Brown Adipose.

http://www.britannica.com/EBchecked/topic/5948/adipose-tissue

Mammals have two different types of adipose: white adipose tissue and brown adipose tissue. White adipose, the most common type, provides insulation, serves as an energy store for times of starvation or great exertion, and forms pads between organs. When muscles and other tissues need energy, substances known as hormones bind to adipose cells and trigger the release of energy-rich fatty acids and glycerol.

Brown adipose, found mainly in newborn animals, generates heat and actually consumes energy. In humans, the percentage of brown adipose found in the body decreases with age. In other animals, however, particularly those that hibernate (e.g., polar bears), it is found in adults and plays an important role in survival. Species that hibernate experience a drop in body temperature and a slowing of metabolism during winter dormancy, which allows them to conserve energy. Brown adipose, by consuming energy, releases heat, which is vital for awakening and emergence from dormancy. Brown adipose tissue typically is tan to red in colour. Its colour and heat-generating properties are imparted by the abundance of organelles known as mitochondria found in brown fat cells. (Mitochondria are the energy-producing components of cells.)