Fig. 2.

Although brain regions generally scale with the rest of the brain, some brain regions exhibit grade shifts in birds and in mammals. a A plot of isocortex volume versus rest of brain size (i.e. brain minus telencephalon) shows that primates (e.g. gray mouse lemurs, rhesus monkeys, and humans) exhibit a disproportionately enlarged isocortex relative to many other mammals. b In contrast, a plot of limbic system volume (i.e. amygdala, hippocampus, schizocortex, and septum) regressed against rest of brain size shows that the limbic system is disproportionately smaller in primates than in many other mammals. c A plot of telencephalon size versus rest of brain size shows that parrots, songbirds, as well as other birds exhibit a disproportionately enlarged telencephalon relative to many other birds (e.g. pigeons, shorebirds, falcons, and galliform birds). a, b Separate linear regressions were fitted through the data in primates and insectivores. c A linear regression was fitted through the data in parrots and songbirds and another linear regression was fitted through the data of the remaining birds. a, b Data are from Stephan et al. [1981]. c Data are from Iwaniuk and Hurd [2005].