The cottonmouth, or water moccasin (Agkistrodon piscivorus), is a thick-bodied, semi-aquatic pit viper found throughout the Southeastern United States. This species is one of the few venomous snakes in North America that spends significant time in aquatic environments, such as swamps, marshes, and slow-moving streams. Its reputation often leads people to believe it is aggressive, but most of its unique actions are tied to defense or biological necessity.
The Defensive Warning Display
The primary reason a cottonmouth opens its mouth is to perform a defensive warning display, a behavior that gave the snake its common name. When threatened and unable to escape, the snake will stand its ground, coiling its body and raising its head slightly. The signature action involves a wide, deliberate gape that reveals the stark, cotton-white interior of its mouth.
This flash of white is a form of aposematism, a visual signal meant to deter a potential predator without using venom. This prolonged, open-mouth posture is a final, non-lethal warning, signifying a preference for avoiding conflict rather than striking. The snake may also vibrate its tail and secrete a foul-smelling musk as part of this defensive repertoire.
Other Physiological Reasons for Mouth Opening
Beyond the defensive gape, cottonmouths, like all snakes, open their mouths for essential biological functions. One reason is thermoregulation; as ectotherms, they can gape to release excess heat from their bodies, similar to panting in mammals. This action is usually brief and lacks the coiled posture of a threat display.
Snakes also open their mouths for mandibular realignment after consuming a large meal. This process helps reset the jaw bones after they have been stretched to swallow prey. Occasionally, a snake may also open its mouth to clear its respiratory passage before or after intense activity.
Identifying the Cottonmouth
Distinguishing a cottonmouth from harmless water snakes is important, as they often occupy the same habitats. The cottonmouth is a heavy-bodied snake, typically 30 to 48 inches long, with a head that is noticeably wider and more triangular than its neck. Like all pit vipers, it possesses heat-sensing pits between its eyes and nostrils.
A prominent dark stripe or mask often runs from the eye back to the jaw, a feature generally absent in non-venomous water snakes. When swimming, cottonmouths tend to hold their bodies mostly above the water’s surface, unlike water snakes which usually swim with only their heads exposed. Juveniles are often lighter in color with vivid crossbands and a distinct, sulfur-yellow tail tip used for luring prey.
Effects of the Venom
The cottonmouth’s venom is primarily hemotoxic, meaning its toxins affect the circulatory system and body tissues. If a bite occurs, the venom works to destroy red blood cells and prevent blood from clotting. Victims typically feel immediate, intense burning pain at the bite site, followed by rapid swelling and discoloration.
These hemotoxic effects cause tissue damage, which can lead to localized necrosis, or the death of living tissue, around the wound. Though fatalities are rare, a cottonmouth bite is a serious medical emergency requiring immediate attention. Antivenom is administered to neutralize the toxins and halt the progression of tissue destruction and internal hemorrhaging.