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Male Bonding: The influence of male relationships in captive chimpanzee social groups

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    Photograph: Emily Taylor

Mary Catherine Mareno
Faculty Sponsor: Dr. Steven Schapiro

In laboratories in the United States, groups of captive chimpanzees are maintained in order to conduct behavioral and medical research that further 1) our understanding of human behavioral and cognitive evolution and 2) the fight against disease. Many studies of wild populations of chimpanzees have examined the importance of male-male bonds to group cohesiveness and have found that variations in the number of males and their age distribution impact the interactions within the group. Male-male interactions serve to maintain functional social groups and seem to be of more critical importance to group cohesiveness in chimpanzees than female-female bonds. In the wild, communities are based around a core of adult males who spend the majority of their time together and generally interact with one another affiliatively. These strong relationships maintain the cooperation needed to organize hunts and to protect their territory and females from other groups.

Considerably fewer data are available on the effects of differences in male composition on interactions within captive social groups. Studies have shown that like in the wild, captive male chimpanzees prefer other males as grooming partners. Yet many primate facilities shy away from housing multi-male groups for fear of increased aggression. To promote species-typical social conditions, which in turn allow for healthier subjects that yield more valid behavioral and medical data, a balance between affiliation and conflict among males must be achieved. However, the strategies to achieve such a balance are not particularly well characterized.

To study the effects of different group compositions, 54 chimpanzees were observed living in five separate social groups at The University of Texas M. D. Anderson Cancer Center. Each social group was comprised of a different number of male chimpanzees with varying age distributions. The group sizes ranged from eight to 14 animals, and the proportion of males in each group varied from 3/8 to 1/2. Age differences between the oldest and youngest male in each group varied from 16 years to 25 years. Each group was observed once a week for 15 weeks. These observations were balanced between morning and afternoon time periods, switching each week. During the observations, the frequencies of 15 behaviors were recorded for each male with special emphasis placed on affiliative and aggressive behaviors. In addition, every three minutes the whole group was scanned to record the social behavior of each individual, using the same 15 behaviors.

The results indicate that group composition does affect the occurrence of aggression and affiliation. Specifically, the number of aggressive conflicts and affiliative interactions per group were neither randomly nor equally distributed across groups. Importantly, higher numbers of males did not lead to increased total aggression.

Group cohesiveness can be utilized to establish group compositions that promote high levels of group cohesion, thereby improving the psychological well being of captive chimpanzees. Using these findings, facilities with chimpanzee can empirically form and manage multi-male, multi-female chimpanzee social groups that mimic natural interaction patterns and encourage a more socially enriched existence.