Your turn
Each lab is one part of the video. Change the ground and see what happens.
Each lab isolates one gait or mechanism. Speeds and scores are simplified to show the idea.
Tap, pick and slide. See what happens!
Tap the snake. Watch a wave of squeezes travel from head to tail.
Tap to trigger a contraction wave; watch it travel along the vertebral column.
Tap the snake. Watch it wiggle!
Switch the ground and the rocks. When does the S-wave work?
Toggle obstacles and a low-friction surface. Which one kills forward motion?
Put the snake on glass. Can it still go?
Pick a move and watch the green grip spots.
Compare the static contact regions in each gait.
Pick a move. Green spots grip the ground.
Make the dune steeper. How much of the body has to touch the sand?
Increase the slope, then switch off the extra contact and watch the snake slip.
Make the sand hill steep. Does the snake slip?
Pick a ground and a move. Which works best?
Pair each terrain with each gait. Scores are illustrative.
Pick a ground and a move. Does it work?
A snake has no legs, no arms and no feet. But snakes glide on grass, swim, climb trees and cross hot sand. How?
Inside, a snake has lots and lots of little back bones, many more than you. Strong muscles run down both sides. When one side squeezes, the body bends.
Most snakes move in a wiggle wave shaped like the letter S. Each curve pushes back on rocks and bumps, and the snake slides forward. On smooth glass there is nothing to push on, so the snake wiggles but barely gets anywhere!
In a thin tunnel a snake squeezes up, then stretches out. Big, heavy snakes creep in a straight line, like a caterpillar. On soft sand, some desert snakes go sideways and leave hook-shaped tracks.
More animals: how fish breathe, how horses sleep, what happened to the dinosaurs.
Snakes move by bending a long, flexible body with muscles that run down both sides, and by gripping the ground with wide belly scales.21 Depending on the ground, a snake uses one of four main moves: serpentine slithering in S-curves, concertina (bunch up and stretch), rectilinear (straight, like a caterpillar) and sidewinding (sideways loops over sand).15 Scientists have also found that the belly scales slide easily forward but resist sliding sideways, and that this is a big part of how slithering works.3
No living snake uses legs to move. Some boas and pythons still carry tiny leftover hip and thigh bones, but they play no part in getting around.1 Instead, a snake has three tools:
Put those together and you get a body that can bend into curves, fold up, stretch out and step its belly scales forward. What it does with that body depends on where it is.
| Move | What it looks like | Best for | What grips |
|---|---|---|---|
| Serpentine (lateral undulation) | S-shaped curves travel from head to tail | Grass, rocks, twigs, water: the most common move | Curves push on bumps; belly scales resist sideways slip |
| Concertina | Bunches up, then stretches out, like an accordion | Tunnels and climbing branches | Folded body presses on tunnel walls or grips a branch |
| Rectilinear (caterpillar) | Creeps in a straight line, no side-to-side bending | Large, heavy snakes such as boas; tight spaces | Patches of belly skin held still on the ground |
| Sidewinding | Lifted loops, moving sideways | Loose, hot desert sand | A few spots of body pressed into the sand |
Sources: Britannica and Jayne (2020).15
This is the way most snakes move. The body forms a series of S-shaped loops, and each loop pushes against whatever it can find: rocks, branches, twigs, pebbles or even dust and sand.1 When there's enough to push against, every point on the body follows more or less the same path, like the cars of a train on a track.5 Snakes swim the same way, by sending waves down the body like fish and eels. Sea snakes add a flattened, paddle-shaped tail.12
In a tunnel there's no room for wide curves. The snake anchors its back end, stretches its head and front end forward as far as it can, then anchors the front and pulls the back end up into tight folds.1 Then it repeats. The name comes from the concertina, a small squeeze-box instrument that opens and closes the same way.1 In a tunnel, the folds push sideways against the walls to anchor. On a branch, the snake grips inward instead.5 Snakes also use concertina to climb along branches and tree trunks, gripping with some loops while others reach ahead.1
Big, stout snakes like boa constrictors can creep forward in a perfectly straight line.106 Their bodies don't bend at all. Instead, muscles pull a patch of belly skin forward, the skin grips the ground, and then other muscles pull the skeleton forward over the gripping skin. Waves of this run down the body, which looks like a caterpillar crawling.16 Scientists think this straight-line crawl may have helped early snakes move cheaply through underground tunnels.6
Loose sand can give way under a push, so S-curves often slip. Some desert snakes, like the sidewinder rattlesnake, lift the front of the body, move it to the side, set it down, and pass that loop down to the tail. By the time one loop reaches the tail, the next has started at the head.1 The snake moves sideways and touches the sand at just a few spots, leaving separate, parallel tracks. Sidewinders make J-shaped tracks.17 It isn't rolling: the belly is always what touches the sand.1 Touching the sand less also keeps most of the body off the hot ground.17 Sidewinding also uses less energy than ordinary slithering.5
