Endotherm

An endotherm generates most of its body heat internally, as a by-product of a high metabolic rate, and can keep its core warm and fairly constant even when its surroundings are cold. Mammals and birds are the classic endotherms, and they evolved the ability separately, an example of convergent evolution. Most endotherms are also homeotherms, meaning their core temperature stays within a narrow band, close to 37 degrees Celsius in humans.

Staying warm is costly, so an endotherm needs far more food than an ectotherm of the same size. Much of the heat comes from the constant work of mitochondria and ion pumps in every cell. Extra heat can come from shivering and from brown fat, a tissue whose mitochondria release energy as heat instead of capturing it as ATP. Fur, feathers and fat slow heat loss, and the hypothalamus in the brain acts as a thermostat: it narrows or widens blood vessels in the skin and triggers shivering, sweating or panting to hold the body near its set point.

The reward is the freedom to stay active at night and in cold climates. Some fishes, such as tunas, and some large flying insects warm parts of their bodies, which shows that endothermy comes in degrees.