Scalable Architecture in Mammalian Brains

Comparison of mammalian brain parts has often focused on differences in absolute size, revealing only a general tendency for all parts to grow together. One attempt to find size-independent effects uses body weight as a reference variable. However this approach introduces additional variability due to independent growth of the body and gives phylogenies of questionable significance. Here we use the brain itself as a size reference to define the cerebrotype, a species-by-species measure of brain compostion. With this measure, across many mammalian taxa the cerebellum occupies a constant fraction of the total brain, arguing against the hypothesis that the cerebellum acts as a computational engine principally serving the neocortex. Mammalian taxa can be well-separated by cerebrotype, thus allowing quantitative neuroanatomical data to be used in testing evolutionary relationships. Primates have successively large neocortices in lemurs and lorises, New World moneys, Old World Moneys, and hominoids, suggesting the action of directed selection pressure. At the same time, absolute brain size can vary up to 50-fold within each taxon while maintaining a uniform cerebrotype. Brains therefore constitute a scalable architecture.