Endotherm

Lore & Background

The evolutionary origins of endothermy remain an active area of research, with various hypotheses suggesting it may have emerged among synapsids during the Permian or Triassic periods. Evidence for endothermy has been found in some extinct groups, such as certain synapsids and marine reptiles, but claims about basal archosauromorphs or the earliest amniotes being endotherms are speculative and not established canon. Many endotherms have a larger amount of mitochondria per cell than ectotherms, enabling them to generate heat by increasing the rate at which they metabolize fats and sugars. To sustain their higher metabolism, endothermic animals typically require several times as much food as ectothermic animals do. In many endothermic animals, a controlled temporary state of hypothermia conserves energy by permitting the body temperature to drop nearly to ambient levels, such as in torpor or hibernation. The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain. Heat loss is a major threat to smaller creatures, which have a larger ratio of surface area to volume. Small warm-blooded animals have insulation in the form of fur or feathers, while aquatic warm-blooded animals often have deep layers of blubber. Endotherms in very cold conditions may have specialized blood vessel structures in their extremities that act as heat exchangers. In hot conditions, many warm-blooded animals increase heat loss by panting, flushing, or sweating. Elephants keep cool by using their huge ears like radiators.

Reader's Guide

Endothermy is significant because it provides decreased vulnerability to fluctuations in external temperature, maintaining a constant core temperature for optimal enzyme activity. Endotherms control body temperature by internal homeostatic mechanisms; in mammals, two separate mechanisms—one increasing and one decreasing body temperature—provide a very high degree of control. The major advantage is that endothermic/homeothermic animals can be optimally active at more times during the diurnal cycle in places of sharp temperature variations between day and night, and during more of the year in places of great seasonal temperature differences. However, this is accompanied by the need to expend more energy to maintain constant internal temperature and a greater food requirement. Endothermy may be important during reproduction, expanding the thermal range over which a species can reproduce, and may also provide protection against fungal infection, as fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The overall rate of an animal's metabolism increases by a factor of about two for every 10 °C rise in temperature, limited by the need to avoid hyperthermia. Ectotherms increase their body temperature mostly through external heat sources, so they depend on environmental conditions to reach operational body temperatures, whereas endotherms mostly use internal heat production through metabolic active organs or specialized heat-producing tissues like brown adipose tissue.

Did You Know?

• Only birds and mammals are considered truly endothermic groups of animals.
• Some reptiles, particularly some species of python and tegu, possess seasonal reproductive endothermy only during their reproductive season.
• The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain.
• Endothermy may provide protection against fungal infection, as fungi are fundamentally ill-equipped to thrive at mammalian temperatures.