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Physics Theory Tackles Mystery of Movement

Animals designed to get as far as possible with least amount of energy, new study says

THURSDAY, Jan. 5, 2006 (HealthDay News) -- Fish gotta swim and birds gotta fly, but why do they move the way they do?

A mechanical engineer thinks he may know the answer: Animals are designed to move as smoothly as possible through the world.

Animals actually move in a similar fashion to the currents of rivers and bloodstreams, which try to get as far as possible while expending the least energy, said Adrian Bejan, a mechanical engineering professor at Duke University's Pratt School of Engineering. Essentially, he said, humans and other species move in the way they do because they're "polished into an optimal configuration so the movement is the easiest."

Put another way, he said, animals move by finding the path of least resistance. And he has the statistics to prove it.

With James Marden, a professor of biology at Pennsylvania State University, Bejan presented the latest take on his "constructal theory" in the January issue of The Journal of Experimental Biology.

Bejan spent much of his career studying not animals but electronics, trying to figure out how to disburse heat in small electrical devices so they don't burn up. He found that in many cases the best way to disburse heat is to let it spread out like the limbs and leaves of a tree or the tentacles of a river reaching for the sea.

Trees and rivers try to get as far as possible while exerting the least amount of energy, he said.

"Animals do the same thing, moving while having to eat the least amount of food," he added.

Bejan adapted his ideas to the study of how animals move through the air and water. His finding: Animal movement "is an optimized rhythm, like a fine-tuned engine that goes beep and bop at the right rhythm."

If animals are designed so efficiently, why do so many seem so clumsy? While animals may have been optimized for efficiency at some point during their development, evolution puts other pressures on them, Bejan said. "It is the animal that's most capable of changing that survives," he added

Robert Full, a professor of integrative biology at the University of California at Berkeley, said the finding is a "very preliminary step" in proposing a new theory to explain animal locomotion. "It doesn't reject other hypotheses, but adds another to the mix," he said.

The goal is to find a rule to explain why animals "show this extraordinary diversity in form -- body shape, number of appendages, and the way their appendages look, from legs to fins to wings," Full added. "We want to know fundamentally how our bodies work."

More information

Learn more about constructal theory from Duke University.

SOURCES: Adrian Bejan, Ph.D., professor, mechanical engineering, Pratt School of Engineering, Duke University, Durham, N.C.; Robert Full, Ph.D., professor, integrative biology, University of California, Berkeley; January 2006 The Journal of Experimental Biology
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