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Last Updated: Thursday, 23 December, 2004, 20:37 GMT
Whales 'suffer from the bends'
By Julianna Kettlewell
BBC News science reporter

Sperm whale head, Michael Moore
Scientists say much more research is needed
A swift ascent from deep water may be just as dangerous for whales as it is for humans, Science magazine reports.

Damage consistent with the bends - every diver's nightmare - has been found in the bones of sperm whales.

An acute form of the illness was also suspected in beaked whales, which were beached after a naval sonar exercise.

This has led researchers to speculate that sonar may distress whales into rising too quickly, resulting in potentially lethal cases of the bends.

These findings will come as a surprise to many people, because it has long been assumed that marine mammals are immune to the illness.

Pits and lesions

Since whales have evolved in the sea for millions of years, it is reasonable to suspect that natural selection would have "found" a physiological solution to decompression sickness.

But there is accumulating evidence to suggest this is not the case.

Michael Moore and his team, of Woods Hole Oceanographic Institution, US, have been inspecting bones from a collection of sperm whales.

There is a very good correlation between naval sonar and the mass strandings of beaked whales, going back to the 1960s
Paul Jepson, ZSL
They noticed that many of the bones contained lesions and pits, indicating the whales may have suffered mild, but chronic, decompression sickness over the course of their lives.

"I see it as the cost of living in a pressure gradient environment," said Michael Moore.

Professor Moore examined bones from whales of a range of ages and found that older individuals exhibited greater damage.

"It is a bit like the ageing process," said Professor Moore. "Old whales show the stresses that they face in their daily lives."

It seems that chronic decompression sickness is not a recent phenomenon in whales - the oldest bones Professor Moore examined were over 100 years old.

He believes the whales avoid a more acute form of the illness by making sure they ascend gradually, pausing frequently.

"Natural selection has in the large part solved the problem by developing dive behaviours that allow the animals to manage the problem," he said.

"Previous research does show that the animals display mid-water stops. At the time it wasn't apparent to anybody why they were stopping - but it fits with this hypothesis."

Beaked whales

Apparently acute forms of the bends have been observed in beaked whales, which beached themselves in the Canary Islands in 2002, following a military sonar exercise.

Severe osteonoecrotic lesions in the chevron bone of a sperm whale, Michael Moore
Researchers noticed that many of the bones contained lesions and pits
Paul Jepson, of the Zoological Society of London, studied the dead whales. He found they had bubbles in their tissues, which were consistent with severe decompression sickness.

It is not certain how they got into this state, but it seems likely sonar was to blame.

"There is a very good correlation between naval sonar and the mass strandings of beaked whales, going back to the 1960s," said Dr Jepson. "The question is how is it making the animals strand?"

One theory suggests that the noxious boom of sonar causes the terrified animals to flee to the surface, triggering an acute form of the bends.

Michael Moore said: "One of the quietest zones in the ocean is right up at the surface - they might be trying to get away from the noise."

Before scientists can be sure, more research needs to be done.

"We don't fully understand how they behave in response to sound," said Dr Jepson. "If we know the mechanism we can try to mitigate it and prevent it being potentially damaging to the animals."

Call for study on impact of sonar
23 Oct 04 |  Cornwall
Sonar 'may cause whale deaths'
08 Oct 03 |  Science/Nature
US Navy sued over new sonars
08 Aug 02 |  Americas

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