A belief suggests that observing a snake’s swimming posture can reliably determine if it is venomous. Many assume that a snake cruising high on the water’s surface, with its entire body elevated, indicates danger. This observation is common where venomous pit vipers share space with non-venomous species. Understanding the truth requires examining the snake’s variable behavior and physiology. Swimming posture is not a fixed trait and should never be the sole basis for identification.
The Myth Versus the Reality of Water Posture
The notion that a snake swimming on top of the water is definitively venomous is an overgeneralization that can be misleading. While certain venomous snakes, such as the cottonmouth or water moccasin (Agkistrodon piscivorus), frequently exhibit high buoyancy, this behavior is not exclusive to them. The cottonmouth often cruises with a considerable portion of its body elevated above the water line, appearing to float effortlessly.
This floating tendency is often contrasted with the typical movement of non-venomous water snakes from the Nerodia genus. These snakes are generally observed swimming with their bodies more actively submerged, often keeping only their heads above the surface as they move through the water. However, this difference is a general trend and not a rigid rule for identification.
The posture of any snake is highly dependent on its current activity, making swimming style an unreliable diagnostic tool. Non-venomous water snakes frequently swim high on the surface, especially when basking on floating vegetation or moving quickly across open water. Conversely, a cottonmouth may submerge its body when diving to hunt or attempting to evade a perceived threat. Relying solely on the height of the snake in the water can lead to dangerous errors.
Biological Factors Affecting Flotation
The ability of a snake to control its swimming posture, regardless of its species or venom status, lies in its specialized respiratory system. Snakes possess a single, elongated lung that runs much of the length of their body, acting as a hydrostatic organ. By inflating or partially deflating this lung, the snake can precisely adjust its specific gravity relative to the surrounding water.
Inflating the lung increases the snake’s overall volume, reducing its specific gravity and allowing it to float high. This mechanism enables both venomous and non-venomous species to maintain a high-riding position with minimal muscular effort. The ability to regulate this internal air volume confirms that swimming posture is a behavioral choice rather than a fixed physiological trait.
The lung structure is divided into a vascular segment near the head for gas exchange and a more distal saccular segment that acts primarily as an air reservoir. This large air sac allows the snake to store oxygen and precisely manage its buoyancy, functioning much like a swim bladder. This physiological control means any snake can choose to swim high or submerge, depending on needs like hunting, thermoregulation, or defense.
Reliable Ways to Identify Water Snakes
Since swimming posture is an unreliable indicator, focusing on distinct physical markers is the only safe way to differentiate between species. Venomous pit vipers, including the cottonmouth, exhibit a broad, triangular-shaped head that is distinctly wider than their narrower neck. This characteristic shape is due to the large venom glands located toward the rear of the jaw.
Non-venomous water snakes typically have a more slender, elongated head that blends more smoothly into the neck region. Another feature unique to pit vipers is the presence of loreal pits, which are small, deep depressions located on each side of the face between the eye and the nostril. These pits are highly sensitive infrared-detecting organs absent in non-venomous species.
The shape of the eye pupil also offers a differentiating characteristic, although this is often difficult to see in the field. Pit vipers possess vertical, elliptical pupils that resemble a cat’s eye in bright light. Conversely, non-venomous water snakes generally have round pupils. Focusing on these morphological traits provides a much more accurate assessment than relying on variable behavior. Given the difficulty of observing these details safely, the safest action is always to assume the snake is venomous and maintain distance.