Bears hibernate not because of the cold but because there is a lack of food during the winter months. Females have their babies during hibernation (mid-winter) and mothers nurse their babies in the den until spring arrives.
Even though a bear becomes pregnant, it does not mean she will have a baby that winter. Bears mate in the spring and after a brief moment of embryo development, the female experiences "delayed implantation"; the embryo stops development for several months. If the mother has enough stored energy (fat) to last her and her babies through the winter, the embryo will implant and continue to develop into a baby. If she does not have enough stored energy, she will reabsorb the embryo and will not give birth that year. This adaptation works to ensure that the female bear will survive the long winter.
Bears do not go into complete hibernation the way rodents do. Their body temperature drops by only 7 or 8 degrees centigrade (instead of freezing like a ground squirrel). Their heart rate slows from 50 beats per minute to about 10 beats per minute. Bears burn about 4,000 calories a day while in hibernation – which is why bears need to put on so much fat (fuel) before they go into hibernation (an adult male can curl up with over a million calories of energy stored in his rolls!).
Bears do not urinate or defecate during hibernation. Instead, bears reabsorb their urine and feces in the form of proteins. Bears can lose 25-40% of their body weight during hibernation – burning their fat for fuel.