It seemed only natural for deep-diving sperm whales to be immune from decompression illness, or the bends—the painful, sometimes fatal condition that human divers suffer when they surface too rapidly. But the whales may be as susceptible as land mammals, according to a new study by WHOI biologists.
Michael Moore and Greg Early examined bones from 16 sperm whale skeletons archived in several museums and detected telltale patches of dead bone (or osteonecrosis), which were most likely caused by nitrogen bubbles that form when divers decompress too rapidly.
Only the bones of whale calves did not show signs of osteonecrosis, the scientists found, and the bone damage became more severe in larger (older) whales—an indication that osteonecrosis caused by decompression illness is a chronic, progressive disease among sperm whales.
When air-breathing mammals dive to high-pressure depths, the nitrogen in their bodies becomes supersaturated in their tissues. If they rise too quickly, the pressure is released too suddenly. The nitrogen reverts to gas, forming bubbles, or emboli, which can obstruct blood flow and lead to bone damage.
Moore and Early launched their study in 2002 after a necropsy of a sperm whale found dead on a Nantucket Beach revealed evidence of osteonecrosis. Intrigued, they decided to examine as many sperm whale skeletons as possible.
Their inventory included whales from the Pacific and Atlantic, and whales that died as long as 111 years ago—so the newly found phenomenon is neither localized nor recent.
Sperm whale dives typically last about an hour, but can be up to two hours, and they go to depths of 1,000 to 2,000 meters in search of their preferred prey: squid. The scientists theorize that the whales normally manage their surfacing behavior to decompression problems. But if a noxious sound—from a sonar, for instance, or seismic airguns used in oil exploration—disrupts their usual behavior and provokes fast surfacing, the whales risk acute problems from nitrogen emboli.
Moore and Early reported their findings in the journal Science. Their study was supported in part by the NOAA Fisheries John H. Prescott Marine Mammal Rescue Assistance Program.
The skeleton from the original specimen is now on display at the New Bedford Whaling Museum.