For a long time, scientists explained slithering as pushing sideways against rocks and branches. But snakes can slither across fairly flat ground too. In 2009, a team led by David Hu measured the friction of milk snakes' skin: they laid the snakes on a board and tilted it until the snakes slid.3 On a cloth surface, the scales slid most easily forward, a bit harder backward, and hardest sideways:
| Direction of sliding | Friction (on cloth) | Friction (smooth board) |
|---|---|---|
| Forward (head first) | 0.11 | about 0.14 |
| Backward (tail first) | 0.14 | about 0.14 |
| Sideways | 0.20 | about 0.16 |
Bigger number = more grip. Data: Hu et al. 2009, PNAS.3 The overlapping scales catch on tiny bumps in the ground, a bit like the teeth of a ratchet. That gives the snake a preferred direction to slide in, so the sideways swing of each curve turns into forward motion.3 On the smooth board the three numbers were almost the same, and a snake placed on a smooth surface failed to slither forward.3
The team also noticed that snakes lift the curved parts of their bodies slightly and press their weight onto the straighter parts. In their model, that made slithering nearly 50% more efficient.3
Glass has no bumps. S-curves have nothing to push against and belly scales have nothing to catch on. Snakes that only slither, placed on smooth glass, whip and thrash around without getting anywhere.1 That is exactly what happens in the "Your turn" toy above when you pick Glass and S-curves. (The speeds in the toy are made up to show which moves fit which ground. They aren't measurements.)
| Ground | Best move | Why |
|---|---|---|
| Grass, rocks, twigs | Serpentine | Lots of bumps to push against1 |
| Water | Serpentine (swimming) | Waves push on the water1 |
| Narrow tunnel | Concertina, or rectilinear | Folds anchor on the walls; straight crawl needs no room56 |
| Tree branch | Concertina | Grip, reach, grip again15 |
| Loose desert sand | Sidewinding | Few contact points, little slipping, less hot sand1 |
Sandy slopes are hard. Push too hard and the sand avalanches under you. In 2014, Hamid Marvi and colleagues studied sidewinder rattlesnakes in the lab on a tilting bed of sand. As the slope got steeper, the snakes laid a longer stretch of their body on the sand.4 When the team gave a snake robot the same trick, it could climb sandy slopes close to the steepest angle sand can hold before sliding.4 In the toy for that chapter, the trend is real but the percentages are made up for illustration.
The four moves are the classic list, but scientists now think it's too simple. Snakes swim differently from how they slither on land, cross gaps between branches in their own way, and climb poles differently from how they push through tunnels. One 2020 review proposed counting at least 11 distinct gaits.5 Engineers study all of them to build snake robots that can squeeze through tight spaces.54
There are thousands of known species of snakes. Only a small share, fewer than 300 species, are venomous. In the United States, fewer than about 10 people a year die from snakebites, but around the world snakebites are a serious danger, so never touch a wild snake.2 In Great Smoky Mountains National Park, for example, only 2 of the 23 snake species are venomous, and both are generally shy and avoid people.8 If you spot a snake, watch it from a distance (binoculars are great for this) and never try to pick up a wild snake.8 From a safe distance you might see which of the four moves it uses.
Curious about other animals? See how octopuses change color, how fish breathe underwater, how horses sleep, or how the dinosaurs died.
Snakes use muscles along both sides of a very long, flexible backbone (with hundreds of vertebrae, most with a pair of ribs) to bend their bodies, and wide belly scales to grip the ground. Depending on the surface they slither in S-curves, bunch and stretch (concertina), creep in a straight line (rectilinear) or sidewind.
Both. Most of the time snakes slither in S-shaped curves (lateral undulation). Big, heavy snakes such as boas can also crawl in a straight line like a caterpillar by moving their belly skin, which is called rectilinear locomotion.
Serpentine motion (lateral undulation), concertina, rectilinear (caterpillar) and sidewinding. Scientists now split these into at least 11 distinct gaits, because snakes move differently in water, in trees and in tunnels.
On a smooth surface there are no bumps for the S-curves to push against and nothing for the belly scales to catch on. Many snakes trying to slither on very smooth glass wriggle without getting far.
No. Snakes don't walk on their ribs. In rectilinear crawling, muscles that run from the ribs to the belly skin pull the skin forward, then pull the body forward over the gripping belly scales.
Loose sand gives way when you push on it. By lifting loops of its body and setting them down to one side, a sidewinder keeps a few short stretches of its body planted while lifting the rest, so it slips less (on steeper sand it puts more of its body down) and keeps most of its body off the hot sand.
No. Snakes are covered in dry scales. The scales help keep moisture in and reduce friction as the snake moves.