Relationships between cerebral indices for 'extra' cortical parts and ecological categories in anthropoids.

The relationships between cerebral indices for 'extra' cortical parts associated with advanced functions [Jerison's 'extra' neurons (Nc), Hofman's 'extra' cortical volume (Ve)] and ecology and social structure were examined for a total of 86 species of anthropoids (28 species of New World monkeys, 48 species of Old World monkeys and 10 species of apes). The species were divided into a total of 39 'congeneric groups' of species which share common ecology and social structure (in most cases, congeneric groups are synonymous with genera). Both Nc and Ve were significantly larger for polygynous congeneric groups than for monogynous congeneric groups in the case of New World monkeys. In the case of Old World monkeys, both Nc and Ve were significantly larger for terrestrial congeneric groups than for arboreal congeneric groups. In the case of apes, although complete analyses could not be performed because of the limited size of the sample for which data were available, both Nc and Ve appeared to be larger for polygynous/terrestrial apes than for monogynous/arboreal apes. These results suggest that the expansion of the cerebral cortex in anthropoids may be associated with terrestriality and polygyny.