6.3 Heat from metabolism
The high rates of metabolism in mammals (and birds) mean that relatively large amounts of heat are produced as a by-product. But this heat is not wasted; it’s used in these animals to warm the body. The fact that a mammal (or bird) keeps its body temperature at a high level (37 °C for humans) ensures that metabolism proceeds at a high and efficient rate, allowing the sustained and elevated pace of life typical of mammals. Animals that depend on internally generated heat (i.e. metabolic heat) to maintain their body temperature are called endotherms. By contrast, ectotherms have a body temperature influenced most sharply by heat from external sources, i.e. from the immediate environment.
You will notice the use of a type of scientific ‘shorthand’ in this section, where letters are used to represent words. The italic letter T is often used to represent ‘temperature’. Here both the temperature inside an animal’s body and the temperature of the surroundings are measured, so we need to differentiate them easily. This is done by adding an appropriate letter, usually subscripted and not italicised, immediately after the T. So here Tb stands for the body temperature of the animal, and Ta for the surrounding temperature. When you see the shorthand version, try to translate it in your head into its real meaning, reading Tb as ‘body temperature’ rather than as ‘tee-little-b’.