Reducing complex diets to simple rules: food selection by olive baboons

Determinants of diet in free-ranging baboons Papio anubis were investigated. Foods and non-foods differed significantly in chemical composition, the former being higher in protein and lower in fibre and phenolics. Within the range of items selected, biomass was found to be the single most important factor affecting percentage of total intake and time spent feeding (Figure 1). When this effect of biomass was statistically controlled, independent effects of chemical composition (primarily protein content) and harvesting rate were found. However, while the harvesting rates of foods were positively related to their percentage contribution to total intake, they were either uncorrelated, or, in one case, actually negatively correlated with the percentage of feeding time. A post-hoc model based on Charnov's (1976) marginal value theorem is developed to account for these latter results, and direct evidence for rate-maximising patch use is then presented. It is argued that the apparent complexity of primate diets may in large part be reducible to relatively simple optimization criteria.