It Takes More Than Faith

Wednesday, May 01, 1996
The Central American basilisk is in most ways a fairly nondescript lizard. Yet in one respect this drab brown reptile is truly remarkable: it runs on water. Biomechanics Thomas McMahon of Harvard and James Glasheen of the University of California at Berkeley have made a detailed analysis of the Jesus Christ lizard, as Basiliscus basiliscus is sometimes called. They believe they have uncovered the secret of its sleight-of-foot locomotion.

The lizard uses three distinct motions to support its body weight on the water, says McMahon. First the basilisk slaps the surface with its foot. The force of this action spreads out the fringes of the lizard’s toes, increasing their width. Next the foot strokes downward into the water, submerging the entire leg and creating a cavity of air in its wake. The inertia of the water resists the plunging foot and helps support the lizard. The slapping action, by widening the lizard’s toes, enlarges the air cavity--which helps prevent the water from dragging the animal under. The trick for the basilisk here, McMahon explains, is to complete the final motion--lifting its foot up to the surface again--before the surrounding water closes the air cavity.

A .08-ounce juvenile can support its weight with almost all slap and a little stroke, says Glasheen, but a 4-ounce adult can just barely support its body weight, and if it doesn’t combine the motions in just the right way, it’ll sink. We couldn’t get any adults heavier than that (males can weigh up to 12 ounces) to run on the water at all. They just swim. When adults abandon water walking, he adds, they stop competing with juveniles for food. Instead they haunt the trees along the banks, leaving the water for the hungry young basilisks.

Apparently the nimble lizards are in a pretty exclusive club. Certain young iguanas are said to have similar abilities, but no one really knows yet, says Glasheen. Some insects also walk on water, but they use surface tension forces to support themselves, not the dynamic forces tapped by the basilisks. Could people join the ranks of water walkers? No way, says Glasheen. We’re just too big, and our feet are too small.
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