Warm-blooded

In biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly control their body temperature by regulating their metabolic rates (e.g. increasing their metabolic rate as the surrounding temperature begins to decrease).

Both the terms "warm-blooded" and "cold-blooded" have fallen out of favor with scientists, because of the vagueness of the terms, and due to an increased understanding in this field. Body temperature types do not fall into simple either/or categories. Each term may be replaced with one or more variants. Body temperature maintenance incorporates a wide range of different techniques that result in a body temperature continuum, with the traditional ideals of warm-blooded and cold-blooded being at opposite ends of the spectrum.

Definitions of warm-bloodedness
Warm-bloodedness generally refers to three separate aspects of thermoregulation.
 * 1) Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or fat burning (Greek: endon = "within", thermē = "heat"). Some writers restrict the meaning of "endothermy" to mechanisms which directly raise the animal's metabolic rate in order to produce heat. The opposite of endothermy is ectothermy.
 * 2) Homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This temperature is often (though not necessarily) higher than the immediate environment (Greek: homoios = "similar", thermē = "heat"). The opposite is poikilothermy.
 * 3) Tachymetabolism is the kind of thermoregulation used by creatures that maintain a high resting metabolism (Greek: tachys/tachus = "fast, swift", metabolēn = "throw beyond"). Tachymetabolic creatures are, essentially, "on" all the time. Though their resting metabolism is still many times slower than their active metabolism, the difference is often not as large as that seen in bradymetabolic creatures. Tachymetabolic creatures have greater difficulty dealing with a scarcity of food.

A large proportion of the creatures traditionally called "warm-blooded" (mammals and birds) fit all three of these categories. However, over the past 30 years, studies in the field of animal thermophysiology have revealed many species belonging to these two groups that don't fit all these criteria. For example, many bats and small birds are poikilothermic and bradymetabolic when they sleep for the night (or day, as the case may be). For these creatures, another term was coined: heterothermy.