Endothermy and Ectothermy

The two great strategies for setting body temperature: making your own heat and defending a steady warmth, or letting the environment and behaviour do the regulating.

Definition

Endothermy is the maintenance of body temperature chiefly by internally generated metabolic heat, while ectothermy is reliance on external heat sources and behaviour to set body temperature; the terms describe the source of heat rather than whether temperature is regulated.

Scope

This topic covers the physiology of the main thermoregulatory strategies: ectothermy, in which body temperature largely follows the environment and is adjusted behaviourally, and endothermy, in which high metabolic heat production maintains a stable, often high temperature. It treats the routes of heat exchange, the mechanisms endotherms use to produce and conserve heat, the thermoneutral zone, intermediate and regional strategies such as heterothermy, and the costs and benefits of each. Coverage is comparative and mechanistic.

Core questions

• By what routes do animals gain and lose heat?
• How do endotherms generate and conserve enough heat to stay warm?
• How do ectotherms regulate their temperature without high metabolic heat?
• What are the energetic costs and ecological benefits of endothermy versus ectothermy?

Key theories

Heat-balance equation

An animal's temperature changes according to the net of metabolic heat production and heat exchanged with the environment by conduction, convection, radiation, and evaporation, so thermoregulation is the management of these terms.

Thermoneutral zone in endotherms

Within a range of ambient temperatures an endotherm can hold its body temperature constant by adjusting insulation and blood flow without extra heat production, and only outside this thermoneutral zone must it increase metabolism or evaporative cooling.

Mechanisms

Heat moves between an animal and its surroundings by conduction, convection, radiation, and evaporation, and body temperature reflects the balance of these with metabolic heat. Ectotherms have low resting metabolism and adjust temperature behaviourally — basking, seeking shade, changing posture — and physiologically by altering blood flow and colour. Endotherms produce abundant heat from a high metabolic rate, retain it with insulation such as fur, feathers, and fat, and regulate loss through vasomotor control; in cold they raise heat production by shivering and, in some, non-shivering thermogenesis in brown fat, while in heat they dissipate it by panting, sweating, or other evaporative means. Within the thermoneutral zone these adjustments suffice without extra metabolic cost. Many animals are heterotherms, mixing strategies in time or across body regions, for example warming only the flight muscles or the brain.

Clinical relevance

Comparative thermal physiology illuminates the responses to cold and heat stress, the role of brown fat in thermogenesis, and the consequences of fever and hypothermia. This entry is educational reference material and not medical guidance.

History

Scholander and Irving's studies of insulation and heat exchange in arctic and tropical animals quantified how endotherms conserve heat, and Bartholomew and others explored the diversity of thermoregulatory behaviour in ectotherms and heterotherms, establishing the comparative physiology of body temperature.

Key figures

• Knut Schmidt-Nielsen
• Per Scholander
• George Bartholomew
• Laurence Irving

Seminal works

• schmidtnielsen1997
• hill2016
• randall2002

Frequently asked questions

Are 'cold-blooded' animals always cold?

No. Many ectotherms keep their bodies quite warm by basking and choosing warm microhabitats; the term simply means their heat comes mainly from the environment rather than from their own metabolism.

Why does endothermy cost so much energy?

Maintaining a high, stable body temperature requires producing heat continuously, so endotherms must eat far more than similar-sized ectotherms to fuel that constant heat production.