Rank-biased access to females in the likely fertile period despite comparable copulation rates in male bonobos at Wamba

ObjectivesFemale bonobos (Pan paniscus) show prolonged sexual swelling and copulate across an unusually extended timeframe, including the postpartum infertile stage. We tested the updated prolonged sexual receptivity hypothesis that the simultaneous presence of multiple receptive females disperses male mating effort and weakens male-male contest competition by examining rank effects on copulation rates and mating partners. Materials and MethodsIn a free-ranging bonobo group at Wamba, we collected copulation and party-composition data using all-day focal follows. Based on daily swelling scores, we operationally defined a likely fertile period (LFP) from detumescence at the end of the maximal swelling phase. We analyzed rank-dependent patterns in copulation rates and mating partners were analyzed using generalized linear mixed models (GLMMs). ResultsCopulation rates did not differ detectably across male dominance ranks, with approximately twice per day on average. In contrast, higher-ranking males were more likely to copulate with LFP females. At the party level, lower-ranking males increasingly copulated with non-LFP females as the number of LFP females present increased. DiscussionThese findings indicate that pronounced male reproductive skew in bonobos appears to arise through rank-biased access to females in the fertile window despite broadly similar copulation rates across males. Subordinate males tendency to shift mating effort toward non-LFP females when LFP females are available may reduce injury risks of direct contest competition while facilitating dominant males to guard fertile females effectively. Therefore, prolonged sexual receptivity in female bonobos could be a key factor yielding the condition where lower male aggression and higher reproductive skew coexist.