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The September release of a male wolf from Michigan’s Upper Peninsula on Isle Royale. The wolf population now numbers 15, with seven females and eight males.

NPS/Phyllis Green

Relocated island wolves outlasting mainland wolves in new Isle Royale home

Island life isn’t for everyone, nor, it seems, for every wolf.

One year into a federal effort to restock the wolf population in Isle Royale National Park in Michigan’s Lake Superior, a pack of eight relocated from a nearby island appears to be thriving, while four of 11 wolves brought from the mainland have died. Another wolf voluntarily departed last winter, returning to Minnesota over an ice bridge.

The U.S. National Park Service (NPS) today released news of the most recent wolf deaths, and the emerging pattern is clear: Wolves relocated as a pack from Canada’s Michipicoten Island Provincial Park have so far been more successful on Isle Royale than wolves brought individually from either mainland Minnesota, Michigan, or Canada’s Ontario province.  The Michipicoten wolves’ provenance as a bonded group was likely crucial to the fact they have all survived so far in the new environment, says wildlife ecologist Rolf Peterson from Michigan Technological University in Houghton, who has studied Isle Royale wolves since 1971. “That’s about the only explanation I can think of,” to account for the difference in the wolves’ fates.

Population ecologist Brent Patterson of Trent University in Peterborough, Canada, has been studying the Michipicoten wolves since a breeding pair crossed an ice bridge from mainland Canada to that island in 2014. Their large size, about 50 kilograms, is another important factor in their survival on Isle Royale, he suspects. Before settling on Michipicoten, where they hunted woodland caribou, the wolves had been preying on moose in northern Ontario, so they came equipped to hunt Isle Royale’s moose. At the time they were moved to Isle Royale, the Michipicoten wolves were food stressed and battered, having eliminated the caribou—but the presence of their pack mates and their large physical stature gave them a leg up in getting through the snow to hunt moose again, Patterson says.

The relocated U.S. mainland wolves, in contrast, were not moose hunters and were generally smaller, although they were considered healthy at the times they were moved to Isle Royale. The circumstances of their deaths have all been different. One Minnesota male died of pneumonia shortly after being moved in fall 2018. The body of another male, from Ontario, was retrieved from a bog in April; it was too decomposed to determine a cause of death. In September, two relocated females died; one from Michigan had an infection and wound from the leg trap used in her capture. The second, introduced from Minnesota in October 2018, died from severe trauma after an attack by another wolf or wolves. (Another Minnesota wolf intended for relocation in 2018 died before its move because of “capture stress.”)

NPS expected some wolf deaths, as well as wolf fights, or other random events to take a toll on the relocated animals, but “all the mortalities are surprising,” says NPS wildlife biologist Doug Smith, who directed a similar relocation of wolves to Yellowstone National Park and has worked on Isle Royale but is not involved in the current effort. In Yellowstone, 41 wolves introduced to restore the predators to the park all survived their relocation. Those wolves belonged to three packs, but individual wolves have also been successfully relocated, Smith says. He points out that moving wolves on a large scale to restore predation is still relatively new. “This is an art, not a science.”

Isle Royale researchers have been watching the movements of the new radio-collared wolves—except for the breeding male from Michipicoten, who slipped his collar in July—and consider their social dynamics to still be in flux. The public can investigate which wolves are hanging out together and where with a new online tool.

The last male wolf of the intensely studied island-born population also died this fall. It dropped dead on a hiking trail, where a ranger found its intact, though emaciated, body on 17 October. Eleven years old, it far outlived most wild wolves and was apparently survived by the 9-year-old island-born female that is both its daughter and its half-sibling. The female had been prodding the male along for several years. Pathologists at the United States Geological Surveys National Wildlife Health Center in Madison noted broken ribs, as well as several puncture wounds they attributed to wolf attack. “It is still a question in my mind what it actually died of,” Peterson says, noting that wolf attacks don’t usually break bones, although moose kicks commonly do. He may get more answers as the frozen corpse arrives this week in Houghton, where he will dissect the body and preserve the skeleton. Other Michigan Tech researchers plan to sequence the wolf’s genome.

*Correction, 5 May, 11 a.m.: Because of an editing error, the story misstated when a female wolf from Minnesota that died was introduced to the island. This wolf was introduced in 2018.

Posted in:

doi:10.1126/science.aba6490

Responses

The story above implies that wolves introduced from Michipicoten Island, Ontario, have experienced greater survival than their mainland counterparts because of “provenance as a bonded group,” larger size, and prior experience hunting moose. We provide the following corrections and clarifications to this story, using data from the Isle Royale wolf introduction project through December 2019.

The story implies that wolves relocated from Michipicoten Island were more successful because they originated as a bonded group. This could only suggest that success of these wolves was because they continued as a group post introduction. However, social network analysis we conducted using relocation data from GPS-collared introduced wolves suggests Michipicoten wolves did not behave as a bonded group. Only two of eight Michipicoten wolves appeared to be part of a bonded group (see https://belantlab.shinyapps.io/wolf-networks/). 

