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THE mystery behind sea mammals鈥� deep, effortless dives has been cracked by
Terrie Williams from the University of California at Santa Cruz. Williams and
her colleagues strapped video cameras, depth and speed recorders to seals, a
dolphin and a whale. The animals start their descent with a few powerful strokes
of their flippers and then plunge effortlessly. As they descend their slightly
buoyant bodies become negatively buoyant because their lungs collapse under the
increasing water pressure (Science, vol 288, p 133). 鈥淚t changes the
volume of the animal,鈥� says Williams. The researchers calculate that animals
diving this way expend no energy.