THE INFLUENCE OF CHIMPANZEE PREDATION ON GROUP-SIZE AND ANTIPREDATOR BEHAVIOR IN RED COLOBUS MONKEYS

The extent to which predation influences the grouping patterns and beh avioural ecology of non-human primates is a matter of continuing debat e, but there are few held studies in which predation has been observed frequently enough to test the hypothesis that predation is an importa nt influence. Group size and anti-predator behaviour of red colobus mo nkeys, Colobus badius tephrosceles, were studied in relation to predat ion by chimpanzees, Pan troglodytes schweinfurthii, in Gombe National Park, Tanzania. Predation by Gombe chimpanzees had a major impact on r ed colobus group size and structure. Mean group size of red colobus li ving in the core hunting area of their chimpanzee predators was 46% sm aller than the mean size of groups living in the periphery of the chim panzees' hunting range. Chimpanzees fed mainly upon immature red colob us monkeys at Gombe, and predation by chimpanzees was the likely limit ing factor on red colobus group size and population size. Despite high levels of predation, there were few differences between large and sma ll groups in likely predator avoidance behaviour: foraging and resting height, or nearest-neighbour proximity, for any age/sex class when th ey were not in the proximity of chimpanzees. When parties of chimpanze es were present the spatial positioning of colobus changed; females an d immatures increased their nearest-neighbour scores to adult males, a nd adult males decreased their nearest-neighbour distances as a part o f their communal defence tactics. Predation by chimpanzees elicits dif ferent detection and defence strategies from red colobus than do aeria l (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.