The production of heat within tissues to raise body temperature or as an adaptive response (adaptive thermogenesis) to ‘burn off’ excess food energy intake. Heat production occurs especially in birds and mammals, animals that maintain their temperature within a narrow range (i.e. endotherms), but is also found in some ‘cold-blooded’ vertebrates and invertebrates. There are two types of thermogenesis, of which the more familiar is shivering. This involves repeated rapid contractions of antagonistic sets of skeletal muscles, which produce little net movement so that most of the chemical energy (in the form of ATP) is converted to heat rather than mechanical work. Many endothermic insects, such as bees and moths, perform synchronous contraction of their flight muscles to generate heat before taking off in cold conditions. Nonshivering thermogenesis takes place in fat cells (adipose tissue) and involves the breakdown of stored fat to generate heat in situ instead of its being transported to the liver for conversion to ATP. This process is activated by the sympathetic nervous system and is accomplished in two ways: nonproductive cyclical active transport of ions across the fat-cell plasma membrane and uncoupling of electron transport from ATP synthesis within the fat-cell mitochondria (see uncoupling protein). The net result is release of energy from existing ATP and oxidation of fat to produce heat instead of ATP. Certain mammals have deposits of a special adipose tissue called brown fat that is adapted to provide the body with bursts of intense heat. Stimulation of brown fat oxidation enables rapid warming during the arousal of hibernating animals, for example. Brown fat deposits are also present in human babies and other neonate mammals, to help protect them against hypothermia, and in adult humans, where they provide a means for cold-stimulated thermogenesis. The latter ability is associated with maintaining a lean body mass. Another form of adaptive thermogenesis is the ability to dissipate excess food energy as heat, instead of converting it to fat. In humans, such an ability can enable an individual to maintain food intake without becoming overweight, and seems to have a strong degree of genetic control. It may have evolved as an adaptation when humans consumed large quantities of energy-rich but nutrient-poor foods. The mechanisms involved are significant for our understanding of the causes and treatment of human obesity.