London, May 9 : Biologists have discovered signs of decompression syndrome - the bends - in several different whale fossils, which suggests that ancient whales were not master divers like their modern descendents, a finding that could revise the evolutionary history of deep diving.
According to a report in New Scientist, 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," he added.
Scientists classify whales into two groups, both intrepid explorers of the deep sea.
While Baleens, such as the gargantuan blue whale, take huge gulps of sea water, and then filter out their meals, 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 sonar can 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.
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," said Beatty.
None of the 331 modern whale vertebrae showed signs of decompression syndrome, while a handful of the thousand ancient whale bones contained such marks.
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.
According to 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.
"As our sampling of living species gets better, these data will better inform our expectations of what to find in the fossil record," he said.