Why Do Animals Hibernate?

Step 1 of 5

Winter is a food problem, not a cold problem.

Insects vanish, plants stop growing, fruit disappears. Staying warm is expensive precisely when food is hardest to find.

Step 2 of 5

So the animal fattens up first.

Autumn is spent eating hard, laying down fat. That fat is the entire winter's fuel supply — there will be no top-ups.

Step 3 of 5

Then everything slows dramatically.

Body temperature can drop close to freezing. Heart rate falls from hundreds of beats a minute to a handful. Breathing nearly stops.

Step 4 of 5

A slower body burns far less fuel.

Chemistry runs slower when cold, so energy use can drop by 98 percent. That is what makes a fat store last for months.

Step 5 of 5

They do wake up periodically.

Hibernators rouse every week or two, warm up briefly, then drop back. It is expensive, and researchers think it may be needed for immune function and sleep.

Bears are not true hibernators — their temperature drops only a few degrees, so they can wake quickly if disturbed.