The answer is found in what surrounds Isle Royale - Lake Superior. Any mammals that reach the island must cross the lake's icy waters, a trip of at least 14 miles. Some mammals are better adapted to making such a crossing than others. Bats can fly over. Good swimmers, like moose, may swim. Mammals that can walk long distances, like the wolves, can cross Lake Superior when it freezes. Other mammals, like bear, do not swim long distances and are hibernating through the coldest months. Some mammals like porcupine stay awake all winter, but would not make such a long journey. Because reaching Isle Royale is so difficult, even mammals that can make the trip rarely do so. Today, only 19 species of mammals can be found on Isle Royale, compared to more than 40 on the surrounding mainland.
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