Several different mammals and species hibernate and in few cases the temperature of the body falls so much that it is almost near freezing. The reason for the decrease in overall body temperature is mostly due to the drop in the heart rate and slow breathing process.
Compared to these species, which have their body temperatures falling down to near freezing, a bear is a bit different. Bears do not hibernate this deeply but their body temperature, heart rate and the metabolism rate falls down significantly.
Most people often wonder how a bear that is hibernating does not excrete faeces or urine during this process. This is because the bears are hibernating at such a low metabolism rate that they can go up to six months without food, water and without having to urinate and excrete faeces.
How do bears manage to do this? They do it by burning only a pound of fat in their body per day. One pound of fat has 3500 calories, which is enough for the bear to survive.
The whole process of burning fat creates water within the digestive system of the bear and that is the reason why they do not need water in order keep themselves hydrated. During this process, bears recycle nitrogen through their urine in order to create protein.
This helps them in keeping the muscle mass that they need for survival. Once the bears are out of hibernation, they are lighter than before and they also rush out to eat in order to get rid of their food cravings. However, their muscle mass is still intact.