The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Juvenile harassment of adults in bonobos and the exploratory aggression and rank improvement hypotheses


Anthropology, University of Oregon

Thursday 1:15-1:30, Ballroom B Add to calendar

Dominance hierarchies in primates are often established during development and are important to lifetime reproductive success. Harassment of mature group members functions as a way for juveniles to explore the nature of dominance interactions and as a mechanism to improve rank. In chimpanzees, where all males out-rank all females, harassment of mature females by juvenile males is the first step in their ascension of the group hierarchy. In contrast, dominance relationships in bonobos are complex and females often hold the very highest rank positions. This study presents data on harassment behavior (N=81) in the Columbus Zoo bonobo colony (N=16 individuals). Similar to chimpanzees, harassment was spontaneous, juveniles harassed significantly more than adolescents or infants (G=24.551; P<0.001) and males harassed significantly more than females (G=6.581; P<0.05). In contrast to chimpanzees, targets were more likely to be males than females (G=6.581; P<0.05) and there was a negative correlation between target rank and amount of harassment received (Spearman r = 0.850, P<0.01). Frequency of types of responses to harassment was dependent on sex, where male bonobos, like female chimpanzees, reacted more with aggressive and submissive behaviors than with neutral behaviors, whereas female bonobos reacted equally with aggressive, submissive, and neutral behaviors (G=4.946, P<0.05). We also present preliminary results of urinary hormonal analyses, including on testosterone and cortisol. This pattern of harassment behavior supports the proposed functions of exploratory aggression and rank improvement while highlighting key difference in the social structures of the Pan species.

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