Crush

		Williams spied on diving animals like this Weddell seal by attaching cameras to their backs

Even taking into account the dive reflex and other physiological adaptations, marine animals routinely make longer dives than expected. According to Dr. Terrie Williams of the University of California, Santa Cruz, who studies these mammals, "there was a real mismatch between predictions based on energy and oxygen reserves and what actually happened."

By attaching cameras to the backs of dolphins, Weddell seals, elephant seals and blue whales as they dove, Williams and her colleagues got a unique glimpse into the world of deep diving mammals. Scientists have learned that these mammals have uniquely designed ribcages. As the animal descends, the ribcage collapses under the increasing weight of the water, compressing the spongy lung tissue. This decrease in the animal's overall volume lowers its buoyancy. But Williams' footage revealed that marine animals of all sorts—from dolphins, to seals, to blue whales—make good use of the change in buoyancy. After a few forceful strokes of the tail or hind flippers at the beginning of the dive, the animals stop swimming and simply glide downward as the change in buoyancy takes over.

According to Williams' article in the April 7, 2000 issue of the journal Science, marine mammals use 2 to 23 times more energy than fish do swimming. Therefore, "turning off the motor," as Williams puts it, during descent can greatly reduce oxygen consumption. In Weddell seals, Williams documented energetic savings up to 60 percent.

The humpback whale can hold its breath for 30 minutes, and dive up to 200 meters

"We didn't know what they did under water before," says Williams, "We now know they're using the environment to make [an energetically] cheaper dive."

While people like Leferme can train themselves to push the limits of human endurance, dolphins and seals have the laws of natural selection on their side. With their specialized circulatory systems, highly developed dive reflex and collapsible rib-cages, these mammals who call the water home will remain the world's true champion divers.

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Photos: NOAA/ Department of Commerce