The influence of chimpanzee predation on group size and anti-predator behaviour in red colobus monkeys

Abstract

The extent to which predation influences the grouping patterns and behavioural ecology of non-human primates is a matter of continuing debate, but there are few field studies in which predation has been observed frequently enough to test the hypothesis that predation is an important influence. Group size and anti-predator behaviour of red colobus monkeys, Colobus badius tephrosceles, were studied in relation to predation by chimpanzees, Pan troglodytes schweinfurthii, in Gombe National Park, Tanzania. Predation by Gombe chimpanzees had a major impact on red colobus group size and structure. Mean group size of red colobus living in the core hunting area of their chimpanzee predators was 46% smaller than the mean size of groups living in the periphery of the chimpanzees' hunting range. Chimpanzees fed mainly upon immature red colobus monkeys at Gombe, and predation by chimpanzees was the likely limiting factor on red colobus group size and population size. Despite high levels of predation, there were few differences between large and small groups in likely predator avoidance behaviour: foraging and resting height, or nearest-neighbour proximity, for any age/sex class when they were not in the proximity of chimpanzees. When parties of chimpanzees were present the spatial positioning of colobus changed; females and immatures increased their nearest-neighbour scores to adult males, and adult males decreased their nearest-neighbour distances as a part of their communal defence tactics. Predation by chimpanzees elicits different detection and defence strategies from red colobus than do aerial (raptors) or terrestrial (felids) ambush predators. This appears to be owing to the social nature of chimpanzee hunting and the effect of chimpanzee party size and composition on hunting success.