Correlations of cerebral indices for 'extra' cortical parts and ecological variables in primates.

Correlations between cerebral indices for 'extra' cortical parts associated with advanced functions (Jerson's extra neurons, Nc; Hofman's extra cortical volume, Ve, and Hofman's extra cortical surface, Se) and ecological variables, namely troop size (TS), size of home range of one troop (HRt), and size of home range per individual (HRi) were examined in 12 species of prosimians, 8 species of New World monkeys, and 27 species of Old World monkeys. All three of the cerebral indices, Nc, Ve, and Se were, among the ecological variables examined, positively and most closely correlated with TS in the prosimians, with HRt in the New World monkeys, and with HRi in the Old World monkeys. These results suggest that the cerebral cortex may have expanded in relation to growth of TS for prosimians, in relation to expansion of HRt for New World monkeys, and in relation to expansion of HRi for Old World monkeys. It is, therefore, possible that the cerebral cortex may have expanded in conjunction with changes in different ecological factors in different taxonomic groups of primates.