Red colobus monkeys display alternative behavioral responses to the costs of scramble competition

Abstract

Food competition is an expected cost of group living. It is therefore puzzling that there is little evidence for competition among group-living folivorous monkeys, for example, daily travel distance does not seem to increase with group size. It is even more puzzling that folivores do not form larger groups despite this apparent lack of food competition. This has become known as the folivore paradox, and to date, there is no broadly accepted theoretical solution. However, there have been no multigroup studies that have controlled for the potentially confounding effects of variation in habitat quality. We studied 9 groups of red colobus monkeys (Procolobus rufomitratus) in Kibale National Park, Uganda, and controlled for spatial and temporal variation in food availability. We found that larger groups occupied larger home ranges than smaller groups and that group size was related to increased foraging effort (longer daily travel distance), increased group spread, and reduced female reproductive success. Our results also suggest that monkeys in larger groups spent more time feeding and less time engaged in social behavior. These results suggest that folivorous red colobus monkeys experience within-group scramble competition and possess a suite of behavioral responses that may mitigate the cost of competition and represent adaptations for group living. The results offer insight into the folivore paradox and the evolutionary ecology of group size.