But a parasitic brainworm that deer tolerate -- and moose don't -- is either directly or indirectly related to one-fourth to one-third, perhaps more, of moose deaths, said the biologists leading the research.
"It's really affecting our moose in the wild," said Michelle Carstensen, who heads the state's $2 million moose mortality project.
Overall, wolves kill slightly more adult moose than the parasite, but a significant portion of those kills were moose already infected by deer parasites and vulnerable to attack, she said.
Deer are relative newcomers to the northeast corner of Minnesota. Over the past 150 years, their numbers have risen along with the humans who built roads, cleared the trees, and created the open landscapes deer prefer.
Tony Swader, a longtime hunter and a member of the Grand Portage band, said tribal elders recall seeing a few deer along the Pigeon River that forms the Canadian border, but anyone who was serious about hunting them used to have to go at least as far south as Duluth.
Moose was historically a primary source of meat and a cultural touchstone for the Ojibwe, and members of Swader's tribe still hunt a few bulls on reservation and treaty lands.
But on the reservation, deer now occupy a fourth of moose territory, said Seth Moore, the tribe's environmental director. And tribal members harvest far more deer now because that's what's there. As a result, the culture is shifting as well, he said.
"Fewer kids learn about moose," he said.
Deer and moose weren't made to share the forest. Deer are native to the southern part of the North American continent, and over hundreds of thousands of years, they evolved to withstand the parasites they carry. Two of those parasites are problematic -- brainworm and liver flukes -- and both spend part of their lives in slugs and snails. Deer eat the snails and slugs while browsing, and the microscopic brainworms make their way to the outside of the deer's brain, where they lay the eggs. After hatching, the larvae move through the digestive tract and out in feces, where they are picked up again by snails. Liver flukes start in the liver, but end up back in snails by the same route.
Moose pick them up the same way -- by eating snails while browsing. But moose, which are believed to have crossed over from Siberia just 70,000 years ago, have not developed the defense mechanisms that deer have, and the parasites can damage their brains, nervous systems and livers.