S1-3.6 Jan. 5 The Mechanism of Rapid Running in Weak Sand GOLDMAN, D.I.*; KORFF , W.L.; WEHNER, M.; BERNS, M.S.; FULL, R.J.; Univ. of California, Berkeley; Caltech; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley firstname.lastname@example.org
Locomotion on granular media is unlike locomotion on rigid ground because during a step the material begins as a solid, becomes a fluid and then re-solidifies. The fluidization and solidification depend on the strength of the material and can affect limb penetration depth, time to re-solidify and propulsive force. Legged animals could use a drag mechanism to paddle feet through sand or could rely on mechanisms by which appropriate limb trajectory and toe use rapidly solidify the material. To study how the fluidizing properties affect speed in rapidly running lizards and crabs, we used a fluidized a bed of of 250 μm glass spheres (30x16 cm2 area by 8 cm deep). A constant flow rate Q of air through the bed below the onset of fluidization set the material strength. We measured the average speed, foot impact depth, and time of foot contact as a function of material strength for a desert dwelling generalist lizard Callisaurus draconoides, the sand-specialist Mojave fringe-toed lizard Uma scoparia, an arboreal gecko Pachydactylus bibroni, a generalist lizard Sceloporus olivaceus and the fastest land invertebrate, the ghost crab Ocypode quadrata. Crabs and geckos decreased speed by three-fold as foot impact depth increased by a factor of two. Surprisingly, the performance of S. olivaceus was comparable to that of the sand adapted U. scoparia suffering only a 10% decrease in speed (always > 1 m/sec) as Q increased to fluidization. C. draconoides maintained high speed (>1.5 m/sec) even on completely fluidized sand. Terrestrial lizards used long toes to solidify the material rapidly relative to stance duration resulting in stance duration being controlled by the animal, not by the substrate’s stopping strength.