Generating and conserving heat
Sustained energy output of an endothermic animal (mammal
) and an ectothermic animal (reptile
) as a function of core temperature
This image shows the difference between endotherms and ectotherms. The mouse is endothermic and regulates its body temperature through homeostasis. The lizard is ectothermic and its body temperature is dependent on the environment.
Many endotherms have a larger number of mitochondria per cell than ectotherms. This enables them to generate heat by increasing the rate at which they metabolize fats and sugars. Accordingly, to sustain their higher metabolism, endothermic animals typically require several times as much food as ectothermic animals do, and usually require a more sustained supply of metabolic fuel.
In many endothermic animals, a controlled temporary state of hypothermia conserves energy by permitting the body temperature to drop nearly to ambient levels. Such states may be brief, regular circadian cycles called torpor, or they might occur in much longer, even seasonal, cycles called hibernation. The body temperatures of many small birds (e.g. hummingbirds) and small mammals (e.g. tenrecs) fall dramatically during daily inactivity, such as nightly in diurnal animals or during the day in nocturnal animals, thus reducing the energy cost of maintaining body temperature. Less drastic intermittent reduction in body temperature also occurs in other, larger endotherms; for example human metabolism also slows down during sleep, causing a drop in core temperature, commonly of the order of 1 degree Celsius. There may be other variations in temperature, usually smaller, either endogenous or in response to external circumstances or vigorous exertion, and either an increase or a drop.
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. The brain generates about 16% of the total heat produced by the body.
Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume. Small warm-blooded animals have insulation in the form of fur or feathers. Aquatic warm-blooded animals, such as seals, generally have deep layers of blubber under the skin and any pelage that they might have; both contribute to their insulation. Penguins have both feathers and blubber. Penguin feathers are scale-like and serve both for insulation and for streamlining. Endotherms that live in very cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have specialised structures of blood vessels in their extremities that act as heat exchangers. The veins are adjacent to the arteries full of warm blood. Some of the arterial heat is conducted to the cold blood and recycled back into the trunk. Birds, especially waders, often have very well-developed heat exchange mechanisms in their legs—those in the legs of emperor penguins are part of the adaptations that enable them to spend months on Antarctic winter ice. In response to cold many warm-blooded animals also reduce blood flow to the skin by vasoconstriction to reduce heat loss. As a result, they blanch (become paler).
In equatorial climates and during temperate summers, overheating (hyperthermia) is as great a threat as cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water evaporation in the breath, and/or flushing, increasing the blood flow to the skin so the heat will radiate into the environment. Hairless and short-haired mammals, including humans, also sweat, since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ears like radiators in automobiles. Their ears are thin and the blood vessels are close to the skin, and flapping their ears to increase the airflow over them causes the blood to cool, which reduces their core body temperature when the blood moves through the rest of the circulatory system.