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Marine mammals stroke-and-glide to depth
Video supplied by cameras attached to dolphins, seals and whales reveals how the marine mammals employ a stroke-and-glide sequence that enables them to perform long, deep dives that seem to exceed their aerobic capacities.
"Basically, they're turning the motor on and off in the course of the dive, and that enables them to reduce oxygen consumption," said Terrie Williams, a biology professor at the University of California at Santa Cruz.
Williams previously believed that dolphins carried out their dives at a constant, efficient speed. But her calculations showed that the mammals would have to exceed their oxygen requirements by more than 25 percent on dives 650 feet deep.
In the field, her subjects bottlenose dolphins routinely went on dives deeper than 650 feet and returned with ample reserves of oxygen. Williams and her colleagues were perplexed.
To solve the conundrum, the researchers attached video cameras to the backs of dolphins, seals and whales in the Pacific and Antarctic oceans. Their findings are reported in today's issue of the journal Science.
"Video images permit direct observation of swimming periods, stroke frequency and glide sequences," the authors write. "Coupled with time-depth recorders, these new tools allowed us to assess the locomotor strategies used by marine mammals throughout their dives."
The study reveals that the marine mammals, despite their independent evolution of swimming methods and differences in body size and propulsive mechanisms, all started their dives with a few powerful swimming strokes and completed their descent mostly in a relaxed glide.
Seals, dolphins, whales and other marine mammals share an anatomical feature that makes the gliding descent possible and protects the animals from getting nitrogen narcosis, or the bends, said Williams.
The lungs of these mammals are designed to collapse progressively with increased water pressure at depth so that air is forced out of the 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," she said. "The progressive collapse of the lungs in marine mammals pre-adapts them for taking advantage of the buoyancy change."
In humans and other land animals, air gets trapped in the air sacs as the lungs are compressed, forcing nitrogen into the bloodstream. This can cause nitrogen narcosis, or the bends, a life-threatening syndrome that afflicts divers who return too quickly from ocean depths.
The researchers say that this form of diving, called intermittent locomotion, allows marine mammals to save their energy for hunting or avoiding predators.
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