KORFF, Wyatt L.; GOLDMAN, Daniel I.; University of California, Berkeley; University of California, Berkeley: Escape at the solid/fluid interface: Impulsive accelerations of lizards on sand.

When an animal accelerates from rest it exerts a force against the physical world; animals that live in fluids like air or water push against a medium that can move and deform dynamically. Studies of terrestrial locomotion assume that the foot-substrate interaction is a no-slip interaction. For some terrestrial animals, their propulsive elements can slip or even deform the natural environment, an example of this is lizards living on sand. Sand, like other granular materials, displays physical properties of both a solid and a fluid depending on the initial conditions of the substrate and how an animal pushes against it. Because little is known about how granular materials respond to objects, like a lizard foot, impulsively accelerating through them, we measured drag forces on a flat plate with dimensions comparable to that of a lizard foot. We controlled the position of the plate as a function of time, x(t), and measured forces on an attached rod instrumented with strain gages. As acceleration increased, peak drag force decreased. We propose that this is because the medium acts more like a fluid as plate accelerations increased. This suggests that because sand fluidizes for sufficiently large perturbations, for effective transfer of momentum between an animal and this substrate, lizards should push off in this intermediate realm or risk even greater foot slippage. Using these insightst from impulsive accelerations, we investigated how two species of sand-dwelling lizards (Callisaurus draconoides and Uma scoparia) accelerate from rest on sand, an important behavior for capturing prey or escaping from predators. To control the initial condition of the substrate, we use a fluidized bed, a collection of grains forced by a flow of air opposite to the direction of gravity.