A walk on the wild side.

In the wilds of Central America, laced with rivers and streams, the agile, whip-shaped basilisk shows why it has earned the nickname Jesus Christ lizard.

It runs on water.

Upright on two legs, the 90-gram lizard can skip briskly on fringed toes across the surface of a river. Until now, scientists had not understood how these creatures support their body weight on the water, says Thomas A. McMahon, a biomechanist at Harvard University.

Motivated by a long-standing interest in running, McMahon and James W. Glasheen, also of Harvard, propose in an upcoming issue of Nature a hydrodynamic model to explain how the basilisk accomplishes this feat.

After studying Basiliscus basiliscus plucked from the jungles of Costa Rica, the researchers found that most of the lizard's support comes from a well-timed foot stroke. With each step, the basilisk slaps the water and strokes its foot down, creating an air-filled cavity, then moves into the next step.

"I've been interested for many years in the mechanics of walking and running, in four-legged animals as well as two-legged ones," says McMahon. "We want to know how the basic physics of walking and running shapes performance. If you change a physical parameter, such as an animal's size or the length of its leg, or the strength of gravity, how do those changes affect performance?"

"It sounds strange to ask these questions about animals," he adds, "even though engineers ask them all the time about machines."

McMahon's team has devised mathematical models to integrate various parameters of walking and running, enabling them to predict how motion changes when, for example, a runner carries extra weight, chugs up an incline, or traverses an unusual surface.

The models have helped the engineers grasp the biomechanics of legged locomotion in a wide range of animals, "improving our engineering sense of how animals work," he says. For instance, the models show that animal legs work like springs, maintaining a uniform response to applied forces, no matter how fast the animal runs.

"This came as a surprise," he says. "Neurophysiologists now want to understand how muscles and nerves collaborate to produce springlike legs."

To test the models, the team had people run on treadmills while suspended from the ceiling to simulate low gravity-and even to run on their hands while lying prone on a horizontal support. The latter, for instance, showed that jogging on one's hands eats up four to five times as much energy as equivalent motion via legs.

In low-gravity experiments, McMahon and his colleagues Rodger Kram and Claire Farley of the University of California, Berkeley found that the rate of energy consumption changes in proportion to apparent body weight during both running and walking but changes less during walking. A simulated gravity reduction of 75 percent caused energy consumption to drop 72 percent during running but only 33 percent during walking.

"Because reducing gravity decreases the energetic cost much more for running than for walking," the team concluded, "walking is not the cheapest way to travel a mile at low gravity."