Homeothermic

Homeothermic the ability of an organism to maintain a constant body temperature (usually because that organism is also endothermic)
(homo‚ homeo = same‚ like‚ alike; thermo = heat) ...

homeothermic Having a nearly uniform body temperature, regulated independent of the environmental temperature; "warm blooded."
homeotic genes Genes, identified through mutations, that give developmental identity to specific body segments.

Many homeothermic, or warm-blooded, animals also make use of these techniques at times. For example, all animals are at risk of overheating on hot days in the desert sun , and most homeothermic animals can shiver.

Mammals are homeothermic / must maintain constant body temp
Warm environment / less energy maintains body temp / more organic matter stored / more transferred to consumer
Small organisms / large surface area:volume ratio / lose high amounts of energy ...

Wakelin Barratt has demonstrated that under certain pathological conditions, a warm-blooded (homeothermic) animal may become temporarily cold-blooded (poikilothermic).

Unlike reptiles, birds are homeothermic ("warm blooded"), maintaining a constant body temperature (usually around 40°C) despite wide fluctuations in the temperature of their surroundings.

See also: Blood, Organ, Animal, Human, Muscle