How Do Snakes Move? Four Gaits Without a Single Leg
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A black mamba can move at about 20 kilometres an hour, faster than most people can run over rough ground, and it does it with a body that is essentially a muscular tube with a spine of up to 400 vertebrae and no limbs at all. Snakes gave up their legs around a hundred million years ago, and instead of being handicapped by the loss they became one of the most successful groups of land animals, with nearly 4,000 species on every continent except Antarctica. They manage it with four distinct ways of moving, and which one a snake uses depends on what it is moving across.
The body
Everything begins with the skeleton. A snake's backbone has between 150 and 400 vertebrae, each carrying a pair of ribs and each able to flex a few degrees against its neighbours, so that the whole can bend into curves impossible for any limbed animal. Thousands of muscles run along the spine in overlapping bands, some spanning a few vertebrae and some thirty or more, and by contracting them in sequence the snake can throw a wave down its body or press any part of itself against the ground. The belly is covered in wide scales, the scutes, one per vertebra, whose rear edges catch on any roughness in the surface and stop the body sliding backwards, like the scales on a cross-country ski. Between them, the muscles, the vertebrae and the scutes let a snake use almost any point of contact as a foot.
The four gaits
Herpetologists recognise four main modes, and most snakes can use at least three:
- •Lateral undulation: the familiar S-shaped wriggle, in which waves pass down the body and every curve pushes backwards against a rock, a stem or a ridge in the ground, so that the snake is driven forward by the sum of the pushes; it is the fastest gait and the one used in water, where the whole body pushes against the water
- •Concertina: for tunnels and smooth branches, the snake anchors its rear by bunching it into folds, extends the front, anchors the front and draws the rear up, like an inchworm
- •Rectilinear: the straight-line creep of heavy snakes such as pythons and boas, in which the belly scutes are lifted, moved forward and set down in waves while the body glides over them, slow and silent, used for stalking
- •Sidewinding: for loose sand, the snake lifts sections of its body clear of the ground and throws them sideways, touching down in only two or three places at a time and leaving a row of separate J-shaped tracks; the sidewinder rattlesnake and the horned vipers of the Sahara evolved it independently
What the scales do
Experiments in which snakes were put on smooth glass, or fitted with a sleeve that covered their belly scales, found that they could barely move at all, which showed that the scutes, not the muscles alone, make the gaits work. The scales are also anisotropic: their friction is low when sliding forward and much higher sideways and backwards, so that a snake pressing a curve of its body against the ground slides along the curve rather than skidding across it. Engineers building snake robots for search and rescue, pipe inspection and surgery have had to copy that property to get anywhere, and the robots that work best have belly plates modelled on a snake's.
Climbing, flying and swimming
Tree snakes climb by concertina and by wrapping, and in 2021 researchers filmed brown tree snakes in Guam climbing smooth metal poles by looping their body into a lasso around the pole and inching upward, a fifth gait nobody had seen. The paradise tree snake of Southeast Asia flings itself from a branch, flattens its body into a ribbon, and glides for tens of metres while undulating in the air, steering as it goes. Sea snakes have a flattened, paddle-like tail and swim by undulation, and some spend their whole lives at sea, breathing partly through their skin. A snake's speed is often exaggerated; most cruise at a walking pace and strike, from a coiled position, faster than the eye can follow, since the strike is not locomotion but a single throw of the front third of the body.
Sensing the way
A snake moves through a world it perceives differently. It has no external ears and hears mostly through its jawbone, which picks up vibrations from the ground; it smells with its forked tongue, which collects scent particles from the air and delivers them to a pair of pits in the roof of the mouth, the fork telling it which side the scent is stronger on; and pit vipers, pythons and boas have heat-sensing organs on the face that image the warmth of a mouse in total darkness. Its eyes, covered by a fixed transparent scale rather than lids, are good at motion and poor at detail. The animal that people find most alien is reading the ground through its belly, tasting the air, and feeling for footsteps, and moving on a set of principles that engineers are still learning to copy.
The takeaway
Snakes move with a spine of up to 400 vertebrae, bands of muscle that can bend any part of the body, and belly scales that grip the ground in one direction and slide in the other, using four gaits: undulation, pushing curves against surfaces and water; concertina, for tunnels and branches; rectilinear creeping on the belly scales; and sidewinding across loose sand. Some also climb by lasso, glide, and swim, and they navigate by ground vibration, a scent-collecting tongue and, in some, heat vision.