When scientists at Michigan Tech University examined the skulls of 662 moose killed by wolves over the past four decades at Isle Royale National Park, they discovered something surprising.
Over those 40 years, the size of those skulls had shrunk by 16 percent, even as the moose population on the Lake Superior island flourished, tripling in the past decade to about 1,600 this year.
The findings, published recently in the journal Global Change Biology, are part of a long-running study on the predator-prey dynamics between moose and wolves on the remote island, located about 25 miles off the tip of northeastern Minnesota.
The biggest reason researchers attributed to the decline in moose size is the increasing density of moose on the island, as the wolf population has collapsed. Researchers now say there may only be one wolf remaining on the island.
The study found that skull size was smaller for moose born in years when moose were more abundant, because more moose means less food for each animal.
"As wolf predation has gone down on the island, moose abundance has gone up, and that's contributed to most of the changes we've seen, maybe 80 percent of the changes we've seen," said John Vucetich, an ecologist at Michigan Tech and one of the study's co-authors.
Moose that were born in warmer years were also found to have smaller skulls, and lived shorter lives.
That's important to understanding what's happening to moose in northeast Minnesota, where the animal's population has dropped by about half over the past decade, said the researchers.
"The moose populations in northern Minnesota have tanked," said Michigan Tech's Sarah Hoy, who led the study. "Climate is considered a main driver, whether it's direct through warmer winter temperatures causing heat stress and influencing the nutritional condition of moose or indirectly by establishing more favorable habitat for white-tail deer."
But the moose population on Isle Royale posed a conundrum for researchers. Why would the moose population there be surging, when the climate is essentially the same as northern Minnesota?
Part of the reason is likely tied to parasites from white tailed deer, which impact moose in Minnesota, but not on Isle Royale, where there are no deer.
But this study shows that moose on Isle Royale have been impacted by climate warming, Vucetich said. "We just haven't seen it in their abundance. What we have seen it in, is in their body size, as indicated by their skull sizes."
The smaller skull size, he added, could indicate that a transition in the moose population is underway on the island.
The National Park Service is on the cusp of hastening that transition toward fewer moose on the island competing for food and other resources. After years of debate, the agency is expected to release its final plan soon to introduce new wolves to the island.
The new study, Vucetich said, "lends support for the notion that wolves are important for keeping a healthy moose population."