The Case of the Shrinking Moose

It’s not that easy to find a dead moose. Jason Duetsch would know. A game warden from Colorado, he has volunteered his vacation time every summer since 2014 to find moose skeletons in Isle Royale National Park, a 45-mile-long chunk of boreal forests, swamps and rocky shores in northern Lake Superior.

For the past three decades, teams of five or six volunteers have paid for the privilege of thwacking through the brush, wading through swamps, plunging down ravines and balancing along muddy beaver dams. They brave frigid cold, sweltering heat and, at times, great clouds of mosquitoes. Sometimes they smell a carcass long before they see it or literally trip over bones. Other times volunteers absent-mindedly picking thimbleberries notice the bleached tip of a femur sticking out of the duff.

“It can be dirty. It can be smelly,” Duetsch said. “But the way you get the data is through a lot of hard work and sweat.”

The work is part of a renowned research program that has tracked wolf and moose interactions on the island since 1958, the longest-running predator-prey study in the world. Over the decades, volunteers and researchers from Michigan Technological University have collected the carcasses of more than 4,600 moose and recorded observations on vegetation and other species. Recently, this hard-won trove has allowed researchers to start answering an intriguing question: Is climate change affecting the park’s moose?

Moose in many parts of the country are not faring well. In Minnesota alone, the population of moose has nosedived by 50 percent over the last 12 years. Moose die from a range of causes, including tick infestations, wolf predation, hunting and parasites transmitted by deer, but scientists are not sure what is bringing about such drastic declines. Climate change is an obvious suspect — not only do moose struggle physically in warmer temperatures, but their parasites thrive. The problem is that it’s difficult to separate the influence of climate change from other factors.

And on Isle Royale, the plot thickens. Unlike their counterparts on the mainland, the park’s moose are protected from hunting and deer parasites because hunting is prohibited and there are no deer on the island. Their habitat has also been spared from logging for the past 100 years. And as wolves — their only predators on the island — have declined, moose populations have skyrocketed. Based on numbers alone, it seemed that the park’s moose were doing just fine, but the Michigan Tech researchers wanted to know if a changing climate was affecting them in less obvious ways.

They thought their warehouse full of skulls might hold some clues. The research team enlisted graduate students to help measure the volumes of the skulls. Head size is a good indicator of the overall size of a moose — and overall size is a good indication of moose fitness. In general, the bigger the better, especially for bull moose, which fight for the opportunity to mate.

Researcher Sarah Hoy then compared trends in skull size to other data like life span and winter temperature. She found that over four decades, moose skulls on Isle Royale have shrunk by about 16 percent — 19 percent among males and 13 percent among females — while average winter temperatures have increased by 6 degrees Fahrenheit. “The shrinkage in size was really quite unexpected,” said Rolf Peterson, who has studied wolves and moose on Isle Royale since 1970 and is one of the paper’s co-authors.

Hoy also determined that moose that experience warm winters in their first year of life typically end up with smaller skulls. In theory, smaller moose could simply reflect successful adaptation — moose get too hot at about 23 degrees in winter, and a smaller size makes it easier for an animal to regulate its body temperature. But the scientists also found that moose with smaller skulls typically had shorter lives. Overall, moose life spans have decreased over four decades, according to the study, which was published in the journal Global Change Biology. In other words, while researchers are not seeing the effects of climate change in the moose’s numbers, they are seeing them in the animal’s fitness.

“Before the study, it looked like things were bad in Minnesota, but they were quite rosy in Isle Royale,” said John Vucetich, a co-author of the paper and professor of wildlife biology. “What we can say now is that’s not quite right. We can see adverse impacts of these warming winters in Isle Royale; we just don’t see it in moose population dynamics. We see it in their bodies, their life histories.”

A warmer climate is not the only cause of moose shrinkage. Hoy’s analysis showed that high moose population density also contributed to smaller moose sizes. More moose means there’s less food for each moose, which translates to poorer health for many individuals. This matters because the National Park Service has proposed a plan to introduce more wolves to the island. Since the first wolves walked over the frozen lake to Isle Royale in the late 1940s, they have struggled with a contagious virus, inbreeding and, at times, low moose numbers. Now, only two wolves roam the island, down from an all-time high of about 50. The agency anticipates releasing a final decision this year. If it chooses to proceed — a position NPCA supports — biologists would release between 20 and 30 wolves over the next three years.

“Without wolves, the moose population is going to continue to rise, and they’re going to eat themselves out of house and home,” said Christine Goepfert, senior program manager for NPCA’s Midwest office. “Bringing in wolves is the best way to help balance the ecosystem there.”

Many questions remain. What is it about climate change that is causing stunted growth? Is it primarily heat stress, too many winter ticks or something else? What factors other than climate change are at play? Will the shorter life spans eventually translate into fewer moose on Isle Royale? If wolves are reintroduced, what effect will that actually have on the moose?

Peterson, Vucetich, Hoy and others will have plenty of reasons to continue the 60-year endeavor — which has passed through three generations of researchers — and keep adding to their collection of moose parts. Each winter, small crews of researchers and volunteers will keep flying over the island to look for fresh moose carcasses against the paper-white snow. And come summer, as the forest turns lush and the moose eat their weight in greenery many times over, volunteers like Duetsch will return and start the dirty, smelly, fruitful process of bone collecting all over again.