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Snakes
Snakes move in a very unique way. To the casual observer, it seems almost magical than an animal with no legs can move so quickly and gracefully. Snakes travel best on surfaces with obstructions and some roughness—this gives their bellies something to grip. As you might deduce, snakes do not do very well on slippery surfaces.
If you put a snake on loose sand and watch it crawl, you'll notice its unique movement. Every part of its belly touches the ground, and it flows along in a series of S-curves. On the back of each curve, you will notice that the sand has been pushed up. The body pivots and pushes sidewise against these piles and is propelled forward. Snakes swim in water with the same motion.
Snakes use several different modes of locomotion to get around. The method used depends on several factors, such as the size of the snake, the roughness of the surface, and the speed of travel. We'll take a look at these different methods next.
Serpentine Locomotion
Serpentine locomotion uses the classic S curve and is the most common method of travel used by snakes. In lateral undulation, waves of sideways bending are propagated along the body from head to tail. The snake's muscles are activated sequentially along the body, relaxing and contracting to form an S shape, as you can see in Figure 3. As the snake progresses, each point along its body follows along the path established by the head and neck, like the cars of a train following the engine as it moves along the track.
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Figure 3 Serpentine locomotion is the most common method of travel used by snakes. Each point of the body follows along the S-shaped path established by the head and neck, much like the cars of a train following the track.
Sidewinding
Many snakes use sidewinding to crawl on smooth or slippery surfaces. This is similar to lateral undulation in the pattern of bending, but it differs in a few ways. First, the snake's body doesn't slide along the ground; as you can see in Figure 4, the snake lifts part of its body while firmly setting down other parts. This allows the snake to get a better grip.
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Next, the parts of the body that are not firmly planted on the ground lift up from the ground, causing the body to roll along the ground from neck to tail, forming a characteristic track.
Finally, because the snake repeatedly lifts parts of the body, it moves diagonally relative to the tracks that it forms on the ground (see Figure 5).
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When you're animating this type of motion, the distance that the snake lifts its body off the ground is usually measured in fractions of an inch, which are practically negligible from the audience's viewpoint. For added effect, you can exaggerate this lift.
Figure 4 In sidewinding, the snake actually lifts parts of its body and sets them down again.
Figure 5 Sidewinding causes the snake to move diagonally relative to the S shape.
Concertina Locomotion
In concertina locomotion, the snake alternately bends its body like an accordion and then lifts and straightens itself out to move forward (see Figure 6). The front part of the body then comes to rest on the surface, and the back part of the body is lifted and pulled up into the accordion shape again. Concertina locomotion is used mostly in crawling through tunnels or narrow passages and in climbing.
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Figure 6 In concertina locomotion, the snake bends its body like an accordion and then lifts and straightens itself out to move forward.
Rectilinear Locomotion
Rectilinear locomotion lets the snake move straight ahead with its body stretched out or perhaps on a wide arc. This type of motion is used primarily by large snakes such as boas and pythons. In rectilinear locomotion, the action is somewhat like rippling the belly: The scales on the snake's belly scales are pulled forward and lifted off the ground, and then set down and pushed backward (see Figure 7). Because the scales are aligned much like a ratchet, this pulls the snake forward.
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Figure 7 Rectilinear locomotion lets the snake move straight ahead with its body stretched out.
Other Motions
Aside from the motions that a snakes uses to travel forward, snakes can perform many other motions. These including coiling up and striking, climbing trees, and swimming. In all these motions, the snake uses its entire body to move and position itself.