Birds and mammals maintain their body temperatures more or less constant and independent of variations in environmental temperature. To maintain a constant body temperature, there is one fundamental requirement: The rate of heat loss must equal the rate of heat production. This simple fact is basic to all considerations of temperature regulation.

Both heat production and heat loss can be varied through a number of physiological mechanisms. Most mammals and birds are very successful in achieving balance and maintaining a constant core temperature with only minor fluctuations. One regular fluctuation is the change in core temperature with the day-and-night cycle, the night temperature being a couple of degrees lower than the day temperature in diurnal animals, and vice versa for nocturnal animals. The body temperature is also increased during physical activity, but we shall not be concerned with any of these fluctuations.

What do we mean by body temperature? This is by no means a simple question, because the different parts of the organism are not all at the same temperature. Body temperature in mammals and birds is usually taken to mean the temperature of the deeper abdominal organs, sometimes referred to as the core temperature, and often measured as the deep rectal temperature.

Scaling of heat loss

We saw earlier that various groups of warm-blooded vertebrates have characteristic ranges of body core temperatures, without any obvious relationship to body size (Table 16.1). In each group, the body temperature is maintained within a few degrees as a scale-independent variable.