Limbless locomotion in complex terrestrial terrain

Abstract

Limbless locomotion is used by animals ranging from micro to macroscopic to move in a wide variety of terrains. Snakes in particular use their elongate bodies to swim in water, climb trees, burrow into sand and soil, crawl across terrestrial terrain from jungle to desert, and glide through the air. The versatility of limbless locomotion makes it suitable for situations that challenge legged or wheeled platforms, and for this reason snake-like robots are an attractive solution for engineering problems like movement in the confined, unstable rubble of a collapsed building. We studied the motion of live snakes as they moved through laboratory models of terrestrial terrain to search for principles governing both the relationship between waveform and performance as mediated by the terrain physics and the neuromechanical strategies for coordinating a many degrees-of-freedom body in complex surroundings. We found the desert-dwelling snake, Chionactis occipitalis, in homogeneous granular matter used a stereotyped waveform to move quickly across the surface. Resistive force theory calculation of sand-swimming performance revealed the self-deformation pattern of the snakes conferred maximum speed given a constraint on peak power. We explored the neuromechanics of these snakes by observing kinematics as they traversed a single row of posts. The interaction resulted in a mechanical diffraction pattern, reminiscent of the diffraction of subatomic particles. Comparison with a geometric model revealed this phenomenon was explained by an unaltered motor program supplemented by passive dynamics. We modeled multi-component terrestrial terrain as arrays of rigid posts of different post-to-post spacing affixed to a low-friction substrate and embedded in granular matter. The performance of the desert snake was a function of both substrate and lattice spacing, and comparison to a generalist snake species (Pantherophis guttatus) suggested that the ability to modulate the waveform in response to the environment was important in variable terrain.