Neuronal scaling rules for primate brains: the primate advantage.

In what concerns cognitive abilities, primates usually outrank other mammals of similar, or even larger, brain size, as illustrated by comparisons between a macaque monkey and a capybara; a chimpanzee and a cow; or a human and a dolphin, whale, or elephant. Such a cognitive advantage is inconsistent with the traditional view of brain scaling in mammalian evolution as a homogeneous phenomenon regarding numbers of neurons and neuronal density, with brains of different sizes viewed as similarly scaled-up or scaled-down versions of a shared basic plan. Here, I will argue, instead, that different neuronal scaling rules apply to different mammalian orders and that the particular rules that apply to primates are such that endow us with an advantage over other mammals that is likely to have important cognitive consequences: a larger number of neurons concentrated per volume in the brain.