Whales dive deep for next breath.

Anthony R. Martin was puzzled recently as he examined data retrieved by satellite from tagged beluga whales. The information clearly showed that in some of the frigid waters near the Arctic Ocean, male beluga whales often plunge almost 1,000 meters before returning to the surface.

"Why do they dive to these ridiculous depths?" wondered Martin, a scientist at the Natural Environment Research Council's Sea Mammal Research Unit in Cambridge, England.

Martin had never before observed such dives. The whales usually dwell in shallower waters, he says. There, they dive to the ocean floor, eating until their need for oxygen forces them to surface. The new observations, however, showed the belugas performing V-shaped dives, never idling in the deep water.

Martin speculates that this unusual diving behavior helps the whales find their next breathing hole in ice-covered waters. "It all makes sense. There's no proof, of course, but it's a darn nice theory," says Martin.

To the surprise of the scientists, many of the male belugas migrate north from the Beaufort Sea through Arctic waters toward Melville Island, explains Martin. In contrast, their female counterparts head to the Amusden Gulf between the Canadian mainland and Victoria Island. While the females encounter largely open water, the males must travel under ice that has accumulated over many years.

Since the males make the V-shaped dives only under the ice pack, says Martin, the strategy may help the whales survive the arduous journey. The dives, he suggests, give a whale its best chance of finding the infrequent breathing holes in the dense ice pack.

Martin compares the dives to the actions of a pilot with severe plane trouble. In order to find an emergency landing site, the pilot climbs as high as possible to enlarge his or her field of view.

Martin speculates that a beluga swims downward until it has used up half its oxygen. At that point, he says, the whale decides whether to return to its previous breathing hole or to strike out toward a newly identified one. From those great depths, whales probably cannot see open water at the surface, but Martin remarks that their amazing hearing may detect lapping water, which signifies a hole in the ice.