Brain, Sociobiology, and Evolution in Primates

Abstract

Socialization in primates can be understood essentially as a function of the information processing ability of the CNS, which can be roughly measured in terms of relative brain size in closely related species groups. Both the cephalization constant (Hemmer 1971, 1974) and the extra neuron number (Jerison 1964, 1973) may be used for relevant quantification, as there is a highly significant correlation of both parameters in primates (7 ape species: r = 0.97, 20 Old World monkey species: r = 0.99; Hemmer 1978). The author has shown in a previous paper (Hemmer 1979) close negative correlations of relative brain size and the social organization as expressed in troop size (r = −0.92) and of relative brain size and reproductive success in captivity (r = −0.90) in the genus Lemur. This has been discussed as reflecting changes in the level of sensibility to stressing influences (psychosocial tolerance) that is caused in over 80% by the informations processing ability. The lemur example led to the conclusion that the sociobiological place of each primate species may be seen as resultant from two opposite processes, both of them depending on progressive cephalization, i.e., decreasing psychosocial tolerance and increasing social learning plasticity, paired with aspiration to social ties. This resulted in an oscillation concept of social group evolution in primates. For progressive primate cephalization a continual change has been postulated between zones of high social tolerance, large social group size, high population density in relatively open habitats and relative evolutionary stability, and zones of low tolerance, small group size, low population density mostly in forest habitats and high evolutionary lability.