Physiologists have long been aware that body mass is the main determinant of basal metabolic rate (BMR) where BMR is the minimum metabolic rate of post-absorptive inactive endotherms in their rest phase and in a thermal neutral zone. Physiologists also realised that BMR scales as a power function of body mass. However, there has been much debate about the other factors that account for BMR variation in different species with the same body mass. Diet and phylogenetic history have received the most attention as factors thought to explain this variation although, their relative importance has been controversial as different statistical methods and selection techniques have been used to categorize the animals. Agustí Muñoz-Garcia and Joseph Williams of The Ohio State University set out to put an end to the controversy. They stringently compiled data for BMR, diet and home range size(a proxy for activity level) of 58 species of carnivores and tested the hypothesis that BMR is associated with home range size and diet using both of the two statistical methods employed in previous studies to determine if a correlation exists between BMR and a factor of interest.
The team applied conventional least-squares regression (CLSR), which does not include phylogeny in its analysis, and phylogenetic independent contrasts(PIC), which does include phylogeny in its analysis, to test for a correlation. Analysing the results, the team found that, regardless of the statistical method they used, exclusive meat-eaters had larger home ranges and higher BMRs than carnivores that included vegetation in their diet. In this way, the team successfully eliminated the dissonance between previous studies that had used only one or other statistical test and convincingly showed that BMR is strongly correlated with home range size and diet; namely, the proportion of meat in the diet.
In an attempt to explain why metabolism is elevated in carnivore species that eat entirely meat compared with those that do not, Muñoz-Garcia and Williams advance a novel, plausible and testable biochemical `muscle performance' hypothesis that provides a possible scenario of how natural selection acts to modify the physiology of carnivores. Muñoz-Garcia and Williams suggest that BMR is elevated in exclusive meat-eating carnivores because natural selection has designed their muscle structure for endurance.
Muñoz-Garcia and Williams reason that vertebrate-eating carnivores will have a higher proportion of endurance fibers with high densities of mitochondria in their muscles since muscle tissue is the primary determinant of total oxygen consumption, accounting for 35% of BMR. Natural selection should have maximized muscle movement efficiency in exclusive meat-eating species because foraging for vertebrate prey requires movement over larger home ranges; highly efficient endurance muscles should have a higher density of mitochondria compared with other muscle fiber types and, as such, have an elevated oxygen consumption. The team argues that the potentially higher proportion of endurance muscles, with their high oxygen demand, in solely meat-eating carnivores accounts for the elevated BMR in these species compared with carnivore species that are partially vegetarian and enthusiastically invites the rest of us to test their predictions.
