SANTA CRUZ, CA--For years scientists have puzzled over the ability of dolphins, seals, and other marine mammals to perform long, deep dives that seem to exceed their aerobic capacities. Now, with the help of sophisticated instruments and video technology, a team of researchers has resolved the paradox and discovered a laid-back diving strategy that appears to be widespread among marine mammals.
The researchers studied Weddell seals hunting beneath the ice in the Antarctic, a northern elephant seal diving in Monterey Bay, a trained bottlenose dolphin diving offshore of San Diego, and a 100-ton blue whale traveling off the coast of northern California. Video cameras mounted on the animals' backs revealed that they all started their dives with a few powerful swimming strokes, then made the rest of the descent mostly in a relaxed glide.
"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, Santa Cruz.
Williams, working with collaborators from several institutions, found that marine mammals take advantage of a change in buoyancy due to increasing pressure with depth, which enables them to sink effortlessly through the water. The researchers describe their findings in the April 7 issue of the journal Science. Williams's coauthors include UCSC biologists Burney Le Boeuf and Donald Croll, Randall Davis and Markus Horning of Texas A&M University, Lee Fuiman of the University of Texas at Austin, John Francis of the National Geographic Society, and John Calambokidis of Cascadia Research in Olympia, WA.
The researchers correlated video images of the diving animals with data from other instruments, including time-depth recorders. With the Weddell seals, the scientists were able to monitor heart rates and measure oxygen consumption during dives. By matching diving behavior with physiological monitoring, Williams was able to calculate the energy savings of prolonged gliding during deep dives.
"It took some amazing technology and logistics, but the result is a very detailed look at diving behavior and energetics," Williams said. "It really gives you a sense of how finely tuned these animals are to the marine environment, anatomically, physiologically, and behaviorally."
The range of animals exhibiting the same diving behavior was striking. Cetaceans (whales and dolphins) and pinnipeds (including seals) evolved independently and use quite different mechanisms to propel themselves through the water. The animals studied varied in body size from the 390-pound dolphin to the largest animal on earth, the blue whale, and are found in a wide range of habitats, from Antarctica to the California Coast.
Despite their diversity, these and other marine mammals share an anatomical feature that makes a gliding descent possible and also protects them from getting the bends, Williams said. Their lungs are designed to collapse progressively with increased hydrostatic pressure at depth so that air is forced out of the alveoli (air sacs) and into the upper part of the respiratory system. As the increasing pressure compresses the animal's body and the air in its respiratory system into a smaller and smaller volume, a marked change in buoyancy occurs.
"The mass of the animal remains the same while its volume decreases, so it starts to sink," Williams explained.
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, or the bends, a painful and life-threatening syndrome that afflicts divers who return too quickly from deep dives.
"The progressive collapse of the lungs in marine mammals preadapts them for taking advantage of the buoyancy change," Williams said.
By resting on the way down, the animals are able to extend their dives. "They're pacing themselves, saving energy and conserving oxygen until they need to expend it for hunting or avoiding predators," Williams said.
The logistics of these studies were often extremely challenging. The blue whale team, consisting of Francis, Calambokidis, and Croll, had to approach the swimming behemoth in a small boat and attach a camera to its back with a suction cup. The Weddell seal team (Williams, Davis, Fuiman, and Horning) set up camp on the frozen surface of Antarctica's McMurdo Sound and drilled through 15 feet of ice in sub-zero temperatures so they could work with the seals at an isolated breathing hole.
The research project was funded by the Office of Naval Research, the National Science Foundation, and the National Geographic Society.
Editor's notes: You may contact Williams at (831) 459-5123 or williams@biology.ucsc.edu .
BROADCASTERS: B-roll of Weddell seals, elephant seal, and bottlenose dolphin is available from Dena Headlee, National Science Foundation, at (703) 306-1070 or 1-888-937-5249 (pager). B-roll of the blue whale is available from Carolyn Ruff, National Geographic Society, at (202) 857-7793 or cruff@ngs.org .
Photographs are available from Tim Stephens at (831) 459-2495 or stephens@cats.ucsc.edu .
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Science