Cellular scaling rules for the brains of an extended number of primate species
What are the rules relating the size of the brain and its struc- tures to the number of cells that compose them and their average sizes? We have shown previously that the cerebral cortex, cerebellum and the remaining brain structures in- crease in size as a linear function of their numbers of neurons and non-neuronal cells across 6 species of primates. Here we describe that the cellular composition of the same brain structures of 5 other primate species, as well as humans, con- form to the scaling rules identified previously, and that the updated power functions for the extended sample are simi- lar to those determined earlier. Accounting for phylogenetic relatedness in the combined dataset does not affect the scal- ing slopes that apply to the cerebral cortex and cerebellum, but alters the slope for the remaining brain structures to a value that is similar to that observed in rodents, which raises the possibility that the neuronal scaling rules for these struc- tures are shared among rodents and primates. The confor- mity of the new set of primate species to the previous rules strongly suggests that the cellular scaling rules we have identified apply to primates in general, including humans, and not only to particular subgroups of primate species. In contrast, the allometric rules relating body and brain size are highly sensitive to the particular species sampled, suggest- ing that brain size is neither determined by body size nor together with it, but is rather only loosely correlated with body size.
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