Competitive ability determines coalition participation and partner selection during maturation in wild male chimpanzees (Pan troglodytes schweinfurthii)

Social mammals often live in groups in which a dominance hierarchy is an important determinant of access to mates. In addition to competing individually, males may form coalitions of two or more to attack or intimidate rivals. Coalition formation could be particularly advantageous for adolescent males by helping them compensate for their physical and social immaturity. However, adolescents may struggle to attract effective coalition partners because of these inadequacies. Here, we examine the behavior of maturing male chimpanzees to test whether coalitions are more frequent among more or less powerful individuals. Our longitudinal study followed 18 males (ages 5 through 25 years) and utilized 1517 coalitions across 12 years of observation of the Kanyawara chimpanzee community in Kibale National Park, Uganda. We found that rates of coalition formation increased across maturation and that this increase was independent of a rise in the overall use of aggression. Juveniles formed coalitions almost exclusively with their mothers, while adolescents partnered primarily with peers and adult males. When adolescents and adult males formed coalitions with each other, the adolescents were more likely to join the adults than vice versa. Finally, adolescents engaged in joint behavior with adult males more often in non-aggressive vocal displays than in aggressive coalitions. Taken together, our results suggest that adolescent males are largely unable to attract the most powerful coalition partners and that they make the best of a bad job by joining adult males in less competitive situations, when the risk of receiving aggression from opponents is lower. Significance statement Adolescent males are at a disadvantage in competing for powerful allies, because they have less strength to contribute to a cooperative dyad. Even when a variety of possible coalition partners are present, the partner choices of higher-ranking males likely limit the number of remaining individuals that are available and willing to form coalitions with adolescents. Accordingly, we found that males sorted with closely ranked coalition partners down the hierarchy, leaving adolescents to form coalitions with partners of similar rank to themselves rather than high-ranking adult males. There was no evidence for rank sorting in joint vocal displays, however. Additionally, adolescents participated in more than twice as many joint vocal displays as coalitions. These data suggest that joint vocal displays represent a viable alternative to coalitions for adolescents attempting to integrate themselves into the adult male social world.