Warm-blooded animals maintain a constant body temperature. This means that they do not depend upon the temperature of their environment.
Warm-blooded animal species can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The only known living homeotherms are birds and mammals, though ichthyosaurs, plesiosaurs and dinosaurs are believed to have been homeotherms. Other species have various degrees of thermoregulation.
Animal body temperature control varies by species, so the terms "warm-blooded" and "cold-blooded" (though still in everyday use) suggest a false idea of there being only two categories of body temperature control, and are no longer used scientifically.
Terminology
In general, warm-bloodedness refers to three separate categories of thermoregulation.
Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism (Greek: ἔνδον endon "within" θέρμη thermē "heat"). Some writers[who?] restrict the meaning of endothermy to mechanisms that directly raise the animal's metabolic rate to produce heat. The opposite of endothermy is ectothermy.
Homeothermy maintains a stable internal body temperature regardless of external influence and temperatures. The stable internal temperature is often higher than the immediate environment (Greek: ὅμοιος homoios "similar", θέρμη thermē "heat"). The opposite is poikilothermy. Mammals and birds are homeothermic.
Tachymetabolism maintains a high "resting" metabolism (Greek: ταχύς tachys/tachus "fast, swift", μεταβάλλειν metaballein "turn quickly"). In essence, tachymetabolic creatures are "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.
The variety of thermoregulation types
A large proportion of the creatures traditionally called "warm-blooded", like birds and mammals, fit all three of these categories (i.e., they are endothermic, homeothermic, and tachymetabolic). However, over the past 30 years, studies in the field of animal thermophysiology have revealed many species belonging to these two groups that do not fit all these criteria. For example, many bats and small birds are poikilothermic and bradymetabolic when they sleep for the night (or, in nocturnal species, for the day). For these creatures, the term heterothermy was coined.
Further studies on animals that were traditionally assumed to be cold-blooded have shown that most creatures incorporate different variations of the three terms defined above, along with their counterparts (ectothermy, poikilothermy, and bradymetabolism), thus creating a broad spectrum of body temperature types. Some fish have warm-blooded characteristics, such as the opah. Swordfish and some sharks have circulatory mechanisms that keep their brains and eyes above ambient temperatures and thus increase their ability to detect and react to prey.[1][2][3] Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed