Not having arms and legs doesn’t stop snakes from moving. They have developed several different ways of moving to deal with particular environments. Each type of snake movement is discrete and distinct from the others.
Lateral undulation is a snake’s only way of moving in water, and the most common way of moving altogether. In this mode, the body of the snake alternately bends to the left and right, resulting in a series of rearward-moving “waves”. While this movement appears rapid, snakes have rarely been seen moving faster than two body-lengths per second, often much less.
This mode of movement has the same amount of calories burned per meter moved as running in lizards of the same mass.
Terrestrial lateral undulation is the most common mode of moving for most snake species. In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc.
Each of these environmental objects, in turn, generates a reaction force directed forward and towards the midline of the snake, resulting in forward thrust while the lateral components cancel out.
The speed of this movement depends upon the density of push-points in the environment, with a medium density of about 8 along the snake’s length being ideal.
The wave speed is precisely the same as the snake speed, and as a result, every point on the snake’s body follows the path of the point ahead of it, allowing snakes to move through very dense vegetation and small openings.
Snakes move forward in water by moving their bodies in a wave-like motion. The waves become larger as they move down the snake’s body, and the wave travels backwards faster than the snake moves forwards.
Thrust is got by pushing their body against the water: this results in the observed slip. In spite of overall similarities, studies show that the pattern of muscle activation is different in aquatic versus terrestrial lateral undulation, which justifies calling them separate modes.
All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves).
This is most often used by colubroid snakes (colubrids, elapids, and vipers). They use it when the environment lacks anything firm to push against, such as a slick mud flat, or a sand dune. Sidewinding is a modified form of lateral undulation in which all of the body segments oriented in one direction remain in contact with the ground, while the other segments are lifted up. This results in a peculiar “rolling” motion. This mode of moving overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping. The static nature of the contact points can be shown from the tracks of a sidewinding snake, which show each belly scale imprint, without any smearing. This mode of moving has very low caloric cost, less than ⅓ of the cost for a lizard or normal snake to move the same distance.
The slowest mode of snake moving is rectilinear moving, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning. In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin.
The ribs of the snake do not move in this mode of moving and this method is most often used by large pythons, boas, and vipers when stalking prey across open ground as the snake’s movements are subtle and harder to detect by their prey in this manner.