Scalable architecture in mammalian brains

Comparison of mammalian brain parts has often focused on differences in absolute size1,2,3, revealing only a general tendency for all parts to grow together2. Attempts to find size-independent effects using body weight as a reference variable1 obscure size relationships owing to independent variation of body size4 and give phylogenies of questionable significance5. Here we use the brain itself as a size reference to define the cerebrotype, a species-by-species measure of brain composition. With this measure, across many mammalian taxa the cerebellum occupies a constant fraction of the total brain volume (0.13 ± 0.02), arguing against the hypothesis that the cerebellum acts as a computational engine principally serving the neocortex3. Mammalian taxa can be well separated by cerebrotype, thus allowing the use of quantitative neuroanatomical data to test evolutionary relationships. Primate cerebrotypes have progressively shifted and neocortical volume fractions have become successively larger in lemurs and lorises, New World monkeys, Old World monkeys, and hominoids, lending support to the idea that primate brain architecture has been driven by directed selection pressure4. At the same time, absolute brain size can vary over 100-fold within a taxon, while maintaining a relatively uniform cerebrotype. Brains therefore constitute a scalable architecture.