Locomotion is the process by which an organism moves from one place to another. For limbless snakes, this process has evolved into four primary gaits, each optimized for specific needs, such as maximizing speed, navigating tight tunnels, or traversing loose terrain. The mode a snake uses reflects the substrate it is moving on, the required speed, and the presence of anchor points.
Lateral Undulation
Lateral undulation is the most recognizable and widespread form of snake movement, often described as serpentine locomotion. It involves the body forming a series of S-shaped curves that propagate backward from head to tail. The snake generates forward thrust by pushing the outer curves against external objects in the environment, such as rocks, grass stems, or rough patches of ground.
Waves of muscle contraction travel sequentially along alternating sides of the body. When the snake pushes laterally against multiple points simultaneously, the resistance of the environment cancels out the sideways forces. This leaves a net force directed straight forward, propelling the snake across the surface. This motion is efficient on moderately rough terrain and allows the fastest terrestrial snakes to achieve their top speeds.
Sidewinding
Sidewinding is a specialized form of movement adapted for traversing loose or slippery substrates like shifting sand or mudflats, where lateral undulation is ineffective. This gait involves the snake throwing its body segments forward, moving at an angle to its overall direction of travel. The mechanism is characterized by a “lift and lay” movement, where the body is lifted off the unstable ground in segments.
Only two or three sections of the body remain in static contact with the ground at any given moment, acting as anchors from which the snake pushes off. Minimal contact with the surface is an adaptation in desert environments, preventing the snake from sinking and reducing exposure to high temperatures. This distinct motion leaves a series of J-shaped tracks in the sand that are perpendicular to the snake’s path.
Concertina Movement
Concertina movement is an accordion-like method used when the snake must move through confined spaces, such as burrows, narrow tunnels, or while climbing vertical surfaces. It is a slow, methodical, and energy-intensive form of locomotion. The movement cycle alternates between two distinct phases to achieve forward progression.
First, the snake anchors the rear portion of its body by bunching it into tight curves that press firmly against the walls or surface. The front section is then extended forward in a relatively straight line. Once the head establishes a new anchor point, the rear portion is pulled forward, compressing the middle section into a new set of bends. This repeated process of anchoring and extending allows the snake to gain purchase in environments lacking external push points.
Rectilinear Crawling
Rectilinear crawling is a straight-line movement used by heavy-bodied snakes, such as pythons, boas, and large vipers, when moving slowly or stalking prey. This mode of travel involves little to no lateral bending of the body, making the snake appear to glide silently across the ground. The propulsion mechanism utilizes the snake’s specialized ventral scales, which function like the treads of a tank.
The movement is driven by waves of muscle contraction that move the belly skin relative to the skeleton. Muscles contract, pulling the skin backward while the scales anchor to the ground, propelling the body forward. Simultaneously, other muscles pull the lifted sections of skin forward for the next cycle. Although this is the slowest method of movement, it is nearly noiseless, which is an advantage for ambush predators.