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Boreal Community Media

Smaller skull size in moose leads to lower life span

May 25, 2018 07:36AM ● By Editor

By Brian Larsen of the Cook County News Herald - May 25, 2018

Warmer winters lead to smaller skull size in yearling moose and a shortening of their life span, noted the researchers who conducted the Ecological Studies of Wolves on Isle Royale between January 2017 and January 2018.

Snowshoe hare and red fox numbers are up, and tracks of pine marten were observed at Windigo and several other nearby locations. Great horned owls, a primary predator of rabbits, were also often seen. One snowy owl was observed and in 2017 trumpeter swans nested (first record) on at least two lakes on Isle Royale with three cygnets produced. The study also looked at loss of balsam fir, one the main food sources for moose, on the island.

And while there are lots of beaver, beaver colonies are counted every other year, with the next count scheduled for October 2018.

These were just some of the interesting facts and observations noted in the research conducted by Rolf Peterson, John Vucetich and Sarah Hoy through the School of Forest Resources and Environmental Science, Michigan Technological University.

This year marks the 60th year of the continuous moose/wolf predation study conducted by Michigan Tech. And while this study was ostensibly about wolves and their interaction with the ecology, with only two remaining wolves on the island, the study focused more on what is happening in the absence of wolf packs. 

Wolves to Isle Royale

Wolves first came across on an ice bridge from Canada in either the late 1940s or as late as 1950, making a winter crossing when a portion of Lake Superior froze. Wolf numbers peaked in 1980 when there were an estimated 50 on the island.

Wolves (mostly) thrived until 2009, when the population started to crash. Their numbers have fallen from four healthy packs of 24 wolves to the current two wolves, an older male and female that are related and no longer mate.

Grey wolves will live up to 12 years but typically die before their 4th birthdays. They die mostly from starvation or from a fight with another wolf or problems caused by inbreeding or disease. A wolf can eat as much as 20 pounds of meat at one feeding. Ravens, who follow wolves, will eat as much as 2 pounds of meat at a feeding. Red fox will also eat some of the wolf kill. With fewer wolves to hunt, the fox and ravens make do in other ways.

This is the third straight year that only two wolves remain on the island.

Since the crash in wolf population, the increase in moose has been worrisome. With little predation taking place, the moose population had been growing at an average rate of 21.6 percent per year until this last year when the numbers fell from an estimated 1,600 to 1,475.

Adding this year’s aerial count (which is conducted each January), the last seven year growth average is 16 percent, but as the most recent study says, “The true average rate of growth is likely 16-22 percent.”

But even at the lower 16 percent growth rate, it was noted that the moose population would double in four to five years. With more moose comes more loss of vegetation due to over browsing, a worrisome trend that is affecting the island’s vegetation. 


An adult male moose can stand seven feet tall at the shoulders and weigh as much as 1,500 pounds. Moose live in the Northern Hemisphere in temperate to subarctic climates. A typical moose weighing 800 pounds can consume up to 71 pounds of food per day.

Moose born in warmer than normal winters tend to have smaller skulls and moose with smaller skulls tend to have a shorter life span than moose born with normal sized skulls, said the 2018 Michigan Tech study.

Between 1960 and 2000, the “mean” skull size of moose on Isle Royale has declined by 16 percent, and life expectancy of those moose with smaller skulls fell from 15 to 10 years.

Researchers say about one fifth of that decline can be attributed to a warming climate and the rest due to increasing moose density on the island due to a lack of predation by wolves.

Warmer winters are also hard on moose because moose are adapted to living in the cold, but are susceptible to heat stress. 


The study takes note that balsam fir on Isle Royale has declined by about 75 percent since 1846, a reduction from 36 percent to 9 percent of the land based on 2010 U.S. Forest Service data. Much of that decline is in the west end of the island due to moose browsing. Hardwoods have largely replaced those trees.

More than 90 percent of fir trees tagged in 1988 have now died without replacement of new balsam fir.

Sometime in the early 2000s decades-old balsam fir sapling on the west end of the island started to grow because browsing by moose was reduced. That occurred presumably because the moose population was reduced by wolf predation. More wolves mean few moose, which leads to the vegetation impacted by moose browsing to improve. That was one of the arguments to reintroducing more wolves on the island. 

Reintroduction of wolves

Four years ago the National Park Service published a Notice of Intent in the Federal Register announcing initiation of a Moose-Wolf-Vegetation Management Plan and Environmental Impact Statement meant to provide direction for future management of moose and wolves on Isle Royale. Following public hearings and a lengthy discourse between scientists, park officials, environmental groups and others who weighed in with an opinion, a decision was made to reintroduce wolves back to Isle Royale. 

In mid-March 2018 the National Park Service announced it had opted to repopulate the 132,000-acre island, relocating 20-30 wolves over a three-year period. Once the three-year period is up, the NPS will have two more years to reintroduce more wolves if it sees fit to do so. After 20 years the plan will be reviewed.

One thing noted in this year’s study of wolves was, “if climate warming is the responsible culprit for driving wolves on the island to the brink of extinction, does it make sense to restore wolves to the island?”

Only time, and continued research will tell. 

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