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How lizards run on water!
December 02, 2004 09:35 IST
So how true are the claims by some Indian yogis of being able to walk on water?
A definite answer to this is yet to come, but biologists S Tonia Hsieh and George V Lauder of Harvard University have explained how some lizards are able to run on water at speeds of about four kilometres per hour or more.
Basilisk lizards (Basiliscus Plumifrons) are proficient water runners, regularly dashing across bodies of water to evade predators. The Harvard scientists have made direct measurements of the force produced by the lizards while they do so using "digital particle image velocimetry".
The technique involves seeding the water with reflective particles and illuminating the area of interest with a thin laser sheet. Particle movement can then be traced and the forces calculated.
Waterfowl are known to slap the water with their feet during take-off. However, the birds that do so obtain a great deal of lift from their wings. But Basilisk lizards, weighing up to 200 grams, are unique in that they regularly run across water, using only their feet for both lift and thrust.
According to the Harvard team, the mechanics of water running by the lizards represent a novel mode of legged locomotion.
In their study, five juvenile basilisks were induced to run across a 0.8m-long water track. Each running sequence was filmed with two synchronised high-speed cameras.
They found that the lizards produced greatest support and propulsive forces during the first half of the step, when the foot moves primarily vertically downwards into the water. They also produced large transverse reaction forces that change from medial to lateral throughout the step.
"These forces may act to dynamically stabilise the lizards during water running," the scientists reported in the Proceedings of National Academy of Sciences.
"This study quantifies the forces produced after the foot impacts the water to understand how basilisks are able to produce both propulsive thrust and lift during bipedal locomotion on water," they said.
"Basilisk lizards, therefore, serve as interesting model organisms for examining the mechanics of this remarkable locomotor feat."
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