Why do some animals hibernate?

Some animals hibernate because winter makes food scarce, and staying active would spend more energy than they can replace. Hibernation lets them live on fat saved from warmer months by slowing breathing, heartbeat, and body heat until spring brings easier meals.

The astonishing part is how different the shutdown can be. A bear may rouse if disturbed, while a ground squirrel can become cold and nearly still, with a heart that barely ticks along. Their bodies also protect muscle and bone during weeks of stillness.