Extreme inbreeding likely spells doom for Isle Royale wolves

Photo taken in 2015 shows the last two wolves known to be on Isle Royale, as of 2016.

Rolf Peterson

Extreme inbreeding likely spells doom for Isle Royale wolves

Like a spent royal family dynasty, the wolves of Isle Royale, Michigan, have reached a genetic dead end. The two wolves remaining on the Lake Superior island, a male and female, are more inbred than any known wild wolves—and more inbred than some infamous human families; their relationship is so close that “we don’t have a word to describe [it],” says wildlife ecologist Rolf Peterson of Michigan Technological University in Houghton. “There is no reason to expect this pair will flourish. “

The two wolves are the only survivors of an iconic population. Since 1958, researchers have tracked the fate of wolves and their moose prey on the island, a U.S. national park and wilderness area, in the world’s longest running predator-prey study, a staple of ecology textbooks .  The National Park Service (NPS) today released the annual report of the Isle Royale predator-prey study, now in its 58th year.

The report documents the genetic identities of the two adult wolves spotted the previous winter: They are father and daughter as well as half siblings, born 2 years apart to the same mother. That close relationship stunned researchers, who had assumed the two were a different mated pair, the last confirmed to have had pups. “I think it surprised everyone,” says Phil Hedrick of Arizona State University, Tempe, who has been analyzing the genetics of Isle Royale’s wolves.

A few years ago, eight to nine wolves roamed the island, but the population dwindled to three last year. The remaining two are the most closely related of the group. The inbreeding coefficient of their potential offspring—a measure that varies between zero for unrelated parents to approaching one after many generations of brother-sister mating—is 0.438.  (By comparison, the inbreeding coefficient among some of the European Habsburgs was 0.25, according to a 2009 analysis.) Some captive and experimental animal populations approach this level of inbreeding, but such populations are prone to abnormalities—and extinction—and managers try to avoid it. Wolves themselves naturally avoid mating with such close kin, but the pair on Isle Royale have no other options. “They are essentially in prison,” Peterson says.

Still unanswered is the identity of the third wolf from last year’s population count, a small, abnormal-looking animal that was presumed to be the pup of the two. The DNA analyzed came from scat collected last year, none of which represented a third wolf.  There was no sign of that small wolf this year. In addition, for the first time in the study’s history, the only evidence of the wolf population was indirect: two sets of fresh tracks in slush.

This year’s study was also beset by weather and aviation delays, with the result that Peterson was the only researcher who made it to the island. Field studies on the moose didn’t happen, and the aerial moose count wasn’t as complete as usual. The estimated moose count of 1300, up from 1250 last year, “almost certainly” underestimates the population size, says Peterson, given the near record number of calves—about 300, or 22% of the population.  In striking contrast to other North American moose populations, Isle Royale’s has continued to grow at an average annual rate of 19% since 2012, when wolf predation essentially stopped.  

Given the impact of all those browsing moose on the 544-square-kilometer island, researchers pressed for the introduction of new animals while the population was still reproducing, to genetically rescue the wolf population. But in 2014 NPS decided against new wolf introduction. However, formal and informal public comments on the park’s management options overwhelmingly supported wolves on the island, and this March the agency announced it plans to study that option. A third round of public comment is currently underway, with the decision expected next year. But although wolves have been voted on the island, time for the two survivors has likely run out.

Wisconsin-based Christine Mlot has been tracking the Isle Royale wolf story since 1993.