Has predation shaped the social systems of arboreal primates

Abstract

I studied antipredator behavior in two species of monkeys to elucidate the role of predation in shaping the social systems of arboreal primates. I compared the responses of monkeys to auditory and visual contact with predators to response elicited by sound playback experiments using the recorded calls of predators. Changes in vigilance and aggregation persisting up to 30 min after predator encounter occurred in both cases. Measures of vigilance shed light on individual perceptions of risk, while aggregation measures—intragroup spatial cohesion and polyspecific associations—permit direct inference about the protective benefits of grouping for the monkeys. They responded to real predator encounters and simulations in similar ways. Thus, sound playbacks of predator vocalizations are effective to simulate predator proximity. Contrary to predictions, predator encounters did not lead invariably to increased cohesion within groups or to increased time spent vigilant. Moreover, behavior in polyspecific associations was no different from that in single-species groups. Only red colobus encountering chimpanzees behaved as predicted by increasing vigilance and intragroup cohesion. The red colobus social system may have developed to protect against chimpanzee attack. In contrast, red-tailed monkey encounters with raptors and chimpanzees involved no change in time spent vigilant, coupled with decreases in intragroup cohesion. I conclude that predation is not a uniform selective pressure and patterns of social behavior within groups do not predict antipredator behavior.