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Some of the most studied wolf packs in the world are in serious jeopardy. Researchers report that the occurrence of debilitating bone deformities in wolves marooned on Isle Royale, an isolated island in Lake Superior north of Michigan, has risen sharply over the past five decades due to inbreeding.

A genetic defect now common in the Isle’s wolves causes bones in the spine, the vertebrae, to grow gnarled and crooked. Also found in domestic dogs – close wolf relatives – the bone malformations  can pinch nerves in the spinal cord, causing pain that makes it tough to walk and can lead to paralysis of the back legs and tail in severe cases, according to research published in February's issue of Biological Conservation.

Back in the 1960s, about a quarter of Isle Royale’s wolves appeared to have the anatomical abnormality, but now the percentage of afflicted wolves has risen to nearly 60 percent of the population. “In normal, healthy wolf populations without inbreeding, you are only supposed to see this kind of defect in about one out of a hundred animals,” says paper coauthor John Vucetich, an assistant professor of wildlife biology at Michigan Technological University (MTU) in Houghton. The deformity, discovered during autopsies of recovered, dead wolves, has grown so rampant, Vucetich says, “we haven’t found a normal wolf in the past decade.”

Vucetich is one of the project leaders of the ongoing Isle Royale Wolf-Moose Study, along with Rolf Peterson, also a professor of wildlife biology at MTU. The project began in 1958 and has monitored the predator-prey relationship of the island’s wolf packs and moose herds ever since, celebrating 50 years of study last summer. Both species are more or less trapped on the 45-mile- (72-kilometer-) long isle; it is thought that some moose swam over from Minnesota around 1900, and that a few wolves reached the island via ice bridges that existed in the late 1940s. The captive populations have since developed an ecological balance: The small number of wolves (24 currently) subsists mainly on the moose that usually number around 1000. In turn, the moose rely on the wolves to help keep their population in check.

For a wolf trying to survive in the wild, physical deficits can leave it unable to hunt or defend itself. This winter, for example, researchers found a wolf apparently killed by a blow to the neck, probably from a moose. Unusually, this middle-aged wolf had advanced arthritis, or joint deterioration, possibly caused by spinal misalignment as a result of the genetic deformities, says Vucetich.

The new results offer the first evidence of the wolves’ closed population leading to a decline in natural fitness. This is important, Vucetich says, because for years some policy makers and conservationists have pointed to the apparent health of the Isle Royale wolf packs as an indication that small animal populations can maintain proper genetic diversity. “Isle Royale is not this robust place that some people thought it was,” says Vucetich.

Now that the wolves’ plight has come to light, the question remains about what, if anything, to do about it. One option is to do nothing and let nature runs its course, however cruel that may be. An interventional option is “genetic rescue,” says Vucetich, involving the introduction of an outsider wolf to the island to shake things up genetically.
Since only the alpha males and alpha females of packs mate, though, the outsider would have to ascend to one of these dominant positions to ever produce offspring, Vucetich says – a tricky coup d’etat in wolf pack politics, especially for an interloper.

A wolf on Isle Royale [top].
A comparison of a normally shaped wolf vertebra and a disfigured one, caused by a genentic defect [right]. Image credits: Isle Royale Wolf-Moose Study

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