The story further implies that wolves from Michipicoten Island were more successful because of their larger size of 50 kg. At the time of introduction to Isle Royale, average body mass of Michipicoten wolves (37.5 + 5.9 [SD] kg; maximum = 44.5 kg) was greater than mainland wolves (32.0 + 6.4 kg; maximum = 45.0 kg from mainland Ontario), but no wolves weighed 50 kg. 

The story reports that wolves relocated from the US mainland were not moose hunters, implying that type of prey hunted influenced their survival. However, moose are an important prey of wolves in northeastern Minnesota (1), including from the initial Minnesota capture location (Grand Portage Natural Resources Management, pers. comm.) as well as mainland Ontario (2). Michipicoten wolves undoubtedly preyed on moose before their arrival to that island in 2014, but did not have access to this prey until introduced to Isle Royale in 2019.

As the story reports, no wolves from Michipicoten died through December 2019 and infers this represents greater success than their mainland counterparts.  We suggest samples are too small to make such comparisons.  Further, the story suggests the number of wolves that had died overall was surprising. However, overall apparent survival of introduced wolves was 0.78 through 2019, similar to annual survival of wolves in Upper Peninsula (UP), Michigan (0.75, 95% CI = 0.70–0.80) (3).     

We also offer clarification to the above story regarding two wolf mortalities. The Minnesota female that died in September 2019 was introduced to the island during the first release in October 2018, not 2019 as reported. Based on attributes including bite marks, broken ribs, and other broken bones, pathologists of the United States Geological Survey’s National Wildlife Health Center concluded that the cause of death of the island-born male wolf and this same Minnesota female was from attack by other wolves. The story suggests that “wolf attacks do not usually involve broken bones”.  However, a review of 40 necropsy records revealed four instances where wolves were killed by other wolves on Isle Royale had broken ribs, and several with broken dorsal vertebrae (Michigan Technological University, unpublished data). Seven of 19 wolves killed by other wolves in the UP had broken bones, including ribs (Michigan Department of Natural Resources, unpublished data).

Introductions of large carnivores are complex and many factors can contribute to their success or failure. Continued monitoring of this population and associated ecological processes will reveal the salient drivers of this effort.
Jerrold L. Belant, Global Wildlife Conservation Center, State University of New York College of Environmental Science and Forestry
Mark C. Romanski, National Park Service, Isle Royale National Park
Elizabeth K. Orning, Global Wildlife Conservation Center, State University of New York College of Environmental Science and Forestry

 
The news story offers the views of three wolf experts and the example of Yellowstone National Park to potentially explain what these experts consider a surprising pattern and number of deaths among the newly relocated wolves.  The letter offers no alternative explanations, says “samples are too small” for comparisons, then compares the “apparent survival of introduced wolves” on Isle Royale—a remote wilderness area with no roads, no hunting, and no people for much of the year—to “annual survival of wolves in Upper Peninsula, Michigan,” where according to the reference cited wolves die from poaching, road kills, and the like. Readers can decide if another national park or the Michigan Upper Peninsula is the relevant comparison. 
 
The letter argues that the Michipicoten wolves did not behave as a bonded group, although the social network analysis only shows wolf movements through 16 October 2019.  But the letter also notes that two of those wolves “appeared to be part of a bonded group”—supporting the expert opinion that wolves moved as a pack might have had an advantage in the translocation. As to why the Michipicoten pack may not have fully reformed on moose-dense Isle Royale, a National Park Service press release (20 December 2019)  offers this possible explanation:  “Loose [wolf] associations are common when… smaller prey items like moose calves, beaver and snowshoe hare are abundant on the landscape.  These animals are easy prey for a single wolf.”   
 
The story reports that Michipicoten wolves were “food stressed” at the time of relocation, which means the wolves were under their normal weights.  The earlier weights of the adult males, according to Brent Patterson’s data presented at the 2015-2017 Midwest Wolf Stewards Meetings and confirmed by email, were more than 50 kilograms.
 
As the reference cited in the letter makes clear, moose are not the primary prey of Minnesota wolves, as they are for wolves on Isle Royale.   The story links to previous reporting that includes this observation from one of the wolf experts:  “Whereas Ontario wolves have experience killing moose, Michigan wolves may have never seen a moose, and Minnesota wolves may have limited experience killing them.”
 
The letter misstates (“not 2019 as reported”) what the story says about the Minnesota female that died last September—2019 is not reported.  The original story discussed the fate of that wolf separately from the Michigan female that also died last fall.  It was edited to read as above. [Editor’s note: The timing has been clarified in the story.]
Science was aware in January 2020 of Rolf Peterson’s review of other Isle Royale wolf necropsies (cited in the letter as Michigan Technological University, unpublished data), and I value the additional context it gives to what the story reports on the cause of death for the last male island-born wolf.
Christine Mlot 

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