Neuromechanical Control and Passive Interaction Leads to Collisional Diffraction of a Sand Snake

Snakes coordinate the interaction of their flexible trunks with heterogeneity to generate propulsion. We hypothesized the snake C. occipitalis controls for its stereotyped travelling wave of body bending, specialized for movement on the sand substrate of their desert habitat [Schiebel et al SICB 2016], by targeting a pattern of muscle activation. We found C. occipitalis traversing a model multi-component terrain—a spatially uniform sand-mimic substrate and a single row of rigid posts perpendicular to the snakes’ initial trajectory—was re-oriented by the array into preferred directions of either ~0° (continuing straight) or ±21±9°. Since the shape changes of the snake were small during interaction, we posited that the motor program was largely preserved upon contact. We developed a model with a similar travelling wave. Following insights by Astley et al [SICB 2016] based on muscle activation patterns measured by Jayne [1988], we assumed external forces on the body were resisted only where active muscles would be lengthened. Thus posts were accommodated by changing curvature so that active muscles would shorten further, mimicking shape changes due to external forces. The model captured the collisional diffraction pattern, suggesting scattering is a result of passive dynamics.