Black bear are large enough to survive a long swim in the cold waters of Lake Superior. They can and do travel freely among the islands. Raccoons have been released as pets on Madeline Island. Even skunks have been introduced there. Star-nosed moles occasionally wander out of their winter burrows and have somehow reached some of the islands.
Bats are the only other mammals that hibernate and are found among the islands.
The little brown bat is particularly abundant. The populations of both deer and beaver exploded after logging occurred. With the regrowth of the forest, the deer population has greatly declined. They are seen occasionally on Stockton, Oak, Basswood, and Sand islands. The beaver have reached the habitat's limit on Outer but are found only sporadically on the other islands.