He says that early work on hibernation was done on small animals like ground squirrels whose body temperatures do drop dramatically, so this characteristic came to define hibernation. Bears do slow down in winter, cutting their metabolic rates by 50-60 percent. Their respiratory rate slows to one breath every 45 seconds or so, and their hearts beats as little as once every ten seconds when they are most deeply asleep. Bears head into their dens because there is just not enough for them to eat in winter, not because they want to get out of the cold. Remarkably, while living off of the layers of fat that they packed on in the summer and fall, they do not excrete any wastes.
Bears produce urea as they metabolize protein during hibernation.
A buildup of urea will poison or kill most animals in a fairly short period of time, but bears recycle urea into amino acids, the building blocks of proteins. Bears also do not lose bone mass during hibernation, or suffer from osteoporosis, even though they hibernate in a very small space where they cannot move around very much. It is almost like they are bedridden, but they still retain a reasonable bone mass. And to top it all, pregnant bears will give birth within the next month or so, even though they have not eaten in months. With most animals, if they are not getting enough food, reproduction is one of the first things that shuts down.