Coordinated scaling of the number of neurons in the cerebral cortex and cerebellum of mammals. (A) The number of neurons in the cerebellum covaries with the number of neurons in the cerebral cortex across all species in a way that can be described as a linear function of slope 4.2 (p < 0.0001, r² = 0.995). (B) Increased relative cortical mass does not reflect an increased relative number of brain neurons. Each point represents the average values for one species (insectivores, filled gray symbols; rodents, filled black symbols; primates, open black symbols; scandentia, open gray symbols). Circles, relative mass and relative number of brain neurons in the cerebral cortex; squares, relative values for cerebellum. All Spearman correlation p-values > 0.2. Data from Herculano-Houzel et al. (2006, 2007, 2011), Azevedo et al. (2009), Sarko et al. (2009), and Gabi et al. (2010); h, human datapoints.