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No two factors other than body temperature and body size have a more profound impact on how animals function. Temperature affects the rate of all metabolic processes and hence how rapidly nerves conduct, muscles contract, enzymes attack substrates, nutrients are absorbed, and tissues are built. As a consequence, temperature affects how animals perceive and react to predators or prey, how fast they can capture prey to digest and extract needed nutrients, and how rapidly they grow and reproduce. The effect of temperature on all of these processes is exponential, so that even a small increment in body temperature (Tb) can have an important effect. It is no wonder that homeothermic birds and mammals, and many poikilotherms, seek to maintain high Tb through the production of metabolic heat or the absorption of solar radiation. Although high Tb confers advantages, it also entails a cost. For homeotherms, maintaining a high and stable Tb requires a constant allocation of energy that provides no direct material benefit. Sometimes this energetic cost of homeothermy is small compared to the benefits. At other times the cost is prohibitive, so that animals may be forced to abandon strict homeothermy to bring energy expenditures into line, either with rates at which they are able to harvest energy or by varying the size of their energy reserves. Size affects the amount of food required to build bodies and sustain cellular integrity: large animals obviously require more food than do small animals. However, the importance of size in biology lies not just in its effect on the absolute requirements of energy and nutrients but, even more so, in its allpervading …