Figure 6.

Neuronal scaling rules for the olfactory bulb differ between eulipotyphlans, artiodactyls, primates and other clades. Top right: scaling of olfactory bulb mass as a function of numbers of neurons in the structure across species. Plotted power functions have exponent of 0.823 ± 0.071, p = 0.0014 (eulipotyphlans, orange), 1.309 ± 0.257, p = 0.0364 (artiodactyls, pink), and 1.185 ± 0.186, p < 0.0001 (in green: scandentia, afrotherians and glires, excluding the capybara; Ribeiro et al., 2014). Bottom right: scaling of neuronal density in the olfactory bulb as a function of numbers of neurons in the structure. Power functions are not significant, but neuronal density is highest in eulipotyphlans and lowest in artiodactyls, which we suggest that branched off the mammalian ancestor when modifications resulted in decreased and increased average neuronal cell sizes, respectively (orange and pink arrows). Each symbol represents the average values for the rest of brain in one species (afrotherians, blue; glires, green; eulipotyphlans, orange; primates, red; scandentia, gray; artiodactyls, pink). The phylogenetic scheme on the left indicates in blue the clades that share the same neuronal scaling rules for the olfactory bulb, and the presumed extension of these shared scaling rules to the common ancestor to the non-artiodactyl clades, while artiodactyls and eulipotyphlans diverged from them.