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Swimming dolphins don’t need to cheat

Measurement finds no sign of long-predicted lack of power

BUBBLE BURSTER  Analyzing the movements of many tiny bubbles when a bottlenose dolphin swims through them has allowed researchers to reject an old notion that muscle power alone can’t explain dolphin speed.  

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Swimming dolphins generate thrust just fine and have no need to compensate for supposedly underpowered muscles.

That conclusion comes from using bubbles in a pool to study dolphin swimming, says Frank E. Fish of West Chester University in Pennsylvania.

The study contradicts a storied 1936 paper by Sir James Gray, who calculated that dolphins didn’t have the muscle to produce the thrust they need to swim as fast as they do. Called “Gray’s paradox,” the work raised hopes of learning dolphins’ tricks to improve torpedoes (and swimsuits).

Scientists hypothesized that dolphins somehow reduce drag by creating smooth, laminar flow in water rushing by their skin instead of the usual turbulent flow. Soviet scientists even analyzed water-skin interactions of naked women being towed through water as stand-ins for dolphins.

To measure thrust from actual dolphins, Fish and his colleagues filmed the animals swimming through a curtain of tiny air bubbles. The frame-to-frame shifts in bubble position let the researchers work out how much thrust the dolphins produced.

A bottlenose dolphin’s flukes can generate high thrust and power without special drag-reducing tricks, the team reports January 15 in the Journal of Experimental Biology. So there’s no need for a “paradox.”

BUBBLE UP  A bottlenose dolphin swims through bubbles rising in a curtain from the bottom of a swimming pool. The dolphin makes a powerful stroke with its tail and leaves a vortex spinning after it, a clue to its swimming power. The second clip shows the motion at half speed.

Credit: Timothy Wei, Paul Legac and F. Fish  

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