So-called warm-blooded animals control the temperature of their body at quite a high level. This ability is called endothermy. All mammals and birds are endotherms (homeotherms or homoiotherms). The basic source of the heat is chemical energy from the body's metabolism. they have a number of temperature-control devices:
- When they are cold they can do things to make themselves warmer. For example, they "shiver", or shake, or run about, or move into a warmer place. This is because all movement by animals creates heat from the chemical reactions of respiration. So polar bears do not freeze because their metabolism produces heat, and movement produces more heat. During hibernation bears exist several degrees lower that they do in active life. Of course, warm-blooded animals have thermal insulation: hair, feathers, or in water blubber (thick fat). These adaptations help keep heat energy inside the animal.
- When they are warm they sweat to become cooler, or pant or open their feathers, or move to a less hot place and lie down.
In the colder climates many mammals hibernate or aestivate. This means they have a two-level metabolism. Their temperature is held at a high level when they are active, and at a lower level when they are hibernating. This has the advantage of saving energy during times when they cannot get enough food to keep up the higher temperature.
The human body has automatic responses to help regulate temperature. When the external environment heats up, arterioles leading to the capillary loops in the dermis dilate (widen). This increases blood flow to the surface of the skin where its heat can more easily radiate away. This process is called vasodilation:) Sweat glands also produce greater amounts of sweat. This liquid is secreted onto the surface of the skin. Sweat needs energy to turn from a liquid into a gas and evaporate. This energy is called the latent heat of vaporisation. The body supplies this heat and so it cools down as the sweat evaporates.
If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. This occurs when the body is exposed to temperatures of approximately 55 °C; any exposure longer than a few hours at this temperature or up to around 70 °C kills. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.
Palaeontologists are fairly certain that some fossil groups were endotherms. Obvious examples are pterosaurs, whose bodies were covered by hair (or hair-like filaments), and the smaller carnivorous dinosaurs which evolved feathers ("dinobirds"). It is thought that feathers originally functioned to keep body temperature higher. They appear on bird-like dinosaurs before there were any flying birds (Anchiornis huxlei), and they appear on dinosaurs which are too heavy to fly at all.