Let us return to the warm-blooded vertebrates, birds and mammals. We have seen that for these the empirical relationship between metabolic rate and body size is known with much greater certainty than for cold-blooded vertebrates and invertebrates, and the reason is undoubtedly related to their relatively constant body temperature. To understand the regular relationship to body size, we should examine those groups that are best known, also in regard to related physiological information.

Body temperature

First of all, the regularity of metabolic rates in warm-blooded vertebrates is undoubtedly related to their relatively constant body temperatures. This eliminates temperature as a variable here, whereas in cold-blooded animals it causes innumerable difficulties in determining a definite metabolic rate, both for the short term and for the long term (acute effects of a temperature change as well as long-term acclimatization).

How constant is the body temperature of birds and mammals? For the moment, let us disregard the small number of mammals and even smaller number of birds that in connection with periods of torpor or hibernation can undergo profound decreases in body temperature. Here we are concerned with the normal or usual temperature of active animals.

Do small and large mammals have similar temperatures, or is the body temperature of mammals related to body size? This question was discussed by Morrison and Ryser (1952), who examined published material supplemented with a considerable number of observations of their own, especially on smaller animals.