Cumulative sperm whale bone damage and the bends
In a study published in the December 24, 2004 issue of the journal Science, Michael Moore and Greg Early at the Woods Hole Oceanographic Institution (WHOI) have documented bone lesions in the rib and chevron bones of sperm whales, most likely caused by tissue damage from nitrogen bubbles that form when the animals rise to the surface.
The WHOI biologists found that the lesions grow in severity with age, and are found in animals from the Pacific and the Atlantic Oceans. The lesions were found in animals that died up to 111 years ago, and there appears to be no increase in the lesion prevalence since the oceans were industrialized.
Moore and Early studied sixteen partial or complete sperm whale skeletons from the Pacific and Atlantic Oceans that had been archived in museums. They found a series of changes in bones attached to the backbone, namely rib bones, and other small bones in the sperm whales tail region. The changes are patches of bone death as a result of obstructed blood supply to the joint surfaces of the bone.
The team found evidence of comparable bone damage to be present with increasing severity as the size of the individual whale increased. Only the calves appeared normal. In looking at the potential causes of the lesions observed, the biologists concluded that such a wide distribution in time and space made nitrogen gas bubbles from a decompression sickness-like syndrome the most likely explanation.
They suspect that sperm whales normally manage their surfacing behavior to minimize problems with such bubbles. They conclude that if such normal behaviors are interrupted by, for instance, noxious acoustic stimuli (such as sonar or seismic survey guns), there is the risk of acute problems from nitrogen emboli, as has been reported in beaked whales exposed to mid-frequency sonar. This study was supported in part by the NOAA Fisheries John H. Prescott Marine Mammal Rescue Assistance Program.
The story behind the study
In June 2002, a sperm whale died on a beach on the northeast corner of Nantucket Island off the coast of Massachusetts. It was towed to New Bedford, MA, and trucked to the city landfill, for a postmortem examination, or necropsy, and to recover the skeleton for display at the New Bedford Whaling Museum. During the necropsy it became apparent that there was a series of changes in some of the bones. The lesions were bilateral and asymmetrical, and had the appearance of progressive patchy bone death in the bone underlying certain joint surfaces. Intrigued by these findings, Moore and Early decided to examine as many sperm whale skeletons as possible.
The sperm whale study was conducted during an ongoing period of intense interest in the role of loud underwater noises that may drive diving marine mammals rapidly to the surface. The realization that marine mammals need to manage decompression issues by managing their surfacing behavior resulted partly from the studys suggestion of chronic decompression damage in sperm whales.
The skeleton from the original specimen is now on display at the New Bedford Whaling Museum.
Shelley Dawicki | EurekAlert!
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