Damon Clark and colleagues of the Department of Molecular Biology, Princeton University compared the relative sizes of different brain areas to total brain size. In the 10 May Nature they report that within a mammalian group the relative sizes of different brain areas are fairly constant, despite huge variations in brain size (Nature 2001, 411:189-193). For example, although the human brain is much larger than that of the lemur — a small insectivorous primate — the brain area proportions were found to be similar, and formed a group with those of other primates.

Clark et al defined a 'cerebrotype', a species-by-species measure of brain composition. For example, in tree shrews the fraction of the brain occupied by the telencephalon is 61% and that devoted to the cerebellum is 13%. The combination of such proportions makes up the cerebrotype for the species. Between mammalian groups, they found differences...