Meeting Abstract
Terrestrial animals locomote over, manipulate, and make ingress into a variety of natural substrates. Movement in flowable substrates like sand and mud that exhibit both solid and fluid-like behavior is particularly complex. Burrowing into such substrates requires the animal to overcome potentially large material stresses and contend with time-varying substrate properties. To develop principles for effective burrowing, we studied the behavior of the Pacific sand crab, Emerita analoga, a versatile marine invertebrate capable of movement in and on complex terrestrial substrates. We found that the sand crabs used a stereotyped sequence of behavioral primitives to burrow into both saturated and dry substrates with average particle sizes ranging from 0.1 mm to 2 mm. We discovered that as volume fraction of the substrate increased, the average time to burrow increased from 3.7±0.7 s to 5.2±1.0 s. When initiating burrowing, the crabs varied their direction of ingress by altering body pitch angle. Across particle sizes, the dynamics of burrowing were substrate dependent, with pitch at penetration decreasing from 35.6±4.9° to 27.7±5.3° with increasing volume fraction. Drag experiments using constant speed intrusion of small plates showed an increase in penetration force with volume fraction for the substrates tested. We propose a terradynamic model for burrowing behavior based on dynamic substrate response to localized shear and compression. We hypothesize that the crabs modulate body dynamics to exploit low penetration resistance in looser substrates, and reduce shear in substrates near the onset of dilatancy, suggesting effective motion strategies for burrowing in animals and robots.
