Some animals evolved to sleep right through the cold. The alpine marmot (Marmota marmota), a small rodent from the mountains of Europe, sleeps through eight harsh months. In the four months of marmot activity, the animals scurry to mate and gorge on food to build fat reserves for the long winter's nap.
While most cold-hating birds migrate, the common poorwill (Phalaenoptilus nuttallii) of western North America enters a state of torpor, similar to hibernation. The bird hides under rocks to sleep through the winter.
Even animals in the tropics hibernate. The fat-tailed dwarf lemur (Cheirogaleus medius) sleeps through the winter of its native Madagascar, even though the air temperatures can rise to over 30 degrees Celsius (86 degrees F), according to a study published in Nature. Hibernating may help the lemur save energy during the dry season when little fruit and other food is available.