The neighbor effect: Other groups influence intragroup agonistic behavior in captive chimpanzees

Abstract

While intercommunity interactions are well documented in wild chimpanzees, the influence of neighboring captive groups on social behavior has not been investigated. This study examined the influence of vocalizations and noisy displays produced in neighboring groups (i.e., “neighbor vocalization”) on intragroup hooting, bluff displays, and agonistic behavior. Chimpanzees from two broadly differing housing conditions were selected in order to examine the effect of neighboring groups: “Wing Groups” (those living in indoor/outdoor pens with auditory access to large number of neighboring individuals) and “Building Groups” (those housed in one indoor building, able to hear the calls and noisy displays produced by a small number of neighboring individuals occupying the building). Data were collected on 58 adult and juvenile subjects living in social groups varying in size from two to 14 individuals. Observation sessions were divided into two categories depending on the level of neighbor vocalization. Wing Groups showed higher rates of hooting, bluff displays, and agonistic behavior against other group members when levels of neighbor vocalization were high. Building Groups showed the same pattern, with results for bluff displays reaching statistical significance. This effect was also found through an analysis of the time periods immediately surrounding incidents of neighbor vocalization: Hooting and bluff displays were significantly more common after than before neighbor vocalizations. In spite of variation between the two housing conditions with respect to overall levels of neighbor vocalization, degree of differences between the high and low vocalization levels, and methods of data collection, a consistent neighbor effect was detected. This effect is congruous with behavioral patterns observed in the wild, and is therefore considered an expression of species-typical behavior as opposed to an artifact of captivity. These results are relevant to captive management, and bear on the influence of housing condition on psychological well-being.