HARVARD GAZETTE ARCHIVES
Biologists Discover How To Walk on Water
Lizards do it; machines can do it, but humans . . .
By William J. Cromie
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
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
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."
1998 President and Fellows of Harvard College