Chimpanzees have been characterized as male bonded, territorial, and hostile to extra-community conspecifics (Goodall, 1986; Manson & Wrangham, 1991; Mitani & Watts, 2001; Nishida & Hosaka, 1996). Male bonding such as that seen in chimpanzees is unusual among primates and mammals in general (van Hoof & van Schaik, 1994), but is common among humans. Like in humans (Hill et al., 1985), when chimpanzee males enter adolescence they begin associating with other adolescent and adult males, and begin to separate from their mothers (Pusey, 1990). However, we know very little about how male chimpanzees become socially mature and learn the sex-specific behaviors necessary to successfully compete with other males for access to valued resources, such as estrous females. Despite a great deal of research on adult male chimpanzee social relationships and our understanding of the male chimpanzee social world (e.g. Goodall, 1986; Mitani & Watts, 2001; Nishida & Hosaka, 1996; de Waal, 1982; Watts, 2002), we still know very little about the development of male chimpanzee behavior.
In this thesis, I use behavioral data to investigate the ontogeny of social behavior in wild male chimpanzees from an unusually large community at Ngogo, Kibale National Park, Uganda. I focused particularly on behavioral development during adolescence and early adulthood, and collected data on 20 adolescent, 3 juvenile, and 6 young adult males. I subdivided adolescent males into three categories: early, middle and late adolescence. By sampling the complete breadth of male age ranges from juvenility to early adulthood, I obtained a more complete picture of the development of male behavior in chimpanzees than previously available.
Adolescent and other young males show age-related, behavioral variation. As males age they associate less with anestrous, adult females and play less. Conversely, they participate more often in sex-typical male behaviors, such as associating, traveling and grooming with other males, coalition formation, mating competition, hunting and patrolling. Young adult males form coalitions with other males, compete for copulations, hunt and patrol significantly more than adolescent and juvenile males. Late adolescent males groom other males and compete for copulations significantly more than middle and early adolescents and juveniles.
Adolescent and other young males establish social relationships with each other that resemble those of prime and old adult males in some ways, but not others. Young males have dominance relationships and can be ranked in a linear hierarchy. Dominance rank among young males is correlated strongly with relative size/age and presumed fighting ability. When age is controlled for, high ranking young males copulate more than low ranking young males. Young males rarely participate in coalitions with each other and do not form alliances. Young adult males participate in coalitions more than other young males and occasionally form alliances with prime adult males, but not with each other.
Young males invest more in social relationships with adult males than the reverse. Adult males do not reciprocate in social investments such as grooming and coalitional and agonistic support with young males. Adolescent and other young males use adult males as behavioral role models. Young males groom and follow high-ranking adult males significantly more than middle or low ranking adult males. Young males target adult males with relative ranks similar to their own as social partners: relatively high-ranking young males interact preferentially with high-ranking adults, while relatively low-ranking young males interact more with low-ranking adults.