April 04, 1996
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Biologists Discover How To Walk on Water

Lizards do it; machines can do it, but humans . . .

By William J. Cromie

Gazette Staff

How hard is it to walk on water?

Harvard biologists have become the first researchers to provide a definitive answer to that question.

As a model they used a lizard that high-steps across the rivers and bayous of Central and South America. Its official name is Basiliscus basiliscus, but everyone calls it the Jesus Christ lizard.

"The name is appealing, but somewhat of a misnomer," admits Thomas McMahon, Gordon McKay Professor of Applied Mechanics and professor of biology. "They actually run across the water at speeds of about six miles per hour."

He and James Glasheen, now at the University of California at Berkeley, did experiments that reveal the lizard's upright running style. The lizards get part of their lift by rapidly slapping a foot down on the water. They also create air cavities in the water surface which they quickly step out of before the holes close. Such potholes stay open less than a tenth of a second.

Could humans do that if they were quick enough?

"We think humans could run on water for a few steps if they had feet the size of snowshoes or open umbrellas," McMahon answers.

A human water-walker would have to raise his or her umbrella shoes up to their ears, like the J.C. lizard does, to get the required slap speed. Then they would need to furl their feet and pull them out of the air cavity in about a quarter of a second.

"For a man or woman to do that would require a sustained power output at least 15 times greater than ever achieved by humans," McMahon notes.

Machines could do it better. "Using the physics we learned from lizards, it's possible to design machines that could run across water," he says.

Taking up the challenge, Parris Wellman, a graduate student in robotics at Harvard, made a mechanical water-walker from a Coke-can body and two cylindrical feet that work like paddle wheels.

For the Young Only

Wellman's water-walker is a clumsy machine, splashing through the water like a person about to drown. A young basilisk lizard, by contrast, glides smoothly across the water, and is gone before you realize what you're looking at. The smallest, lightest lizards take 20 steps per second.

McMahon and Glasheen videotaped young basilisks slapping across the surface on their five-toed feet. They are not web-footed but have fringes of skin around their toes that help keep them up. With a circular motion, like that of a swimmer, the lizard raises a leg as high as its head, then smacks it straight onto the water.

"The smack generates enough of an upward impulse to keep the youngest, smallest lizards from sinking," McMahon explains. "You can get some feeling for that by smacking your flattened palm down on the water so hard it stings."

Adult lizards, which are about one foot long, don't do it so smoothly, however. They push a leg into the water up to their bellies. An air pocket forms above the reptile's foot as it strokes downward. This combination of slapping and stroking can move a 3-ounce lizard across the water for a distance of about 30 feet.

"Adult lizards get about 20 percent of their lift from the slap and the rest comes from stroking," McMahon says. "Older lizards give up water-running altogether and stay on land," where they also run in an upright position.

Why run on the water in the first place?

"These reptiles have many enemies, which water-running helps them avoid," McMahon answers. "They can run faster than they can swim."

Curiosity about the physics of water-running motivated McMahon and Glasheen to do this research. They had no practical application in mind. However, both think the results could be used by humans for a new sport -- pool-running. McMahon envisions "a series of floating disks, about 25 inches across, scattered on a pool surface like lily pads."


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