Selection for a secondary sexual trait in male hamadryas baboons

Abstract Male hamadryas baboons (Papio hamadryas) are easily recognizable by the striking patches of pink skin on their hindquarters. Within their multilevel societies, hamadryas males compete to acquire females and assemble them into ‘one-male units’ (OMUs), becoming ‘leader’ males who monopolize reproduction within those units. OMUs include a number of females, whom the leader male largely controls, and sometimes subordinate ‘follower’ males. The number of offspring produced varies widely among OMUs, suggesting the potential for strong selection pressure on traits of leader males. In this study, we use morphometric and demographic data from wild hamadryas baboons at Filoha, Ethiopia to test the hypothesis that the prominent paracallosal skin of males is linked to fitness outcomes and has evolved through sexual selection. Our results demonstrate a positive relationship between the relative paracallosal skin size of a leader male and both the number of females in his OMU and the number of offspring born into it. These findings suggest that males with larger paracallosal skin are more successful in attracting, defending, and reproducing with females. This study provides the first evidence that the distinctive paracallosal skin of male hamadryas baboons is likely to have evolved through sexual selection.