Engines of Our Ingenuity

No. 505:
SHARK'S TAILS

by John H. Lienhard

Click here for audio of Episode 505.

Today, we learn a lesson from a shark's tail. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The shark is such a powerful symbol of menace! Zoologists tell us he's a primitive fish. The shark has been around for a hundred million years. We look at him and see eerie, primordial forces of evil. We're so caught in the symbolic power of the beast, we have trouble seeing him for what he is.

His tail, for example: It's all wrong. Most fish have symmetric tails. But the shark's tail is unique among fishes. It's long on the top and short on the bottom. Doesn't the swimming force act downward? Doesn't it drive him down into the water? Worse yet, the shark is heavier than water. Don't his pectoral fins constantly have to push him upward as he moves?

We conclude that the shark suffers extra drag forces -- that he's inefficient unless he's diving. We've believed that for years. The development of this primitive fish seems to have been arrested, before it was finished.

Yet he's been with us too long. He's survived when other species have not. So zoologist Keith Thomson grew curious. He asked a diver to follow a shark with a camera in the New England Aquarium. The diver gave him a set of photos that showed the precise motion of the shark's tail.

He does far more than swish his tail back and forth. He controls the upper tip with the grace of a seagull. He changes the direction of the thrust by tilting the tip, just so.

There's more. We find we've missed a simple point of me chanics as well. Think about the last time you used a screwdriver on a stuck screw. The longer the handle of the screwdriver, the better you did with it. That's because you can tilt it off the axis of the screw, without slipping out of the slot. That gives you a lot of extra leverage.

The shark's seemingly crazy tail is like that. It gives him an extra grip when he has to turn suddenly. He's not only fast, he's maneuverable as well. He is a formidable foe.

So we look further and find still more. His brain is large. His sense of smell is highly developed. The shark, like the cockroach, is ancient. He survives for good reason. In the end, we have to acknowledge a superb piece of design, after all.

We've said before in this series that good design is timeless. You don't try to improve upon the wheel or the violin. So it is with the shark. And that is why he's lasted -- ever since life on Earth was young.

I'm John Lienhard at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Thomson, K.S., The Shape of a Shark's Tail. American Scientist, Vol. 78, November-December, 1990, pp. 499-501.

For more on sharks, see the website:

http://www.ncf.carleton.ca/~bz050/HomePage.shark.html


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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