Brown bears (Ursus arctos) hibernate for 5-7 months without eating, drinking, urinating, and defecating. Nonetheless, they emerge healthy and alert in spring. Important physiological responses in hibernating bears include reduction in body temperature, cardiac output and metabolic rate (oxygen consumption), compared with those of active bears. Winter hibernators repeatedly cycle between cold torpor and rewarming, supported by nonshivering thermogenesis in brown adipose tissue. In contrast, summer animals are homeotherms, undergoing reproduction, growth, and fattening. This life history confers variability to brown adipose tissue recruitment and activity.
Reference: H.V.Carey, M.T Andrews, S.L. Martin. Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature.Physiol Rev. 2003;83:1153-1181.