Systematic Study of Limb-body Coordination during Sandfish Burial

Paez, V. Margot

Goldman, Daniel I.

School of Physics Georgia Institute of Technology Atlanta, GA

mpz@gatech.edu

Sarah Sharpe

Exponent, Inc.

23445 North 19th Ave

Phoenix, AZ

The ~10 cm long sandfish lizard (Scincus sp.) can bury into dry granular media within a half-second. Unlike its subsurface sand-swimming behavior, in which propulsion is generated by a head-to-tail traveling-wave of body bending, sand-diving uses both undulatory body motion (typically 1-2 cycles) and a stereotyped pattern of limb use (and disuse). In previous work (Maladen et al, ICRA 2011), a limbless sandfish robot could bury 75% of its body within 5 undulation cycles. To gain insight into the importance of limb use, we performed biological and robophysical experiments. In animal studies, when all limbs were bound (taped to the body), the animal could not effectively bury; hind limb binding increased burial (~4 cycles) but not effectiveness, whereas forelimb binding decreased burial probability (40% success rate using ~6 cycles). We also found from animal studies that body angles increased directly, and limb angles decreased inversely proportional to total body submerged during burial. To systematically study the morphology, timing and coordination of the sandfish’s limbs and body undulations, we constructed a new sandfish robot with five position-controlled servo motors which generate a travelling body wave and mounted to the body four rotational servo motors with 3D printed limbs. With body and limb angles constant, we found body-limb coordination was necessary for preventing limb-limb and limb-body collisions. Prior to coordination, the robot was unable to bury. With coordination, burial increased but was below the 75% burial seen in the limbless robot. This suggests that timing of limb usage and varying of limb angle from the long body axis, in combination with limb-body coordination, may play a role in sandfish burial.