Effect of spatial crowding on aggressive behavior in a bonobo colony

According to a previously proposed coping model (De Waal [1989) Zoo Biol. I(suppl):141-148), gregarious primates will react to crowding by adjusting their behavior in order to limit the amount of aggression displayed. Depending on the duration of the crowding, this can be achieved by the use of different strategies. Chimpanzees (Pan troglodytes) react to a medium-term duration of spatial crowding by an active tension-reduction mechanism whereby allogrooming markedly increases while aggression rises only slightly. Under comparable circumstances, a similar increase in grooming is found in bonobos (Pan paniscus). However, to test the coping model for bonobos, data on aggressive behavior during crowding are indispensable. This study provides such information by comparing the aggressive repertoire of a well-established bonobo colony during a crowded period in winter with those that occurred during an uncrowded control period in summer. Given that during winter the aggression rate differed between food and nonfood contexts, we accounted for a potential effect of food context on aggression. During the crowded period, a significantly higher total frequency of aggression was found than during the control period. However, this increase was small in relation to the reduction in space when compared to similar experiments in other species. Together with observations of increased grooming under crowded conditions, our data confirm the hypothesis that bonobos cope with the increase in tension created by crowding (medium-term duration) by applying a tension-reduction strategy. Our data further show that although not all aggressive behaviors appear to be influenced by a high-density condition, bonobos do not specifically limit their aggressive behaviors to mild, nonprovocative aggressions in a crowded environment. (C) 2004 Wiley-Liss, Inc.