Rectilinear Locomotion in a Snake (Boa occidentalis)

1. What they did

Snakes move in four main ways: serpentine (side-to-side waves), concertina, side-winding, and a less common “rectilinear” movement, in which the whole snake creeps forward in a straight line without bending sideways. Earlier authors disagreed about how rectilinear movement works: some compared it to a snail or a caterpillar, and one old idea held that snakes “walk” on their ribs.

Lissmann studied two young Boa occidentalis, 57 and 60 cm long, in the laboratory:

• Filming. The snakes crept over a glass plate with a tilted mirror underneath, so the side and the belly could be filmed at the same time. Natural markings, scales and white paint marks were used to follow individual points on the body.
• Measuring forces. A snake crept from a fixed glass tube into a second tube hung on steel springs. Levers magnified the movement of the springs, which was photographed to show the pulling and pushing forces the snake made.
• X-rays. Small lead disks were stuck on the side of the belly and along the back, and a series of X-ray photographs showed whether the ribs moved relative to the backbone.
• Mechanical reasoning. From these results and the layout of the muscles, he worked out how the belly skin muscles and the two sets of muscles linking the ribs to the belly skin might act through each cycle.

2. What they found

• The boas used rectilinear movement slowly, often when approaching live prey. It could also be triggered by putting the snake on a smooth surface.
• Waves of muscle contraction and relaxation ran backwards along the belly, from head towards tail. Usually two complete waves were on the body at once. Sometimes a wave faded out before reaching the tail, as in earthworms.
• Each point on the belly moved forward in “steps” of 2 to 3 cm. While a stretch of belly was fully contracted, it stayed still on the ground (and on a very smooth surface could slip backwards). It then slid forward, speeding up as it relaxed and slowing as the next contraction came.
• The back, and with it the backbone, moved forward at a fairly steady speed. The head also moved at a steady speed.
• The force recordings matched those of an earthworm: a pull while the anchored stretch of the body was in the movable tube, changing to a push once that stretch had passed into the fixed tube.
• The X-rays showed the tips of the ribs kept the same position relative to the backbone. The skin slid past the rib tips instead: the same stretch of skin covered six more ribs when stretched than when contracted.
• In the mechanical model, the belly skin muscle is the main power source. The muscles linking ribs to belly skin probably lengthen and shorten in a regular rhythm, passing force between the belly and the backbone. The muscles running forward from the ribs to the skin and those running backward probably work against each other and take turns.

3. What they concluded

• Rectilinear movement in this boa is basically like an earthworm’s. The terms “snail principle” and “caterpillar principle” used by other authors are not appropriate.
• The ribs do not “walk”; this agrees with three earlier authors and contradicts the rib-walking theory.
• The backbone and everything firmly attached to it can be pictured as a cylinder gliding at steady speed inside a tube of skin and belly muscle, which moves forward at varying speeds and grips the ground in patches.
• The author says it is hard to prove with these methods exactly which muscles supply the forward force.
• Rectilinear movement is probably a unique kind of vertebrate movement, because it does not use the skeleton as levers.

6. Other technologies, skills or innovations

• Simultaneous filming of the side and belly through a glass plate and tilted mirror.
• A pair of glass tubes, one fixed and one hung on springs, with magnifying levers to record pulling and pushing forces.
• Lead disks on the skin and along the back, tracked on a series of X-ray photographs.