The American Black Bear (Ursus americanus) has the ability to sustain a high core temperature throughout the duration of its hibernation cycle, even as outside temperatures fall to -20?C. This ability is largely due the conversion of chemical energy into heat in specialized tissue known as brown fat. We demonstrate temperature variation in a hibernating black bear on a macroscopic scale, without attempting to demonstrate local temperature variation. In this first glimpse of the physical processes underlying thermoregulation in a hibernating black bear, we have incorporated heat generation within a layer of brown fat. Our model indicates that brown fat tissue is capable of providing the energy need to maintain a high temperature. However, our model also points to the importance of the thick fur layer, as well as that of the fat layer, in providing basic insulation. At steady state, a temperature drop of over 40?C occurs in these two layers, keeping the body core at a temperature high above that of the surroundings. Without the insulation provided by these essential layers, along with thermogenesis in brown fat, it is unlikely that the bear would survive a 100-day hibernation cycle.