Cellular signatures in the primary visual cortex of phylogeny and placentation.

The long-held view that brain size can be used as an index of general functional capacity across mammals is in conflict with increasing evidence for phyletic differences in cellular organization. Furthermore, it is poorly understood how the internal cellular organization of the brain covaries with overall brain size variation. Using design-based stereology, we quantified glial cell and neuronal densities in the primary visual cortex of 71 mammalian species (spanning 11 orders) to test how those cellular densities are influenced by phylogeny, behavior, environment, and anatomy. We further tested cellular densities against mode of placentation to determine whether a relationship may exist. We provide evidence for cellular signatures of phylogenetic divergence from the mammalian trend in primates and carnivores, as well as considerably divergent scaling patterns between the primate suborders, Strepsirrhini and Haplorrhini, that likely originated at the anthropoid stem. Finally, we show that cellular densities in the mammalian cortex relate to the variability of maternal resources to the fetus in a species.