Meeting Abstract

97.2  Sunday, Jan. 6  Burrowing biomechanics of the ghost crab. SPRINGTHORPE, D.*; GRAVISH, N.; MAZOUCHOVA, N.; GOLDMAN, D.I.; FULL, R.J.; Univ. of California, Berkeley; Georgia Institute of Technology; Temple University; Georgia Institute of Technology; Univ. of California, Berkeley dspringthorpe@berkeley.edu

Burrowing encompasses a wide range of behaviors, including substrate liquefaction, crack propagation, and lateral-undulatory ‘swimming’ and variants of digging, where animals manipulate the substrate with teeth, limbs or head. Digging to construct permanent or semi-permanent burrows can involve a combination of specialized postures, locomotion in confined environments, and goal-directed control of the substrate. Here we present the first description of such a behavioral suite for free excavation in damp sand (gravimetric water content: 0.16) by the ghost crab, Ocypode quadrata. Observations enabled by a novel method of x-ray imaging with detailed leg and body markers showed that crabs excavated circular burrows using a hook-and-pull motion at average rate of 0.65 cm/min, corresponding to an average mass transport rate of 10 g/min. During excavation, crabs employed a particular posture to anchor themselves within the burrow by pressing against burrow walls with their chelae and the rear of the cephalothorax. Crabs rotated up to 180° within the sagittal plane while excavating. After the substrate was collected, crabs manipulated and transported the sand with both the chelae and walking legs. Sand packets, to be transported to the burrow entrance or compacted within the burrow, were carried by these limbs or passed under the body. Results not only quantify the biomechanics of excavation, but also reveal new insights relevant to the field of mobile manipulation. Further experiments, using the techniques we have developed will likely lead to a new generation of bio-inspired robots capable of excavation and subterranean, confined space locomotion.