DULUTH (AP) _ Scientists studying the moose population on a national park in Lake Superior have discovered that the animal has been shrinking over the years.

Reports that Michigan Tech University scientists studied more than 660 moose skulls from Isle Royale National Park, which is located about 25 miles (40 kilometers) off the tip of northeastern Minnesota. 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.

Scientists also discovered that moose born in warmer years also had smaller skulls and lived shorter lives.

The moose population on the island has tripled in the past decade to about 1,600 this year.