Secret of Mammal Dives Found




SANTA CRUZ, California -- How ocean mammals can dive thousands of meters deep without running out of air has long confounded scientists. A new study using wired-up seals, dolphins and a whale has the answer: They let gravity do most of the work.


By attaching video cameras to the animals' backs, a team of marine biologists learned that ocean mammals exceed their apparent aerobic capacity by starting their dives with a few powerful swimming strokes, then gliding for most of the rest of the descent in a relaxed position with their lungs somewhat compressed.


Humans - unless they carry diving weights - must kick and paddle in a near frenzy to descend into deep water, quickly burning oxygen as they go.


But the researchers found that marine mammals take advantage of a change in their buoyancy that occurs with increasing pressure and depth. That enables them to sink effortlessly through the water.


The groundbreaking and somewhat controversial research released Thursday could not have been done without the help of a trained bottlenose dolphin diving near San Diego, a 100 metric ton blue whale of the Northern California coast, Weddell seals hunting beneath the ice in the Antarctic and a northern elephant seal diving in Monterey Bay.


The findings of the experiment were published in Friday's issue of the journal Nature.


"Basically, they're turning the motor on and off in the course of the dive, and that enables them to reduce oxygen consumption by 10 to 50 percent compared with what they would need if they swam all the way down," said Terrie Williams, an associate professor of biology at the University of California at Santa Cruz. She collaborated with researchers from several institutions.


Ocean mammals' lungs are designed to collapse progressively with increased pressure, forcing oxygen from the alveoli, or air sacs in their lungs, and into the upper part of their respiratory system. This allows their bodies to be compressed into smaller volume, while their mass remains the same, causing them to sink.


In humans and other land animals, air gets trapped in the alveoli as the lungs are compressed, forcing nitrogen into the bloodstream. The result can be nitrogen narcosis - the bends - a life-threatening condition afflicting divers who surface too quickly.


Figuring out ways to attach the cameras to the animals was a challenge.


In the end, the researchers used a large suction cup on the blue whale, and a neoprene wetsuit band with a series of small suction cups on the belly of the dolphin.


They glued patches onto the fur of the seals; the patches fell off when the seals molted.


Animal rights activists objected to the attached cameras.


But Williams said their ultimate goal is conservation.


"My feeling is, gosh, if we don't start learning about these animals, we're going to lose them," she said.