INTRODUCTION

Ecological hypotheses for the evolution of great ape intelligence relate selective pressures for increased intelligence to biological and environmental parameters such as body size, metabolic rate, life history, diet, home range size, habitat stratification, and predation risk (Clutton-Brock & Harvey 1980; Dunbar 1992; Gibson 1986; Milton 1981, 1988; Sawaguchi 1989, 1992). Of these, diet is the ecological selective pressure most frequently invoked to explain the emergence of great ape cognitive abilities. A correlation between diet and relative brain size in primates has long been established; frugivorous primates tend to have relatively larger brains than closely related folivorous taxa (Clutton-Brock & Harvey 1980; Milton 1981, 1988; Sawaguchi 1992). This pattern was most often explained in terms of the differing nutritional properties of fruits and leaves. A high-energy, fruit-based diet, it was thought, released energetic and metabolic constraints, allowing accelerated neonatal brain growth and maintenance of relatively greater adult brain mass (Jolly 1988; Martin 1981). However, the expansion of energy-hungry brain tissue will occur only where it confers an immediate adaptive advantage (Dunbar 1992). In other words, adequate energy supply is a necessary precondition for, but not in itself a sufficient stimulus to, increased encephalization.

Researchers seeking such a stimulus have tended to focus upon the adaptive role of intelligence in solving the unique foraging problems posed by primate diets.