The key to snakes' agility -- hundreds of vertebrae and ribs -- is closely related to the key to their locomotion: ventral scales. These specialized rectangular scales line the underside of a snake, corresponding directly with the number of ribs. The bottom edges of the ventral scales function like the tread on a tire, gripping the surface and propelling the snake forward.
Snakes have four basic methods of movement:
- Serpentine - This S-shape movement, also known as undulatory locomotion, is used by most snakes on land and in water. Starting at the neck, a snake contracts its muscles, thrusting its body from side to side, creating a series of curves. In water, this motion easily propels a snake forward because each contraction pushes against the water. On land, a snake usually finds resistance points in the surface -- such as rocks, branches or dents -- and uses its scales to push on the points all at once, thrusting the snake forward. Speckled rattlesnake using rocks as resistance points
- Sidewinding - In environments with few resistance points, snakes may use a variation of serpentine motion to get around. Contracting their muscles and flinging their bodies, sidewinders create an S-shape that only has two points of contact with the ground; when they push off, they move laterally. Much of a sidewinding snake's body is off the ground while it moves.
- Caterpillar - A much slower method of movement is caterpillar or rectilinear locomotion. This technique also contracts the body into curves, but these waves are much smaller and curve up and down rather than side to side. When a snake uses caterpillar movement, the tops of each curve are lifted above the ground as the ventral scales on the bottoms push against the ground, creating a rippling effect similar to how a caterpillar looks when it walks.
- Concertina - The previous methods work well for horizontal surfaces, but snakes climb using the concertina technique. The snake extends its head and the front of its body along the vertical surface and then finds a place to grip with its ventral scales. To get a good hold, it bunches up the middle of its body into tight curves that grip the surface while it pulls its back end up; it then springs forward again to find a new place to grip with its scales.