Undulatory Swimming in Sand: Subsurface Locomotion of the Sandfish Lizard
Ryan D. Maladen,1
Yang Ding,2
Chen Li,2
Daniel I. Goldman1,2,*
The desert-dwelling sandfish (
Scincus scincus) moves within
dry sand, a material that displays solid and fluidlike behavior.
High-speed x-ray imaging shows that below the surface, the lizard
no longer uses limbs for propulsion but generates thrust to
overcome drag by propagating an undulatory traveling wave down
the body. Although viscous hydrodynamics can predict swimming
speed in fluids such as water, an equivalent theory for granular
drag is not available. To predict sandfish swimming speed, we
developed an empirical model by measuring granular drag force
on a small cylinder oriented at different angles relative to
the displacement direction and summing these forces over the
animal movement profile. The agreement between model and experiment
implies that the noninertial swimming occurs in a frictional
fluid.
2 School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.
* To whom correspondence should be addressed. E-mail: daniel.goldman{at}physics.gatech.edu
