Mitigating memory effects during undulatory locomotion on hysteretic materials.

Undulatory swimming in flowing media like water is well-studied, but little is known about loco-motion in environments that are permanently deformed by body-substrate interactions like snakes in sand, eels in mud, and nematode worms in rotting fruit. We study the desert-specialist snake Chion-actis occipitalis traversing granular matter and find body inertia is negligible despite rapid transit and speed dependent granular reaction forces. New surface resistive force theory (RFT) calculation reveals how this snakes wave shape minimizes memory effects and optimizes escape performance given physiological limitations (power). RFT explains the morphology and waveform dependent performance of a diversity of non-sand-specialist, but overpredicts the capability of snakes with high slip. Robophysical experiments recapitulate aspects of these failure-prone snakes and elucidate how reencountering previously remodeled material hinders performance. This study reveals how memory effects stymied the locomotion of a diversity of snakes in our previous studies [Marvi et al, Science, 2014] and suggests the existence of a predictive model for history-dependent granular physics.