Endotherms

Endotherms are animals that maintain their body temperature largely through internal metabolic heat production rather than mainly by environment. This physiological strategy allows relatively stable body temperatures across a range of external conditions, supporting high levels of activity and broad geographic distribution. The term typically applies to birds and mammals, which have high basal metabolic rates, insulating features such as fur, hair, or feathers, and efficient circulatory and thermoregulatory systems. Some endotherms also exhibit heterothermy, by entering torpor or hibernation to conserve energy during resource-scarce periods.

Endotherms regulate heat through several mechanisms: metabolic heat is produced by cellular respiration; insulation reduces heat loss; superficial or peripheral vasoconstriction and vasodilation adjust heat distribution; evaporative cooling through sweating or panting helps prevent overheating; behavioral strategies such as basking, burrowing, or migrating also contribute.

The evolutionary origin of endothermy occurred independently in the lineages leading to birds and to mammals. In addition to these major groups, some fish show regional endothermy or whole-body endothermy, such as tuna, certain sharks, and the opah, enabling active swimming in cooler waters.

Ecological implications include the ability to remain active in cold climates and at night, sustain flight, or hunt during diverse conditions. These advantages come with high energy demands, requiring abundant food resources and efficient energy management.