INHIBITION OF SOCIAL-BEHAVIOR IN CHIMPANZEES UNDER HIGH-DENSITY CONDITIONS

This is the first study to investigate the short-term effects of high population density on captive chimpanzees (Pan troglodytes). Subjects of the study were 45 chimpanzees living in five different groups at th e Yerkes Regional Primate Research Center. The groups were observed un der two conditions: 1) when they had access to both the indoor and out door sections of their enclosures; 2) during cold days when they were locked into the indoor runs, which reduced the available space by more than half. Under the high-density condition, allogrooming and submiss ive greetings decreased, but juvenile play increased. Remarkably, the rate of various forms of agonistic behavior, such as aggression, bluff charge, bluff display, and hooting, occurred less frequently under th e high-density condition. This general decrease in adult social activi ty, including agonistic behavior, can be interpreted as an inhibition strategy to reduce opportunities for conflict when interindividual dis tances are reduced. This strategy is probably effective only in the sh ort run, however. Behavioral indicators of anxiety, such as rough scra tching and yawning, showed elevated rates, suggesting increased social tension under the high-density condition. (C) 1997 Wiley-Liss, Inc.