Meeting Abstract
We typically consider arthropod pincher-like appendages or chelae as structures to manipulate objects such as food, predators, competitors and barriers to movement. We discovered a behavior in the ghost crab, Ocypode quadrata, where crabs required chelae to manipulate their body over tall, vertical obstacles. Ghost crabs hurdled narrow, vertical walls up to 16 cm high, approximately 8x hip height and greater than their sideways leg span, using both their legs and chelae. To explain this exceptional hurdling behavior, we presented ghost crabs with obstacles ranging from 2 -16 cm high. At low heights, crabs simply raised their hip height and stepped over the obstacle. For obstacles greater than 6cm, however, ghost crabs adopted a different strategy where chelae became critical to the crabs’ capability. We tested the ability of crabs to manipulate their body using their chelae by immobilizing the chelae and then challenging crabs to hurdle a 10 cm obstacle. Immobilization reduced the success rate to zero compared to unconstrained controls which all scaled the hurdle. Quasi-static models of mobile manipulation during the hurdling behavior suggest that chelae are critical because they provide the torque necessary to manipulate the animals’ body over the complex, three dimensional terrain they encounter on the upper beach and in the supratidal zone. Ghost crab hurdling using pincer-like claws provides biological inspiration for the design of legged robots where graspers could not only be used to move objects, but manipulate the body itself for enhanced mobility.
