Bayesian competitiveness estimation predicts dominance turnover among wild male chimpanzees

It is predicted that asymmetries in competitiveness (or resource holding potential (RHP)) would determine the outcomes of animal contests, but it has been difficult to estimate RHP in practice. In long-living animals, individual RHP changes as they age, making it necessary to quantify the individual trajectories of RHP rather than estimate a single representative value. Based on a Bayesian approach and a literature review, we estimated the age-related RHP trajectories of wild male chimpanzees, Pan troglodytes, and studied whether estimated RHP predicted the turnover of alpha males. A model containing a quadratic relationship between age and RHP fitted to the longitudinal changes in male dominance rankings well. When alpha male turnover occurred, the estimated RHP of the new alpha male was higher than were those of previous alpha males, which was explained by age. An exception to this pattern involved cases in which males formed coalitions. In these cases, a temporary increase in the RHP of the new alpha male, which was not attributed to age, was necessary for turnover. Cross-validation analysis showed that the turnover of alpha males was predictable except when males achieved alpha status via coalition formation; this exception is explainable by the instability of coalition partners in this species. This study provides a predictive framework for the occurrence of dominance turnover using incomparable long-term data and sophisticated statistical operations.