Figure 9.

Scaling of numbers of neurons in the olfactory bulb as a function of numbers of neurons in the cerebral cortex varies across clades, while numbers of neurons in the cerebellum vary coordinately with numbers of neurons in the cerebral cortex across all clades. Each symbol represents the average values for the structures indicated in one species (afrotherians, blue; glires, green; eulipotyphlans, orange; primates, red; scandentia, gray; artiodactyls, pink). (A) Scaling of numbers of neurons in the olfactory bulb as a function of numbers of neurons in the cerebral cortex across species. Power functions plotted have exponents 2.129 ± 0.428, p = 0.0156 (eulipotyphlans, in orange), and 0.771 ± 0.188, p = 0.0046 (afrotherians, glires and scandentia, in green). The phylogenetic scheme on the left indicates in blue the clades that share the same neuronal scaling rules for the allocation of neurons in the olfactory bulb relative to the cerebral cortex, and the clades that have diverged from the presumed ancestral scaling rules (artiodactyls, eulipotyphlans, and primates). Primates are considered to also diverge from the ancestral scaling rules given their non-conformity to the relationship that applies jointly to afrotherians, glires, and scandentia. (B) Scaling of numbers of neurons in the cerebellum as a function of numbers of neurons in the cerebral cortex across species. The phylogenetic scheme on the left indicates in blue that all clades share similar neuronal scaling rules for the allocation of neurons in the cerebellum relative to the cerebral cortex. Power functions plotted are overlapping and have exponents 0.867 ± 0.108, p < 0.0001 (primates, in red), 0.904 ± 0.110, p = 0.0038 (artiodactyls, in pink), and 1.066 ± 0.111, p < 0.0001 (afrotherians, glires and scandentia, in green). The ensemble of species can be fitted by a linear function of slope 4.12 (p < 0.0001, not plotted).