Contest competition for fruit and space among wild chimpanzees in relation to the vertical stratification of metabolizable energy

Abstract

Variance in the quality of food sites exploited by group-living animals should allow dominant individuals to gain feeding advantages. In some cases of eating in large patches, however, the amount of agonism can be puzzlingly low. We studied this problem in wild adult chimpanzees, Pan troglodytes, eating ripe fruits in large tree crowns, a context in which little aggression has previously been recorded. Observations were conducted from within the same tree crown as feeding chimpanzees, which allowed us to obtain high-quality data on food distribution, feeding behaviour and aggression. We assessed energy availability and chimpanzee behaviour in relation to their feeding depth. Deeper food sites were associated with less metabolizable food energy (kcal/m3) and lower energy intake rates (kcal/min). Energy availability also varied by tree type (higher in drupe trees than in fig trees) and fruit ripeness (higher in riper fruits). Males were dominant to females, and within both sexes aggressive interactions conformed to a linear hierarchy. Overall higher-ranking individuals used aggression and dominance to access superior feeding sites and achieve higher rates of energy intake, but the effects of dominance varied according to tree type, sex and party composition. Our study shows that covert contest competition over feeding sites gave dominant individuals an important advantage even in very large patches.