Ancient whales were not master divers like their modern descendents. Biologists have discovered signs of decompression syndrome – the bends – in several different whale fossils, a finding that could revise the evolutionary history of deep diving.

A team of paleobiologists surveyed hundreds of modern and ancient whale skeletons for decompression syndrome, which occurs when quick pressure changes force air or fat bubbles out of blood vessels.

Such damage would have been common when whales first began plunging into the depths of the ocean, says Brian Beatty, of New York College of Osteopathic Medicine in Old Westbury, US, who led the study. However, whales eventually evolved to cope with frequent visits to their new world.

“Playing around in the shallows with Flipper is one thing,” he says. “But going out in the open ocean and diving hundreds and thousand of meters to go and get a fish or a squid, is like going up in space in terms of changes in pressure.”


Dive tactics

Scientists classify whales into two groups, both intrepid explorers of the deep sea.

Baleens, such as the gargantuan blue whale, take huge gulps of sea water, and then filter out their meals. While toothed whales like orcas and sperm whales prey on sharks and giant squid in the ocean’s bowels. The two lineages split apart roughly 45 million years ago.

Modern whales of both branches have evolved exquisite adaptations to fight the bends. Some exhale before they dive to clear their lungs of nitrogen gas that could form bubbles, and many whales allow ample time between dives.

Some researchers have suggested that military sonarcan startle whales into changing their diving behaviour, causing decompression syndrome.

To determine when the anti-bends adaptations first arose, Beatty and colleague Bruce Rothschild, of University of Kansas Natural History Museum in Lawrence, examined samples of ancient and modern whale vertebrae.

Minor damage

When gas or fat bubbles form in the blood vessels that feed bone cells, the vessels can burst and seal off the oxygen supply to the cells, resulting in tiny lesions that can be detected by X-ray.

“It’s a measure of small regular damage and not necessarily something traumatic,” Beatty says.

None of the 331 modern whale vertebrae showed signs of decompression syndrome, while a handful of the thousand ancient whale bones contained such marks.

Beatty views the damage as flirtations with the deep ocean, before more modern whales overcame decompression syndrome.

But baleen and toothed whales may have evolved such changes independently. Signs of decompression were found only in very ancient specimens of toothed whales, while more recent baleen whale fossils showed damage, suggesting that baleen whales only evolved their defences much later.

Tagging evidence

“Maybe baleen whales and toothed whales independently arrived at the same conclusion – that going out into the open water and going deep was a good idea,” Beatty says.

Most researchers have assumed that the common ancestor of toothed and baleen whales was a deep diver.

“They have come up with quite a surprising story,” says Erich Fitzgerald, a palaeontologist at Museum Victoria, in Melbourne, Australia.

Beatty’s hypothesis is in accord with “our long-standing understanding of the different diving habits between baleen and toothed whales”, says Nick Pyenson, a paleontologist at the University of California, Berkeley, US.

Tagging modern whales as they dive will help researchers understand how their ancestors evolved to cope, Pyenson adds. “As our sampling of living species gets better, these data will better inform our expectations of what to find in the fossil record,” he says.

Journal reference: Naturwissenschaften (DOI: 10.1007/s00114-008-0385-9)

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