What heat-producing tissue is especially associated with newborn and hibernating mammals?

Brown adipose tissue is the heat-producing tissue especially associated with newborn mammals and hibernating mammals.

Also called brown fat, this tissue specializes in non-shivering thermogenesis: it generates heat rather than merely storing energy. Its cells contain many mitochondria, whose iron-rich components help give the tissue its brown color. The key protein UCP1 allows fuel oxidation to produce heat instead of capturing all the energy as ATP.

Newborns benefit particularly because they lose heat quickly, have relatively little muscle, and cannot rely on shivering as effectively as adults. In human infants, brown fat is concentrated around areas including the upper back, spine, and shoulders, where it helps defend body temperature.

Hibernating mammals use brown fat in a different but related context. It helps produce heat during periodic arousals from torpor and supports the return to normal body temperature. Adult humans retain some active brown fat, but its amount generally declines with age. Brown fat should not be confused with white adipose tissue, whose main role is energy storage.