Mechanics of undulatory swimming on the surface of granular matter.

Elongate, limbless animals from the microscopic C. elegans to eels and snakes use flexural waves of the body to move. The swimming of such organisms immersed in homogeneous fluids is well-studied, but little is known about movement on deformable terrestrial materials. We used as a model system the sand-specialist snake C. occipitalis (~40 cm and 20 g) slithering quickly (body segment speeds of 30-100 cm/s) on the surface of homogeneous granular matter (GM). Surface drag measurements revealed that the ratio of thrust to drag forces, a critical component determining animal performance, was largely independent of drag distance, speed, or depth over an order of magnitude. As a result, resistive force theory (RFT) accurately predicted snake performance without accounting for the observed interface complexities like hysteresis. RFT revealed that the observed stereotyped waveform of the snake conferred maximum speed given a limit on peak muscle power. Our study suggests that surface sand slithering is analogous to low-Re swimming in a frictional fluid even at the highest observed speeds. Therefore, terrestrial “swimmers” may not need to contend with changing material dynamics as they increase speed.