Machine Design

Gray’s Paradox Solved: The Secret of the Speedy Dolphin

James Gray, a 1930s era British zoologist studied dolphins and how they swim. He concluded they didn’t have the muscle power to swim at the speeds they did, more than 20 mph. This conundrum was dubbed Gray’s Paradox. Gray could not figure out how dolphins could overcome the drag at those speeds. This problem has puzzled the scientific community for 70 years.

Finally, it has been conclusively proven that Gray was simply wrong. Dolphins just have more muscle power than Gray gave them credit for.

A team of scientists from Rensselaer Polytechnic University and the University of California, Santa Cruz, unraveled the mystery. They modified and combined forcemeasurement tools developed for aerospace research with digital particle-image velocimetry, which captures images at up to 1,000 fps, to measure the force exerted by a dolphin. They videotaped a pair of dolphins swimming through a section of water filled with hundreds of thousands of air bubbles. A computer then tracked movements of the bubbles, colorcoding results to show speed and direction of the water flowing around and behind the dolphins. This let the team calculate precisely how much force the dolphin generated. (To see a computerenhanced video of a dolphin swimming, check out

It turns out dolphins exert about 200 lb of force every time they flap their tail, 10 times more than what Gray hypothesized, and can put out between 300 and 400 lb when really trying. (Olympic swimmers peak at about 70 lb of force.)

“Dolphins can certainly exert enough force to overcome drag, something the scientific community has known for some time,” says researcher Timothy Wei. “But this is the first time anyone has quantitatively measured the force and solved the paradox for certain.”

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