An explanation of the relationships between mass, metabolic rate and characteristic skeletal length for birds and mammals.

A theory explaining the relationships between metabolic rate and body mass for birds and mammals is developed in terms of the mitochondrion theory of aerobic metabolism, the sliding filament theory of muscle contraction, simple models of vertebrate anatomy and activity, and other propositions. Both body mass and metabolic rates are shown to result from a homogeneous vertebrate design quantitatively expressed in terms of a set of nearly invariant parameters with five degrees of freedom: propulsion technique, mitochondrion capability, non-skeletal muscle mass exponent, characteristic skeletal length, and sturdiness factor. The first three of the degrees of freedom are phylogenetic group specific. The last two vary considerably even within a single species. The theory is shown to agree satisfactorily with placental mammal, marsupial mammal, passerine bird and nonpasserine bird data. Algorithms for determining metabolic rates and body mass as functions of skeletal characteristic length are developed and quantitative estimates of the constants occurring in the algorithms are given.