Drivers of female power in bonobos

Abstract

In mammals, female dominance over males is a rare phenomenon. However, recent findings indicate that even in species with sexual dimorphism biased towards males, females sometimes occupy high status. Here we test three main hypotheses explaining intersexual power relationships, namely the self-reinforcing effects of winning and losing conflicts, the strength of mate competition, and female coalition formation. We test these for bonobos (Pan paniscus), one of our closest living relatives, where females have high status relative to males despite male-biased size dimorphism. We compiled demographic and behavioral data of 30 years and 6 wild living communities. Our results only support predictions of the female coalition hypothesis. We found that females target males in 85% of their coalitions and that females occupy higher ranks compared to males when they form more frequent coalitions. This result indicates that female coalition formation is a behavioral tool for females to gain power over males.

Fig. 1. The figure shows the relationship between the proportion of males within a community/year and the percentage of all intersexual conflicts in which females received submission from males.

Colors depict communities Ekalakala (EKA), Fekako (FEK), Kokoalongo (KOK), Bompusa West (LKB), Bompusa East (LKE), Eyengo (LOM) and circle size represents number of conflicts.

Fig. 2. The figure shows the relationship between two different indices of degree of female power over males and the frequency of female coalition formation.

a shows the relationship between frequency of all female coalition formation and the proportion of conflicts in which females received submission from males submitted to females; and b shows the relationship between frequency of all female coalition formation and the average proportion of males in a community outranked by each female. The dashed line and the grey polygon show the fitted model and its confidence limits for the percentage of days with more than two maximally tumescent females being at its average (model estimates from the models with random slope of FFC). Area of dots is proportionate to number of agonistic interactions observed between sexes (a; range 15–235) and the overall number of conflicts with submissions (b; range 58–493) during a given year. Colors depict the communities Ekalakala (EKA), Fekako (FEK), Kokoalongo (KOK), Bompusa West (LKB), Bompusa East (LKE).