Correspondences Male chimpanzees exchange political support for mating opportunities

Male chimpanzees, Pan troglodytes, differ from males in most other mammalian taxa because they remain in their natal communities throughout their lives, form close bonds with one another, and cooperate in a range of activities [1]. However, males also compete fiercely for status within their groups [2,3], and high rank enhances male reproductive success [4,5]. Males rely partly on coalitions to achieve and maintain status [2,3,6–9], and shifts in male alliances can have dramatic political effects [2,3,6]. It is not known what benefits are obtained by low-ranking coalition partners. Here we report that the highest-ranking (alpha) male in one well-studied community of chimpanzees rewarded his allies by allowing them preferential access to mates. During the 22-month study period, the Kanyawara community of chimpanzees contained ten mature males. The males were ranked in a linear dominance hierarchy that remained stable during the study period. The highest-ranking male, MS, had held the alpha position for four years when the present study began in 2001 and retained this position throughout the study. Six parous females became sexually receptive and began to cycle regularly during the study period. Parous females are more attractive than nulliparous females as mating partners [10]. Females are most likely to conceive when their sex skins are fully swollen, and male mating activity is highest during this period [10]. Thus, we considered females to be sexually ‘attractive’ on a given day if they were parous, their sex skins were maximally swollen, and they were the objects of male–male mating competition on that day. Only matings and interferences in copulations with attractive females are considered below. (For additional information see the Supplemental data available online with this issue). The alpha male, MS, monopolized access to attractive females. MS was responsible for twice as many copulations as any other male (32%, N = 234 copulations by all males), and he was also responsible for the majority of interferences when other males tried to mate (52%, N = 109 interferences by all males). Overall, males interfered in 34% of the 323 mating attempts observed, and interference usually prevented copulations from being completed (89/109 = 82%). The mating success of lowerranking males was strongly affected by their support of the alpha male. MS received 58 acts of support from other males, and all but one of the mature males supported MS at least once (range: 0–19 supports). MS was most tolerant of the mating attempts of males that supported him most frequently in conflicts (rs = 0.800, n = 9, p = 0.010), and the males that most often supported MS also copulated more often than other males (rs = 0.812, n = 9, p = 0.008). These results might be an artifact of male dominance rank because MS was most often supported by high-ranking males who also have high mating success (support x rank: rs = −0.800, n = 9, p = 0.010; mating success x rank: all males: rs = −0.802, n = 10, p = 0.005; excluding MS: rs = −0.686, n = 9, p = 0.041). The sample of mature males is too small to conduct multivariate analyses, but we can assess the relative importance of dominance rank and support of MS on male mating success by examining the partial regression plots. Figure 1 shows that support of MS is strongly linked to male mating success, when the effects of male rank on mating success are controlled statistically. At the same time, male dominance rank has relatively little impact on male mating success when support for MS is controlled statistically (Figure 2). These results show that the alpha male in the Kanyawara community selectively tolerated mating by his allies and exchanged mating tolerance for support in conflicts. The alpha male in this community was a singularly important trading partner because of the disproportionately high value of his mating tolerance. In chimpanzee communities, alpha males rely on support from subordinates to maintain their status [3,6–8]. The most valuable currency controlled by the alpha male is access to mates. By selectively rewarding his allies with tolerance of their mating efforts, the alpha male may discourage potential rivals from mounting challenges against him: rivals will not only have to defeat the alpha male and his allies, they will also forfeit future mating opportunities if their challenges are unsuccessful. The ability to form these kinds of strategic coalitions may allow some male chimpanzees to maintain alpha status for relatively long periods of time.