BioEd Online

Sperm whales get the bends, suggests a study of their skeletons. And environmentalists fear that this could put them at risk if their diving patterns are disrupted by sonar testing.

Zoologists had previously assumed that these mammals do not suffer from this disease, which can cripple human deep-sea divers. But Michael Moore and Greg Early of the Woods Hole Oceanographic Institution in Massachusetts have discovered progressive bone damage in whale carcasses retrieved from the Pacific and Atlantic oceans.

The wear and tear seems to be a hallmark of osteonecrosis, a chronic disease that can be caused in long-term scuba-divers by the nitrogen bubbles that form in the body when surfacing too rapidly.

The whales' pitted, eroded bones show that they may suffer from osteonecrosis over the course of their lives. Moore and Early studied 16 whale skeletons spanning a period of 111 years. They report in the journal Science¹ that they found cavities up to 2 centimetres across in a range of bones. The larger skeletons (belonging to older whales) showed the worst damage.

Under pressure

The most likely cause is the repeated change in pressure caused by diving to catch prey and then returning to the surface for air, says Moore. "The only stressor known to cause this kind of bone damage is the bends," he says.

This implies that the whales stave off the effects of the bends not through some in-built physiological mechanism, but rather by carefully managing their diving patterns much as scuba-divers do. Surface too fast, and they could risk more severe damage.

Disrupted diving seems to be what caused a group of beaked whales to wash up on the Canary Islands in 2002 after a military sonar test in the area. The prevailing theory is that they surfaced too fast after being scared or disoriented by the cacophony of noise.

"Noise is one example of something that could disturb [whales'] behaviour," comments Paul Jepson of London's Institute of Zoology, who advanced this theory in a paper in Nature last year². "Nobody doubts that there's a causal link, we just don't know the mechanism yet."

Moving the military

Some of the carcasses studied by Moore and Early date from before the advent of military sonar, showing that the damage they suffered is likely to be natural. But if whales do routinely suffer the bends, the potential for them to be disturbed by military exercises could be even greater than environmentalists had feared.

Sperm whales are among the deepest divers, and one question that needs to be answered is whether other whales and dolphins suffer from similar problems, says Moore. "It's not an unreasonable presumption," he says. "It will be useful to examine other marine-mammal skeletons."

Meanwhile, the military is still left with the issue of whale strandings apparently caused by sonar, says Jepson. In April 2004, military experts and scientists attended a workshop in Baltimore specifically to discuss the issue of beaked whales.

Potential solutions are not easy to find, but could include staying away from known whale migration routes or habitats. "The military accepts the problem, and they have a 'can-do' attitude," says Jepson.