The nonshivering thermogenesis is a widespread phenomenon in mammals. It refers to the processes that produce heat by means other than shivering. It is not certain whether birds produce heat by nonshivering. One of the well-known sites of nonshivering thermogenesis in mammals is the brown adipose tissue, also called brown fat.
Figure: Brown fat deposition in the bat
The brown adipose tissue contains great numbers of mitochondria and is richly vascularised. Release of norepinephrine into the tissue by the sympathetic nervous system results in a great increase in oxidation of lipid and release of heat. Brown fat, like nonshivering thermogenesis is specially prominent in new born individuals, hibernators and cold acclimated adult mammals. The brown fat tends to occur in discrete masses, located in the neck, interscapular region, axillae and abdomen. Among hibernators brown fat is believed to help in rewarming the body during emergence from hibernation. New born mammals use brown fat in routine thermogenesis. The principal mechanism of heat production by brown fat is by uncoupling the oxidative phosphorylation that occurs in the mitochondria. Thus, oxidation of food-stuffs results in the production of heat.