The cottonmouth (Agkistrodon piscivorus) is not a rattlesnake, although the two are frequently confused due to their shared appearance and venomous nature. They represent distinct genera within the same family. Cottonmouths, also called water moccasins, belong to the genus Agkistrodon, while all rattlesnakes are classified under Crotalus or Sistrurus. This difference in classification means they are separate species with unique biological and physical traits.
Shared Family Ties: The Pit Viper Connection
The confusion between these snakes stems from their shared evolutionary history as members of the Viperidae family. Both the cottonmouth and the rattlesnake belong to the Crotalinae subfamily, known as the pit vipers. This taxonomic relationship means they share several defining anatomical features.
All pit vipers possess a pair of loreal pits, which are specialized, heat-sensing organs located between the eye and the nostril. These infrared-detecting organs help the snakes accurately locate warm-blooded prey, even in total darkness. Both snakes also share the characteristic viper design, featuring a heavy, thick body and hinged, retractable fangs. These long fangs fold back against the roof of the mouth when not in use and are deployed rapidly to inject venom.
The Defining Feature of Rattlesnakes
The most obvious trait distinguishing rattlesnakes (Crotalus and Sistrurus) is the complex structure at the tip of the tail that gives them their name. This acoustic warning device, the rattle, is unique to these two genera and is composed of a series of interlocked, hollow segments made of keratin.
The distinctive rattling sound is produced when the snake vibrates specialized tail muscles at high speed, causing the hollow segments to knock against one another. A new segment is added each time the snake sheds its skin, a process that can occur several times a year. The primary function of the rattle is to deter potential threats and predators by issuing a clear warning signal.
Unique Characteristics of the Cottonmouth
The cottonmouth possesses distinguishing physical traits that separate it from its rattlesnake relatives. The head is thick and blocky, distinctly wider than its neck, giving it a triangular appearance. A broad, dark stripe runs through the eye and along the side of the head, acting as a facial mask.
Its common name comes from its unique defensive display, which involves coiling its body and opening its mouth wide to reveal the stark, pale white lining. Adult cottonmouths often exhibit dark, indistinct dorsal patterns of crossbands, which can fade almost entirely, making older individuals appear nearly black. Unlike the rattlesnake’s specialized tail, the cottonmouth’s tail is short and tapers abruptly to a point, lacking any warning structure.
Practical Identification: Habitat and Behavior
For field identification, recognizing differences in environment and demeanor provides the most practical distinction between the two snakes. The cottonmouth is a semi-aquatic species, preferring wetlands, swamps, sluggish streams, and lake shores. It is unique among American venomous snakes for spending significant time in water, often swimming with its body floating along the surface.
Rattlesnakes are typically terrestrial and prefer drier habitats, such as rocky outcrops, deserts, and scrublands. They generally avoid water unless necessary for crossing. Differences in defensive behavior are also telling when an encounter occurs.
A rattlesnake’s primary defense is issuing its loud rattle as a warning before resorting to a strike. The cottonmouth often behaves differently, sometimes freezing or coiling in place when approached. If the threat persists, it will initiate its defensive gape, flashing the white interior of its mouth rather than immediately striking.