Brain size scales with body size across large groups of animals, but exactly why this should be the case has not been resolved. It is generally assumed that body size is a general proxy for some more important or specific underlying variable, such as metabolic resources available, surface area of the body, or total muscle mass (which is more extensively innervated than is, e.g., adipose tissue). The present study tests whether brain size in mammals scales more closely with muscle mass (and other components of lean body mass) than with total fat. Felsenstein’s independent comparisons method was used to control for phylogenetic effects on body composition in organ weight data taken from a previously published comparative sample of 39 species in 8 different orders of mammals, all collected and processed by the same researchers. The analysis shows that the size of the central nervous system (CNS) is more closely associated with components of fat-free weight than it is to fat weight. These results suggest a possible explanation for why metabolic resources and brain size both share the same general relationship with body size across mammals. They also suggest that some measure of lean body mass is a more appropriate scaling parameter for comparing brain size across species than is overall body weight.

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