DULUTH — Scientists studying the moose population on a national park in Lake Superior have discovered that the size of the animals has been shrinking over the years.

Michigan Tech University scientists studied more than 660 moose skulls from Isle Royale National Park, which is located about 25 miles off the tip of northeastern Minnesota, Minnesota Public Radio reported . Scientists discovered that the skulls have shrunk by 16 percent over the past 40 years.

The study found that moose skulls were smaller if they were born in years when moose were more abundant. An increase in the moose population means there’s 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 the university who co-authored the study.

Researchers believe there may only be one wolf remaining on the island.