavioral characteristics is key to distinguishing it from numerous harmless water snakes.
Primary Physical Markers for Identification
The head of a cottonmouth is distinctly broad and triangular, possessing a thick neck that sharply narrows into the body. This spade-shaped head is characteristic of vipers, though some non-venomous snakes can flatten their heads when threatened. A key feature is the presence of heat-sensing facial pits located between the eyes and the nostrils, a trait shared by all pit vipers.
The eyes feature vertical, cat-like pupils, which are difficult to discern from a distance or in low light. A broad, dark stripe runs through the eye, often bordered by thin, pale lines. The snake’s body is notably thick and heavy, often described as chunky or robust for its length, which typically ranges from 30 to 48 inches in adults.
Coloration varies significantly, ranging from dark olive, brown, or black to lighter shades. Adults often appear uniformly dark because their crossbands fade, but juveniles exhibit a noticeable pattern of reddish-brown crossbands. Young cottonmouths also possess a brightly colored, sulfur-yellow or greenish tail tip, which they use as a lure to attract prey. The ridged, or keeled, scales along the back contribute to the snake’s rough, non-glossy appearance.
Distinctive Behavioral and Postural Clues
The most recognizable action is the defensive display that gave the species its name: the “cottonmouth.” When alarmed or cornered, the snake coils its body and opens its mouth wide, exposing the stark white interior lining. This mouth-gaping is a warning signal intended to startle a potential threat into retreating before a strike occurs.
When swimming, the cottonmouth maintains a unique posture where most of its body floats on the surface of the water. It appears to ride high on the water, with its head often held at an elevated, upward-tilted angle. This semi-aquatic viper is relatively sluggish compared to other aquatic snakes, preferring to stand its ground or slowly retreat rather than flee rapidly.
Cottonmouths may also exhibit other warning behaviors when threatened, such as vibrating their tail rapidly. They frequently release a strong, foul-smelling musk from glands at the base of the tail, which is another defensive tactic aimed at deterring predators. This behavior, along with the mouth display, serves as a clear indication that the snake is stressed and should be left alone.
How to Distinguish Cottonmouths from Water Snakes
The most common source of misidentification involves non-venomous water snakes (Nerodia species), which share a similar habitat and general body type with the cottonmouth. Water snakes have a narrower, more rounded head that is not significantly wider than their neck. While they may flatten their heads defensively to mimic the viper’s shape, their pupils remain round, a clear distinction from the cottonmouth’s vertical, elliptical pupils.
A primary difference is visible in their swimming style. Water snakes swim with most of their body submerged just beneath the surface, with only their head visible above the water line. The cottonmouth, however, floats conspicuously on the water, with nearly its entire body resting upon the surface.
Behavioral differences also provide a means of differentiation when the snake is encountered on land. When approached, water snakes typically flee quickly into the water, or they may strike and profusely musk if handled. The cottonmouth, by contrast, is more likely to remain coiled and stand its ground, relying on its threat display of mouth-gaping rather than immediate flight.
Typical Habitats and Encounter Safety
Cottonmouths are almost always found in or very near water throughout the southeastern United States, from swamps and marshes to slow-moving streams and drainage ditches. They prefer habitats with dense vegetation and can be found basking on logs, rocks, or stumps near the water’s edge. They are proficient swimmers and have been documented in brackish water and salt marshes.
Encountering this snake requires adherence to basic safety protocols. The safest measure is to immediately increase the distance, giving the snake ample space to retreat unprovoked. Never attempt to handle, move, or harass any snake that you suspect may be a cottonmouth.
If a bite occurs, immediate medical attention is necessary. The affected limb should be kept still and below the level of the heart. Note the time of the bite and avoid traditional, ineffective measures such as cutting the wound or applying a tourniquet. Prevention is best achieved by recognizing the snake and leaving it undisturbed.