The only warm-blooded ("homeotherm") animals are mammals and birds. The rest are denominated cold-blooded or poikilotherm. Indeed, poililotherm ("with variable temperature") is a proper denomination.

Birds and mammals generate warmth on their own, by burning calories, to maintain a constant body temperature.

The poikilotherms depend on the temperature of the ambient. Reptiles, like lizards, snakes, crocodiles and turtles are known to rely on sunbaths to raise their body temperature.

But many "cold blooded" animals can have a body temperature higher than that of the environment. Many insect species, especially butterflies and moths, vibrate their flight muscles before taking off, so that the body temperature can be higher than that of the environment.

In the case of tuna, researchers discovered a countercurrent heat exchanger. A complicated system of blood vessels, a tuna swimming in the winter can maintain its active swimming muscles 14°C warmer than the surrounding water due to the warmth generated by muscles. This is required to reach the top speed necessary for hunting.

The great white shark and its relatives (like mako sharks) have a "rete mirabile" ("wonderful net" in Latin), a close web-like structure of veins and arteries, located along each lateral side of the shark, that keep heat by warming the cooler arterial blood with the venous blood that has been warmed by the working muscles. This way, these sharks have temperatures of 14°C above the water, while the heart and gills remain at sea-temperature, enabling them to hunt such rapid and agile prey like marine mammals.

The metabolism of the leatherback turtle, the largest in the world, is about four times higher than of other large reptiles; because this species too has counter-current heat exchangers, and a fatty insulation like in whales, this reptile has a body temperature up 18 °C (32 °F) above that of the surrounding water, explaining its presence in such cold waters like off Norway, Labrador, and Alaska, where other reptiles would freeze.

Some say that the leatherback can generate its own body heat (like a mammal). Some larger poikilotherms, because of the large volume compared to the surface area ratio, can keep relatively high body temperatures, a phenomenon called gigantothermy (inertial homeothermy), seen from sea turtles and large sharks, and probably encountered in many dinosaurs and ancient sea reptiles (like ichthyosaurs and plesiosaurs). As many desert lizards can raise their body temperature to over 42° C (a value at which most homeotherms would be dead), and some fish living in thermal waters inhabit waters warmer than 50°C, the question is: are cold blooded animals really 'cold blooded'?