Snakes: Making waves
These adaptations and loss of structures suggest that the snake's ancestors had previously adopted a burrowing existence, and their surface dwelling is secondary. The loss of legs for locomotion on the surface has been overcome with the development of flank muscles that flex in alternate bands so that their body is drawn up in a series of S-shaped curves. As the contractions travel in waves down the body the flanks are pressed against obstacles on the ground such as stones and the snake is able to push itself forward. When snakes hunt they are able to creep up on their prey without oscillating its body. The scales on the underside are shaped like narrow rectangles running across the width of the body and overlapping one another with their free edges to the rear. The snake is able to hitch these scales up and forward in groups by contracting its belly muscles. The back edges catch the ground and as the contractions pass downwards in waves, the snake advances smoothly and silently with no lateral movement.
Crude animated line-drawing showing the locomotor pattern of sidewinding. The light brown areas are the tracks left behind, and also indicate where the body of the snake touched the ground.
