SYDNEY – The whales that have been beaching themselves in Australia in recent months are from extremely social species, known to follow pod members into danger.
That may help explain why the animals accompany each other in what turns into a mass beaching, but as Australian officials work to rescue survivors from the latest group to strand itself, scientists still cannot explain what draws the deep-sea animals so close to shore in the first place.
"What makes them strand is still mysterious," said Mark Hindell, a whale researcher at University of Tasmania's School of Zoology.
"There are as many different reasons for strandings as there are strandings. There are so many factors, you need so many things to line up in order for a stranding to occur," he said.
Five large pods, totaling more than 500 animals, have beached themselves in Australia since November, with most of them dying.
The latest group — 87 long-finned pilot whales and five bottlenose dolphins — stranded on a beach in Western Australia state Monday. Before rescuers could respond, more than 70 whales and one dolphin had died.
By Tuesday evening, 14 whales and four dolphins had been helped back to sea — some of them after being trucked overland to a beach with deeper, calmer waters.
As usual, there was no explanation for why the whales ended up on that beach.
"In certain years the whales will be closer to land and more available to strand," Hindell said. "But the big question is, why they are coming so close?"
Scientists have offered some theories: The whales may be chased by predators such as killer whales, or they could be following prey themselves. The sonar they use to navigate the dark seas could be hindered by natural geomagnetic factors such as iron ore deposits. They may swim into an area where sandbars or peninsulas block their exit. Or they may follow one ill or injured pod member and refuse to leave it.
Human activity such as undersea exploration for petroleum or the sonar of submarines also can interfere with whale and dolphin navigation.
Whatever the reason, once one animal heads for the dangerous shallows, the rest are likely to follow.
"Certain species of whales are more prone to mass strandings because the social bonds between them are incredibly strong," said Mike Bossley of the Whale and Dolphin Conservation Society. "If one animal is in trouble, the others won't leave him."
Pilot whales and sperm whales, both particularly social species, have been stranding themselves on Australian coastlines since last November.
Out of about 520 beached whales, more than 470 have died. Some are battered by rocks and surf, while others die of dehydration and overheating, while still others have their organs crushed by their own body weight after leaving the weightlessness of water.
The mass strandings occur most often in the island state of Tasmania, in Australia's southeast, and in Western Australia.
Marine researcher Karen Evans said the timing is right for an increase in beachings. In 2004 she co-authored a study concluding that beachings peak in a 10-year cycle linked to climate changes in the oceans.
"We're in a peak period now," said Evans, of Australia's Commonwealth Scientific and Industrial Research Organization. "What happens in that period is the climate factors increase the prey field near the shore, forcing whales closer to shore and thereby increasing the probability that they will strand."
She said the research did not provide a direct reason for strandings, but that it did show a cycle dating back to the 1920s that could help state governments prepare resources for peak periods of beachings.