Drafting dolphins ride the wakes with ease.

As a swimming instructor, Terrie M. Williams found the awkwardness of the human body in water quite frustrating. So when she became a wildlife physiologist, she decided to study just how efficient marine mammals are in their aquatic endeavors. And when she looked at the vital signs of dolphins trained to swim alongside a moving boat, she discovered that they owe their smooth technique not only to their streamlined shape but also to where they swim.

For fast speeds, the animals shift positions to take advantage of the physics of the water, moving along with no apparent effort, Williams and her colleauges report in the Feb. 27 Nature.

Working at the Naval Oceans Systems Center Hawaii Laboratory in Kailua, the researchers equipped two bottlenose dolphins with harnesses that monitored the heart rates of the free-swimming animals. The team also measured the lactic acid buildup in the animals' blood and counted how often the dolphins surfaced to breathe.

Traveling at 2 meters per second, the dolphins used about 1.3 joules of energy per kilogram of body weight; their hearts beat about 76 times a minute. That makes them twice as energy efficient as seals and sea lions and about 10 times as efficient as humans, Williams says.

When the boat sped up to 3 meters per second, the dolphins seemed to strain a little, but they still swam out to the side as trained. However, at 4 meters per second, "there was nothing we could do to make them stay in position," Williams told Science News. Instead, the dolphins would drift back to a particular spot in the boat's wake, where differences in water pressure made swimming easier. Then they traveled--without seeming to flick their tails at all--about one-half meter under the surface, coming up only for a breath. "They got a real advantage just by knowing enough to ride that pressure wave," she adds.

These surfing dolphins breathed 5.5 times per minute, compared with 8.8 times for dolphins traveling at the same speed but without the aid of a wake. The lactic acide levels of the faster-breathing animals shot up to three times those of their wave-riding counterparts. Thus, like bike riders who "draft" by pulling up close behind another cyclist or a vehicle, these animals can sustain high speeds with less effort. By using the water to their advantage, they move twice as fast for only 13 percent more energy, says Williams.

Scientists and seafarers have long wondered why dolphins sometimes travel in the bow waves of ships or in the wakes of small boats or larger marine mammals with seemingly little effort. The new results demonstrate that this behavior is not playful but energy efficient, Williams says.