Dolphin propulsion
Puka, a retired Navy dolphin, was one of two bottlenose dolphins that helped scientists solve a long-standing mystery in dolphin propulsion known as Gray's paradox. (Frank Fish)
January 22, 2014, 6:05 a.m.
Does dolphin skin have secret powers that allow the flippered mammals to outrace boats? Scientists looking to answer this question have found that dolphins achieve impressive swimming speeds based on muscle power alone.

The findings, published in the Journal of Experimental Biology, solve a longtime mystery on the nature of dolphin propulsion.

Researchers have wondered how dolphins manage to swim so fast at least since the 1930s, when British zoologist James Gray marveled at reports of one dolphin's apparent speed as it outraced a boat. Gray calculated that the dolphins simply didn’t have the muscle power to swim that fast; they must somehow use a trick of fluid mechanics to overcome the drag that would hold them back. This observation became known as Gray’s paradox.

The answer to Gray’s paradox was thought to lie in dolphins’ smooth skin. Could it manipulate water flow to reduce drag and improve speed? (It’s a reasonable idea – after all, speedy mako sharks have skin covered in tiny toothlike scales that help them make hairpin turns by controlling flow separation.)

The lure of such potential drag reduction spawned a host of research, said lead author Frank Fish, a biomechanist at West Chester University in Pennsylvania. This was particularly true in the 1960s during the Cold War, when both Russia and the U.S. coveted the dolphin’s supposed secrets.
“Cold war paranoia afflicted both Pentagon and Kremlin in the form of wildly exaggerated estimates of the speeds of each other’s submarines,” Duke University biomechanist Steven Vogel wrote in the book “Comparative Biomechanics: Life’s Physical World.”

Researchers tried to pick apart the secrets of dolphin skin in a number of ways, wrapping rubbery artificial skin around test torpedoes and even dragging naked young women (or "nekkid leddies," as referenced here) through the water to see how their skin responded to the drag. (Women have more fatty tissue under their skin than men do, which gives their skin more "dolphin-like" properties, Fish said.)

Nowadays, to watch how animals affect the flows around them as they move through water, researchers often fill a water tank with 10-micron-wide glass beads and shoot a laser sheet through the water to illuminate the beads and watch how the animals' movement affects the beads and thus disturbs the flow.

You can do this with jellyfish, not so much with dolphins, Fish said – there are concerns about what would happen if the laser hit them in the eye or if they ingested the beads.

“It’s one thing to work with a fish, it’s another thing to work with a dolphin – we tend to protect them,” Fish said. “Dolphins are very pampered animals, when we keep them.”

Luckily, Fish said, engineer Timothy Wei of the University of Nebraska-Lincoln had been working with other “pampered animals” – Olympic swimmers – and had come up with an ingenious and low-cost solution to track them as they swam.

Instead of using glass beads, Wei used air bubbles. Here’s how: They got a garden soaker hose that’s typically used to water lawns and pumped air through it from an oxygen tank. The tiny bubbles that came out of the hose’s pores created a sheet of bubbles that, when illuminated by sunlight, could act just like the reflective glass beads in the laser sheet.

The scientists had Primo and Puka, two retired Navy dolphins, swim along the length of the bubble wall. After watching the patterns created in the bubbles, the scientists realized that the bottlenose dolphins were producing an incredible amount of power – enough to overcome the enormous drag they were experiencing.

So the answer to Gray’s paradox? There was no paradox, Fish concluded.

“First off, we can stop looking for a magic mechanism to reduce drag,” Fish said. “There may be ways to reduce drag, but the dolphin [skin] isn’t going to show us those.”

In any case, he added, “it basically starts to tell us things about how well designed these aquatic athletes are.”

It could mean that flippered robots could theoretically be an alternative to the propeller-driven kind, said Fish, who said he’s currently working on creating a manta ray robot.

In the meantime, the bubble method of tracking animals’ flow patterns might be useful in testing larger animals in the open ocean – it’s certainly more portable than the laser-and-beads method, Fish said.

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