Bears do hibernate, though not as deeply as some other hibernating animals.
Deep hibernators spend most of the winter with drastically lowered body temperatures. Bears lower their body temperatures only slightly and continue to burn about 4,000 calories a day, resulting in a Zen-like state of watchful rest. They can quickly rouse themselves in response to intruders and can even nurse their young during hibernation.
But bears’ state of hibernation, which is also called denning, can outlast that of deeper hibernators, like the ground squirrel. Ground squirrels do not sleep straight through the winter but rouse themselves every couple of weeks to urinate or defecate, presumably with complex metabolic triggers.
Bears can spend up to five months without eating, drinking, defecating, or urinating.
Scientists now suggest that recycling is what permits bears to engage in what seems like a physiological impossibility and emerge healthy.
They seem to be able to use products of bone degradation to build new bone and to use urinary wastes to make protein.