Short answers, pulled from the story.
Hibernation is a state of minimal activity and metabolic reduction in which an animal's body temperature drops, breathing and heart rate slow drastically, and metabolic rate decreases significantly. It differs from normal sleep in its depth and duration; hibernating obligate hibernators such as ground squirrels can sustain body temperatures near ambient for weeks at a time, which is physiologically distinct from ordinary sleep.
Yes. Research in 2011 on captive black bears and a 2016 field study on 14 brown bears by Alina L. Evans at Inland Norway University of Applied Sciences confirmed that bears undergo substantial metabolic suppression during winter, meeting the current scientific definition of hibernation. Their body temperature declines only 3-5 degrees Celsius, far less than in rodents, but their heart rate variability drops dramatically once they enter their dens.
The fat-tailed dwarf lemur of Madagascar was the first primate found to hibernate, overturning the long-held assumption that no primate undergoes hibernation. It hibernates in tree holes for seven months of the year in temperatures that can exceed 30 degrees Celsius, demonstrating that hibernation is not exclusively an adaptation to cold.
Obligate hibernators enter dormancy every year on a seasonal schedule regardless of temperature or food supply; examples include many ground squirrels and the white-tailed prairie dog. Facultative hibernators enter dormancy only when cold-stressed or food-deprived; the black-tailed prairie dog and chipmunk are examples.
The earliest suggested instance of hibernation is in Thrinaxodon, an ancestor of mammals that lived roughly 252 million years ago. Current evidence suggests hibernation likely evolved at the same time as endothermy rather than as a later adaptation to cold climates.
Bears maintain bone mass by elevating CART hormone levels, which inhibit bone-degrading cells, and by suppressing sclerotin, a protein that halts bone formation. They prevent muscle wasting through urea nitrogen salvage, a process in which gut microbes break down urea into ammonia, which is then used to synthesize essential amino acids including valine and leucine.