Watch Your Step

Sidesteppin'. As it dashes across the water, the basilisk lizard pushes side to side with its feet to keep from toppling.

When sprinting across water, the basilisk lizard slaps its large hind feet through the surface and then strokes the water back, generating an upward force that keeps the animal afloat. Now, new measurements show that the basilisks also generate large sideways forces that keep them from tripping over the liquid.

To keep from sinking, a basilisk must work hard. In 1996, biologists James Glasheen of the University of California, Berkeley, and Thomas McMahon of Harvard University in Cambridge, Massachusetts, videotaped basilisks and measured the forces on a model basilisk foot dropped into water to figure out the relative contributions of the slapping and paddling that keep the lizard's head above water.

But a basilisk must also remain steady, and new measurements show that the lizard pushes nearly as hard side to side to avoid tripping. Biologists Tonia Hsieh and George Lauder, also of Harvard, peppered the water their lizards ran on with reflective beads. They shined laser light off the beads to track the motion of the water and determine the forces generated by the lizard's foot.

Because the basilisk's foot enters the water at an angle, the lizard generates almost as much forward thrust during the downward slap as it does during the backward stroke, the researchers report online 15 November in the Proceedings of the National Academy of Sciences. Moreover, the basilisk generates sideways forces nearly equal to its body weight. Because the water gives way, the lizard naturally tends to sway toward its weight-bearing foot, and if it lists too far, it will topple just like a person running in soft sand. The sideways pushes allow the lizard to continually catch itself, Hsieh says.

The new work "certainly adds a lot," says Glasheen, now a venture capitalist with Technology Partners in Palo Alto, California. He notes, however, that Hsieh and Lauder analyzed the physics in a slightly different way that makes it difficult to compare the two studies: "I would have loved if they challenged our data directly."

Related sites
George Lauder's group page
Basilisk fact sheet