Although popular terminology distinguishes between warm-blooded animals (mainly mammals and birds) and cold-blooded animals (all the others), this description is inaccurate. Instead, scientists favor a distinction between endotherms (from the Greek endon, “within”), which generate most of their own heat through internal processes; and ectotherms (from the Greek ektos, “outside”), which get most of their heat from the environment. Contrary to popular belief, ectotherms do not just passively adopt the local ambient temperature. Like endotherms, ectotherms actively regulate their body temperature, but they do so through behavioral means, such as by moving to warmer or cooler locations. And both endotherms and ectotherms will seek out a preferred environmental temperature, which varies from species to species.
The advantages of endothermy come at a cost
No one knows whether endothermy arose in a common ancestor of the birds and mammals or arose separately in these two lines. What we do know is that endotherms pay substantial costs for maintaining a high body temperature and keeping it within narrow limits. Much food energy is used up in the production of heat, and elaborate regulatory systems are required.
Why evolve such a complicated and costly system, compared to the more economical system of ectotherms? One obvious advantage is greater independence from environmental conditions, allowing endothermic animals to forage in a wider variety of settings. A second advantage of endothermy involves the use of oxygen. Ectotherms rely on bursts of intense anaerobic muscular activity—activity stoked by chemical reactions that postpone the requirement for oxygen—so after a few minutes, the animal must rest and repay the oxygen debt. But because they need to constantly stoke the chemical reactions through which they generate heat (discussed next), endotherms evolved a greater capacity for oxygen utilization. This improved oxygen capacity enables intense aerobic muscular activity—activity without an oxygen debt—over much longer periods of time (A. F. Bennett and Ruben, 1979). Ectotherms can escape from and sometimes even pursue endotherms over short distances, but in a long-distance race the endotherm will always win.
Endotherms generate heat through metabolism
The utilization of food by the body is known as metabolism. The breaking of chemical bonds in food releases energy as heat; this food energy is measured in kilocalorie (kcal) (1 kcal, often but incorrectly called a calorie, is enough heat to raise the temperature of a liter of water 1°C). An adult human may generate 600 kcal per hour when exercising strenuously, but only 60 kcal per hour when resting.
When the human body is at rest, about a third of the heat it generates is produced by the brain. As body activity increases, the heat production of the brain does not rise much, but that of the muscles can increase nearly tenfold, so when we are active, our bodies produce a much higher percentage of our body heat. Muscles and gasoline engines have about the same efficiency; each produces about 4 or 5 times as much heat as mechanical work. Some of the main ways the human body gains, conserves, and dissipates heat are shown in Figure 1.
Figure 1 Thermoregulation in Humans
Some of the primary ways that our bodies gain (left), conserve, and lose (right) heat, and their neural controls.
The rate of heat production can be adjusted to suit conditions, by altering the metabolic rate of thermogenic (“heat-making”) tissues. Located in the back and around organs of the trunk, brown fat (adipose tissue that looks brown because it is full of mitochondria) breaks down molecules to produce large amounts of heat, under the control of the sympathetic nervous system. A more familiar thermogenic tissue is the skeletal muscle that cloaks your body; when your body temperature drops below about 36°C, the nervous system instructs muscles to commence shivering. The fivefold increase in oxygen uptake that accompanies extreme shivering shows how metabolically intense this response is.
The rate at which an animal loses heat is directly proportional to the ratio of its surface area to its volume or weight, as Table 1 shows. Small animals with large surface-to-volume ratios, such as the canary, must eat nearly constantly and maintain high metabolic rates in order to maintain the target body temperature. A large animal, like an elephant, has a much lower surface-to-volume ratio, and because it loses heat more slowly, it can afford a lower metabolic rate per gram of body weight.
Bennett, A. F., and Ruben, J. A. (1979). Endothermy and activity in vertebrates. Science 206: 649–654.