This is the time of year when black bear cubs are born, tiny creatures that weigh less than a pound, without much hair and eyes shut tight. Instinctively they nurse and snuggle within the warmth of their mother's fur.
Naturalists often shy away from the term "hibernation" when speaking about bears, reserving that term for a few species like the woodchuck that reduce their body temperature to just above freezing. For bears, the term "seasonal lethargy" has been used, or "winter dormancy." However, respected bear biologist Lynn Rogers gives a green light to considering bears true hibernators.
Rogers cites a trend away from focusing on body temperature as more is learned about other extreme changes in a bear's winter physiology. A bear's winter dormancy is impressive as some very fundamental systems shut down. This includes their digestive system: nothing in, and nothing out.
As for the nothing out part, a bear's body wastes get recycled into protein and other essential and sustaining compounds. In a fall feeding frenzy that leads up to hibernation, bears build generous reserves of fat. It's fat that's metabolized to fuel a bear through the winter months, not protein. Lean body mass, including bone, muscle and other organs, remains constant while fat reserves are depleted. Even for a mother bear nursing two three cubs, or sometimes four.
In spring, after almost complete inactivity for as long as seven months in some regions, bears emerge from hibernation. It takes a few weeks for appetite and other systems to get going, but bears are soon in high operating gear again.