Scaling of organ masses in mammals and birds: phylogenetic signal and implications for metabolic rate scaling
The persistent enigma of why the whole-body metabolic rate increases hypoallometrically with body mass should be solved on both the ultimate and proximate levels. The proximate mechanism may involve hyperallometric scaling of metabolically relatively inert tissue/organ masses, hypoallometric scaling of metabolically expensive organ masses, a decrease in mass-specific metabolic rates of organs or, more likely, a combination of these three factors. Although there are in data in the literature on the scaling of tissue/organ masses, they do not take phylogenetic information into account. Here, we analyse the scaling of tissue/organ masses in a sample of 100 mammalian species and 22 bird species with a phylogenetically informed method (PGLS) to address two questions: the role of phylogenetic signal in organ/tissue size scaling and the potential role of organ/tissue mass scaling in interspecific metabolic rate scaling. Strong phylogenetic signal was found for the brain, kidney, spleen and stomach mass in mammals but only for the brain and leg muscle in birds. Metabolically relatively inert adipose tissue scales isometrically in both groups. The masses of energetically expensive visceral organs scale hypoallometrically in mammals, with the exception of lungs, with the lowest exponent occurring for the brain. In contrast, only brain mass scales hypoallometrically in birds, whereas other tissues and organs scale isometrically or almost isometrically. Taking into account that the whole-body metabolic rate scales more steeply in mammals than in birds, the mass-specific metabolic rate of visceral organs must decrease with body mass at a much faster rate in birds than in mammals. To explain this striking difference, there is an urgent need to study the metabolic rates of tissues and organs to supplement measurements of the whole-body metabolic rate.