High speed x-ray observation of a sand swimming lizard

We use high-speed x-ray imaging to reveal how a small (10~cm) desert dwelling lizard, the sandfish ({\em Scincus scincus}), swims within a granular medium, and how its locomotion is affected by the volume fraction $\phi$ of the media~\footnote{Maladen et. al, Science, {\bf 325}, 314, 2009}. We use an air fluidized bed to prepare 0.3~mm glass beads (similar in size to desert sand) into naturally occurring loose ($\phi=0.58$) and close ($\phi=0.62$) packed states. On the surface, the lizard uses a standard diagonal gait, but once below the surface, the organism no longer uses limbs for propulsion. Instead it propagates a large amplitude single period sinusoidal traveling wave down its body and tail to propel itself at speeds up to $\approx 1$ body-length/sec. For fixed $\phi$ the animal increases forward swimming speed $v_f$ by increasing temporal frequency $f$. For fixed $f$, $v_f$ is independent of $\phi$, despite resistance forces that nearly double from loose to close packed states. Surprisingly, the greatest sandfish velocity (and $f$) occur in the close packed state.