The Mechanism of Locomotion in Snakes

1. What they did

Four main ways of moving over land had been described in snakes: serpentine (gliding in S-shaped curves), concertina, side-winding (also called crotaline), and rectilinear (moving forward in a straight line, as boas do). Gray set out to explain the mechanics: how the muscles along a snake’s body turn into forward movement.

All the observations were made on the common grass snake. Gray changed the surface and surroundings the snake moved over and photographed the results against a grid, at intervals of a fraction of a second:

• Pegs. A smooth metal plate with smooth, upright glass pegs. White spots painted on the snake’s back showed how each part of the body moved.
• Channels. Straight glass tubes, and channels that were straight, circular, or curved in an S-shape.
• Smooth and rough surfaces. A painted metal plate, sand with or without partly buried pebbles or glass beads, and sandpaper.
• Towing. He compared the force needed to pull an inert grass snake backwards with the force needed to pull it forwards.

He also filmed a slow-worm in a curved channel. He treated the snake’s backbone as a chain of rigid rods hinged together, with the muscles as elastic elements between them, and worked out the forces with diagrams and two mathematical appendices.

2. What they found

Serpentine movement

• The grass snake glided in S-shaped curves through grass and over any surface with enough projections to push against: pegs, or sand with pebbles. Every part of the body followed the path of the head and neck.
• Serpentine gliding always depended on something for the body to push against sideways. The pushing comes from the side muscles of the body, and friction with the ground only slows the snake down.
• Three conditions are needed: (1) the body must be in curves whose bend increases towards the head; (2) the muscles must contract on the side towards which the curve is increasing; (3) at least three points of resistance must press against the sides of the body. The forward force is the combined push from all of these resistances.
• A snake cannot glide this way along a straight or circular path, because the muscles cannot shorten there. In a channel shaped in S-curves, the snake glided easily.
• Long, shallow curves give fast progress but produce little forward push, so they need more muscle effort.

Concertina movement

• In a straight or circular channel, the snake switched to concertina movement. It bunched its body into short, tight curves pressed against the walls. These held the snake in place while other parts of the body were pushed or pulled forward. Each part moved forward in distinct steps, then came to rest.
• The narrower the tube, the more curves the snake made, and the shorter they were.
• On fairly smooth open ground the snake sometimes used a modified concertina movement. Pulling an inert grass snake backwards took four or five times more force than pulling it forwards, because the back edges of the belly scales catch on the ground. This let resting parts of the body serve as anchors.

Side-winding

• On a smooth, even surface (such as a painted metal plate) the grass snake’s movement became irregular. Serpentine movement gave way to side-winding, concertina movement, or a mix of the two.
• During side-winding, two sections of the body lay still on the ground while the rest moved, and the whole snake travelled sideways. A side-winding snake leaves a series of more or less parallel straight tracks, set at an angle to the direction the head is moving. Each part of the body is laid down on a track line, rests there, then is lifted and moved to the next line. Each track line is as long as the snake. The lines are parallel throughout only if the curves of the body are the same all along it.
• Side-winding works like the caterpillar track of a tank or tractor. It depends on the snake shifting its weight onto the resting sections, which increases their grip and reduces the drag of the moving sections.
• The grass snake could not side-wind over sandpaper, which shows that some moving sections must still touch the ground in this species.

Rectilinear movement

Rectilinear movement depends on active movements of the ribs and belly scales. Gray never saw it in the grass snake, and none of the three other types depended on active rib or scale movement.

3. What they concluded

• Three of the four main types of snake movement (serpentine, concertina and side-winding) can be produced in the same snake simply by changing its surroundings.
• The muscle cycle during side-winding is essentially the same as during serpentine gliding. What changes is the kind of resistance the ground offers.
• Gray disagreed with Mosauer, an earlier researcher, who had said the ground gives no push to a true side-winder. That could only hold in theory: a snake could not start from rest without grip between its resting sections and the ground.
• A gliding snake and an eel moving through grass use the same mechanism. The eel’s smooth surface and mucus keep friction to a minimum, which suits it ideally to this kind of movement.
• How much work each part of the body does would need experiments, to be described in another paper. Comparing body shapes and speeds across snake genera would be of great interest.