Harassment of Mature Female Chimpanzees by Young Males in the Mahale Mountains

Abstract

Juvenile and adolescent male chimpanzees sometimes threaten older, apparently stronger individuals such as mature females. I label the behavior harassment. Harassment comprises 25 behaviors, 14 of which are accompanied by the use of objects such as branches: Clubbing, flailing and throwing are the most common. Females respond to harassment with 10 behaviors, including scream, avoid, ignore, and retaliate. Females tend to respond to harassment by juvenile males by ignoring them. However, they are more likely to retaliate, scream or avoid in response to harassment by adolescent males. I propose the rank improvement hypothesis that harassment initiates the process of male domination of females, and compare the predictions derived from it with those of the exploratory aggression hypothesis. Males stopped harassing females significantly earlier when females ignored them versus when they did not ignore them. This is not consistent with the exploratory aggression hypothesis. Males harassed adult females significantly longer when females retaliated than when they did not, which is consistent with the rank improvement hypothesis. Although the observations are congruent with my hypothesis, we need more data to test it.