Snake locomotion involves navigating diverse environments without limbs, relying entirely on a muscular body and flexible spine. This absence of appendages led to the evolution of several distinct gaits, which snakes employ depending on the terrain, required speed, and purpose (such as hunting or escaping). This adaptability allows a single species to move effectively over surfaces ranging from rough tree bark to shifting desert sand. Snakes generate forward thrust by manipulating friction and strategically anchoring portions of their body.
Lateral Undulation
Lateral undulation, also known as serpentine movement, is the most common form of snake locomotion. It involves the snake throwing its body into a series of S-shaped curves that travel from the head toward the tail. Propulsion is generated by pushing laterally against external anchor points, such as rocks or uneven ground surfaces. The horizontal bends contact multiple points simultaneously, and the lateral forces exerted against these objects cancel each other out, resulting in a net force that propels the snake forward.
The snake achieves this wave-like motion by sequentially activating its large dorsal muscles. Since this method requires external resistance points, it is highly effective and fast on rough terrain. On perfectly smooth surfaces, such as glass, the snake slips and moves little, demonstrating its reliance on friction. However, even on low-resistance surfaces, the snake can generate some thrust by exploiting the frictional properties of its ventral scales, which resist sideways slip more than forward movement.
Concertina Locomotion
Concertina locomotion is an “anchor and pull” technique used to navigate confined spaces, climb, or move over slick surfaces lacking lateral push-points. The movement resembles an accordion, where the body alternately bunches and extends. The snake starts by gathering the posterior section of its body into tight S-shaped coils, which serve as a static anchor point gripping the substrate to resist backward movement.
While the rear is anchored, the snake extends its head and the anterior body forward in a relatively straight line. Once the front section reaches its extension limit, it anchors itself by forming new coils or pressing against a wall. The snake then pulls the trailing, anchored body forward to meet the new front anchor point, completing the cycle. This gait is metabolically costly compared to lateral undulation, but it is necessary for climbing vertical surfaces or moving through narrow passages where lateral movement is impossible.
Rectilinear Movement
Rectilinear movement, sometimes described as “caterpillar” movement, is a slow, stealthy method of progression where the snake moves in a straight line without visible side-to-side undulations. This gait is achieved through a coordinated wave of muscle contractions involving the ventral scales, or scutes, on the snake’s belly. The snake uses specialized costocutaneous muscles connecting the ribs to the skin.
In this process, sections of the belly are lifted slightly and pulled forward by one set of muscles, momentarily disengaging the scutes from the ground. A different set of muscles then pulls the underlying skeleton and the rest of the body forward. The scales act like tire treads to grip the surface and prevent backward sliding. The resulting motion is a continuous, almost imperceptible glide, making it the preferred method for heavy-bodied snakes, such as pythons and boas, when stalking prey or moving slowly. This movement requires minimal spinal flexing, which is advantageous after consuming a large meal.
Sidewinding
Sidewinding is a specialized gait used primarily by snakes in environments with loose, shifting substrates, such as desert sand or mud. This movement minimizes the contact area between the snake’s body and the hot, unstable ground. The snake creates a series of parallel, diagonal tracks by throwing its body into vertical and horizontal loops.
At any moment, the snake maintains only two or three sections of its body in static contact with the ground, while the rest of the body is lifted and moved forward. The contact points act as anchors, and the body is lifted and rolled forward to a new position, one segment at a time. This process results in the snake moving diagonally, at an angle to the body’s axis, which generates the necessary propulsive force on challenging surfaces. Increasing the frequency of these cycles allows the snake to increase its speed.
Specialized Movement: Water and Air
Snakes that move in water employ aquatic lateral undulation, a modified version of their terrestrial serpentine movement. Swimming snakes generate thrust by pushing against the water, with muscle activity timing adjusted compared to land movement. Sea snakes, such as the yellow-bellied sea snake, are adapted with specialized features like a laterally flattened, paddle-shaped tail and a ventral body keel, which enhance propulsion and maneuverability.
In the air, certain arboreal species, like the paradise tree snake (Chrysopelea paradisi), exhibit gliding. These “flying snakes” launch themselves from trees and flatten their bodies by spreading their ribs, transforming their cylindrical shape into a flattened, aerodynamic wing. While gliding, they perform a continuous, complex undulation, wiggling both side-to-side and up-and-down. This aerial undulation is necessary for stability, preventing the snake from tumbling and allowing for controlled, extended glides between branches.