The timing and causes of a unique chimpanzee community fission preceding Gombe's "Four-Year War".

Our analysis suggests that the community began to split during a time of an unusually male-biased sex ratio and a protracted dominance struggle, and that adult males remained with those with whom they had preferentially associated prior to the split. We discuss the costs and benefits of group membership in chimpanzees and contrast these results with group fissions in humans and other taxa.