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To get through a long winter without food, hibernating animals — like the 13-lined ground squirrel — can slow their metabolism by as much as 99 percent, but they still need important nutrients like proteins to maintain muscles while they hibernate.

If you had to guess which part of the world has the highest levels of atmospheric mercury pollution, you probably wouldn’t pick a patch of pristine Amazonian rainforest. Yet, that’s exactly where they are.

For many, an ideal forest is one that looks the same as it did before European colonizers arrived. 

When rain falls, it picks up pollution from streets, farms and other manmade features as it winds toward the ocean.

The planet was different 700 million years ago. In the Cryogenian period, Earth featured a single supercontinent known as Rodinia, complex organisms had yet to explode on to the scene, and the globe was an ice-covered snowball.