Depending on where they live, most American black bears go into dens for at least part of the winter. Hibernation enables them to live in places where food is not abundant year round, instead of migrating as animals such as elk do.
It also benefits pregnant females in conserving energy and nurturing helpless newborns. For this reason, female black bears hibernate for part of the winter wherever they live. Males might not hibernate at all if they live in southern states such as North Carolina.
Contrary to popular belief, weather doesn't seem to affect the time that bears go into their dens. They are just as likely to begin hibernating on a warm December day as during a blizzard.
What does affect their timing is food. If food is scarce, bears might den earlier. If food is abundant, they might delay denning so they can continue feeding.
Like hibernators such as chipmunks, a bear's respiration rate drops to as slow as eight breaths a minute during deepest sleep. Unlike other hibernators, though, a bear’s body temperature falls only slightly perhaps because of the bear’s large body size and the fact that it metabolizes fat reserves while hibernating.
Bears also don't need to wake up to eliminate body wastes and eat from food they have stored in their dens. Instead, they metabolize their body wastes into useable products and obtain the food they need from their fat reserves.
Idaho's black bears begin moving to their dens in mid-October and may sleep for 4-7 months. In general, male black bears are the last to begin hibernating and the first to emerge in the spring. Females with new cubs are the last to emerge from their winter homes.