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Whales get bent
I GASP FOR AIR JUST WATCHING COMPETITIVE FREEDIVERS, as they hold their breath for more than eight minutes or descend to depths of up to 170m.
But even these superhuman feats barely register on the scale of the diving capabilities of some whales. Sperm whales are the current record-holders, capable of diving to 2000 or even 3000m and of staying under water for up to two hours at a time.
Disappearing into the cold, dark ocean depths to catch deepwater squid or large fish, they behave more like submarines than air-breathing mammals.
So the results of some recent ground-breaking research came as a bit of a shock. Marine biologists have always assumed that sperm whales and their relatives are immune to the bends, but now it seems that rising to the surface too quickly from deep water may be just as dangerous for them as it is for us.
Damage consistent with decompression sickness - every diver's nightmare - has been found in the bones of sperm whales examined by researchers from Woods Hole Oceanographic Institution, in Massachusetts.
For the past decade or so, Michael Moore and Greg Early have been studying the partial or complete skeletons of 16 different sperm whales. The animals had been collected over a period of more than a century, from both the Pacific and Atlantic Oceans, and were being held in various museums.
The researchers discovered that many of the bones contained distinctive lesions and cavities - areas where the bone itself had died because of a lack of blood supply. Such wear and tear is the hallmark of osteonecrosis, a chronic disease that can strike people who have been diving for many years, and which is caused by the nitrogen bubbles that form in the body if you surface too quickly.
It seems that the whales had been suffering from something very similar - some form of DCS.
Intriguingly, the Woods Hole team found that the bones of older whales were more severely damaged than those of the younger ones - which Professor Moore puts down to "the cost of living in a pressure gradient environment".
In other words, the whales had been suffering from the cumulative, long-term effects of deep, repetitive diving rather than just one serious bout of the bends.
What this means is that, although sperm whales dive so much and for so long that they are likely to suffer some ill-effects, they probably have their own in-built dive computers to stave off more serious cases of the bends.
They rise up from the depths gradually, make safety stops along the way, and then spend long enough on the surface for their bodies to recover properly before setting off on another dive. There was already some evidence that they make midwater stops on their way back to the surface, so this hypothesis does seem to hold true.
Meanwhile, some alarming research in the Canary Islands has identified a more acute form of the bends in a family of rarely-seen cetaceans known as the beaked whales - and this time military sonar may be to blame.
In September 2002, a mass stranding of three species of beaked whales (totalling 15 animals) occurred during European naval exercises off the Canaries. Paul Jepson, of London's Institute of Zoology, studied the dead animals and discovered bubbles in their tissues consistent with severe DCS.
In the early 1980s, the US Navy identified the new, quieter generation of submarines being built by the Soviet Union as a special threat - and selected Low Frequency Active Sonar (LFAS) as the best response. LFAS produces extremely loud, low-frequency sounds to detect submarines at great distances, and has been developed in the years since.
These sonic booms are so implausibly loud that, during tests by the US Navy off the coast of California, they were detected on the other side of the North Pacific. The European naval exercise in the Canaries had been testing a similar system - apparently, as far as the whales were concerned, with dire consequences.
There is a growing body of evidence that (not surprisingly) exposure to such loud noise can frighten deep-diving whales into fleeing to the surface much faster than their physiology can allow. They get the bends and, in some cases, they die.
More research needs to be done before scientists can be sure, but it makes the military exercises that have been in the news recently even more disturbing than we had previously feared.
Zoologist, conservationist, writer and broadcaster Mark Carwardine writes each month in DIVER. Visit his website, www.markcarwardine.com
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