Biomechanics. Crab's downfall reveals a hole in biomechanics studies.

325 NEWSFOCUS PHOENIX, ARIZONA—When it comes to running on sand, the ghost crab is an Olympic champion. With legs that are a blur to the naked eye, Ocypode quadrata scoots up to 2 meters per second on hard-packed sand. But soften up the sand a bit, and the gold medal instead goes to the zebra-tailed lizard, an animal that spends little time on the grainy material. This surprising observation, reported earlier this month here at the annual meeting of the Society for Integrative and Comparative Biology, comes courtesy of physicist Daniel Goldman of the Georgia Institute of Technology in Atlanta. Goldman has jumped into the field of bio-mechanics by employing a device physicists have long used to examine granular materials. That's allowed him to study how animals move over different kinds of surfaces, an approach that Goldman and others feel has been neglected to a large extent. " It's nice to see practical and theoretical applications of granular physics applied to an organismal bio-mechanics problem, " says Andrew Biewener, a biomechanicist at Harvard University. " It creates an entirely new field of investigation, " which will advance both basic biology and robot engineering. Until now, most researchers have studied how animals walk, run, trot, and otherwise move using hard, nonskid platforms. " When we studied forces, the last thing we wanted was to have slippery surfaces, " says Cather-ine Loudon, a biomechanicist at the University of California, Irvine. And this approach has proved useful, as researchers have made progress analyzing how muscles and tendons make different gaits possible (Science, 21 January 2005, p. 346). But in the wild, organisms must contend with mud, gravel, and ground littered with debris. Sand can be particularly challenging, as its grains give way briefly underfoot, transforming the surface from a solid to a virtual liquid. Goldman wants to understand how organisms deal with this complexity. " We can't predict how animals will move until we understand the substrate, " he says. Goldman and Wyatt Korff, now at the Cali-fornia Institute of Technology in Pasadena, built a " fluidized " bed, a box of glass beads that were stand-ins for sand. The bed's underside has a porous membrane, and by pumping air at different speeds up through the membrane , Goldman can change how tightly packed the beads are, thereby controlling the properties of the " sand. " More air results in looser packing and, …