Warm-blooded animals

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 homeotherms are birds and mammals. Other species have various degrees of thermoregulation.

Animal body temperature control varies by species, so the terms "warm-blooded" and "cold-blooded" give a false idea of there being only two categories of body temperature control, and are no longer used scientifically. The terms are still in everyday use.

In general, warm-bloodedness refers to three separate categories of thermoregulation.

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. Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed.

...
Wikipedia