Evolution of metabolic scaling among the tetrapod: effect of phylogeny, the geologic time of class formation, and uniformity of species within a class.

The metabolic scaling in the animal has been discussed for over 90 years, but no consensus has been reached. Our analysis of 2126 species of vertebrates reveals a significant allometric exponent heterogeneity. We show that classes of terrestrial vertebrates exhibit the evolution of metabolic scaling. Both the allometric coefficient "a" and the allometric exponent "b" change naturally, but differently depending on the geological time of group formation. The allometric coefficient "a" shows the measure of the evolutionary development of systems that forms resting metabolism in animals. Endothermic classes, such as birds and mammals, have a metabolic rate that is in an order of magnitude higher than that in ectothermic classes, including amphibians and reptiles. In the terrestrial vertebrate phylogeny, we find that the metabolic scaling is characterized by 3 main allometric exponent values: b = 3/4 (mammals), b > 3/4 (ectotherms, such as amphibians and reptiles), and b < 3/4 (birds). The heterogeneity of the allometric exponent is a natural phenomenon associated with the general evolution of vertebrates. The scaling factor decreases depending on both the external design and the size (birds vs mammals) of the animal. The metabolic rate and uniformity of species within a class increase as the geological start date of formation of the class approaches the present time. The higher the mass-specific standard metabolic rate in the class, the slower metabolic rate grows with increasing body size in this class. Our results lay the groundwork for further exploration of the evolutionary and ecological aspects of the development of metabolic scaling in animals.