Functions of Alliances in Contests Within Wild Gorilla Groups

Abstract

AbstractPrimates frequently support other individuals in aggressive contests among group members. Several functions of such alliances have been suggested, some of them dependent on continued residence of females in the group of their birth. The distribution of alliances in the gorilla (Gorilla gorilla), in which both sexes emigrate, was analysed to investigate constraints on the use of alliances by primates. Functions were inferred on the basis of the identity of participants, it being assumed, for example, that a mother supported its infant against an adolescent male for a different reason than did an adolescent female who supported a dominant female against a subordinate female. Gorillas ally, we suggest, to protect relatives from harm, to provide them with access to otherwise unobtainable resources and, perhaps, to emphasise their dominance over rivals; it seems that individuals might also ally to obtain resources for themselves, and to establish supportive relationships with other group members. The first two inferred functions were the most common, involved in over 80% of alliances. The major contrast with female-resident primates lay not so much in the range of functions for which alliances were used, but in the lack of evidence that gorillas attempted to use alliances to manipulate the dominance status of their kin. Possible reasons for the contrast are that i) the widespread distribution and low quality of the gorilla's main food plants meant that ability in defence competition was not sufficiently advantageous for the benefits of raising the rank of kin to outweigh the costs; ii) the large difference in age among immature gorillas in the study area, which was mostly a consequence of the small group size, made it too costly to use support in contests to raise an individual's rank above that of older peers; iii) the leading male prevented the unequal investment among his offspring that would be entailed if individuals used alliances in contests to raise or reinforce the rank of kin. Currently, these possibilities cannot be distinguished.