"Cold-blooded" animals (ectotherms) have their body temperature regulated by interactions with the environment, while "warm-blooded" (endothermic) animals have an internal temperature kept constant by homeostatic (stability-ensuring) mechanisms. The terms "cold-blooded" and "warm-blooded" are being phased out as they are misleading — "cold-blooded" animals don't have blood that is necessarily cold, it just varies in temperature based on the outside. Another name for ectotherms are "poikilotherms," which means animals with varying body temperature.

"Cold-blooded" animals are in the majority, and include all invertebrates, reptiles, and everything besides mammals and birds, the only two groups of animals that are warm-blooded. However, part of the reason that the "warm-blooded" and "cold-blooded" terms are no longer scientifically acceptable is that there is no clear-cut distinction, and animals actually fall on a continuum from totally environmentally-regulated temperature to internally regulated. For instance, placental mammals have an average body temperature around 38°C (100°F), marsupials have an average body temperature of about 35°C (95°F), and monotremes (platypus and echidna).

The primary difference between warm-blooded animals and cold-blooded animals is that warm-blooded animals need about 3-10 times more food to survive than cold-blooded animals. Accordingly, they must be 3-10 times better at obtaining food, putting them on an entirely different metabolic and evolutionary level. Some might say that warm-blooded creatures are "superior" because they tend to have far greater stamina, however, they can't move faster than cold-blooded animals (for short bursts), and they starve far more easily than cold-blooded creatures. In terms of biomass, cold-blooded creatures, like all invertebrates, have had greater evolutionary success than warm-blooded creatures, though there is uncertainty about whether the dinosaurs, for instance, were warm-blooded or cold-blooded.

There are several advantages to being endothermic — greater stamina, having only one set of body enzymes that works optimally at a set temperature, and the ability to increase body temperature during freezing weather. A spider caught in a blizzard will freeze solid for sure, but a human being has at least a chance of survival. Cold-blooded animals need to maintain several sets of enzymes for their biological processes, because enzymes are very sensitive to temperature, but warm-blooded animals can just maintain one set. The most useful advantage of being warm-blooded is the stamina boost, however. A warm-blooded creature can outrun a cold-blooded predator as long as they avoid the initial attack, a warm-blooded predator can outrun cold-blooded prey, and warm-blooded animals can forage for a longer time.