One Giant Leap for Dolphinkind

by Michael Mullaney on October 21, 2009

By his own accounts, professor Tim Wei is not exactly the world’s strongest swimmer. “I’m pretty good at not drowning,” he’s told me.

But when it comes to the flow mechanics aspects of swimming, he’s the Michael Phelps of the academic research crowd.

Along with helping the U.S. Olympic swimmers shave a few seconds off their lap time, Wei has done some fascinating research that – for all intents and purposes – solves the 70-year-old mystery of Gray’s Paradox.

The paradox, observed in the days of yore by Sir Gray, was that dolphins can swim faster than their physiology allows. Because his tests showed that dolphins simply aren’t strong enough to swim faster than 20 mph, but he observed them doing so, Sir Gray posited that the skin of the squeaky sea mammals must have some special, nuanced hydrophobic properties that allow them to glide effortlessly through the water. Sir Gray was on to something, but after 75 years, new technology – pioneered by Wei – has finally advanced to the point where the paradox could be fully explained.

Here’s a very nice segment from KBS ‘s Science Café (which is the South Korean equivalent of PBS’s Nova) that aired recently, where they interview Wei about his research. (You can’t see me, but I was in the room when they were filming.)



So the answer to Gray’s Paradox is that dolphins can, in fact, produce enough thrust to achieve speeds of 20 mph. Here’s why:

Wei videotaped two bottlenose dolphins, Primo and Puka, as they swam through a section of water populated with hundreds of thousands of tiny air bubbles. He then used sophisticated computer software to track the movement of the bubbles. The color-coded results show the speed and in what direction the water is flowing around and behind the dolphin, which allowed researchers to calculate precisely how mush force the dolphin was producing.

Wei also used this technique to film dolphins as they were doing tail-stands, a trick where the dolphins “walk” on water by holding most of their bodies vertical above the water while supporting themselves with short, powerful thrusts of their tails.

The results show that dolphins produce on average about 200 pounds of force when flapping their tail — about 10 times more force than Gray originally hypothesized.

Below is the actual DPIV video. Click here to see a comparison of still photos with and without the DPIV overlay.

For more info on Wei’s research, check out our story and the AP story on his work with the U.S. Swim Team, as well as our story and the AP story on his Gray’s Paradox study. Also, check out the video here which has Tim talking about the work – sans the overdub.