What happens to animals during hibernation?

The answer

During hibernation, animals enter a state of drastically reduced metabolism to conserve energy through winter food scarcity. The Arctic ground squirrel's core body temperature drops to as low as minus 2.9 degrees Celsius, the lowest recorded for any mammal, while its heart rate slows from 200 to about 3 beats per minute and its breathing nearly stops. Black bears can hibernate for up to 7 months without eating, drinking, urinating, or defecating, recycling urea back into usable protein and maintaining bone density despite months of immobility. Hibernators periodically warm up for brief arousal episodes that are metabolically expensive but appear essential for sleep, as paradoxically, hibernation is not sleep; it is so deep that animals must warm up periodically to actually sleep.