Among mammalian species, female social dominance is rare. However, one of our closest living relatives, bonobos, are known for the relatively high statuses females hold in social groups. This puzzles researchers as the males are often bigger and stronger than the females. A new study from the Max Planck Institute for Evolutionary Anthropology has analyzed the dominance relations between male and female wild bonobos, taking particular interest in the high social status of some females.
It is not female alliances that help females win conflicts with males, the study reveals, nor does the context of the conflict seem to be relevant for the outcome either. Instead, the researchers found the determining factor was sexual attractiveness. If the females in the conflicts with males displayed sexually attractive attributes, including sexual swellings, then they tended to win the conflict more easily. The males in these conflicts behave in a less aggressive way as well.
Intersexual dominance relations in bonobo populations have never been thoroughly studied in the wild. However, several theories exist for how females reach their elevated status. One theory proposes bonobo female dominance is facilitated by females forming coalitions which suppress male aggression. Another theory suggests an evolutionary scenario in which females prefer non-aggressive males which renders male aggressiveness a non-adaptive trait.
Recently, a study from the Lui Kotale bonobo project from the Max Planck Institute for Evolutionary Anthropology reported on the outcomes of intersexual conflicts in a bonobo community near the Salonga National Park in the Democratic Republic of the Congo. The analysis of male/female conflicts revealed a sex-independent dominance hierarchy with several females occupying top ranks.
The team also discovered only two factors have a significant influence on the outcome of intersexual conflicts. The first is female motivation to help their offspring, and the second is attractiveness. They found when females defend their offspring against male aggression, often alone but sometimes in groups, males defer to females. They found it even more interesting females are more likely to win conflicts against males during times when they exhibit sexual swellings indicating elevated fecundity.
Martin Surbeck of the Max Planck Institute for Evolutionary Anthropology says, “In those situations, males also aggress females less often, which is different from chimpanzees, our other closest living relatives.” The study findings suggest in bonobos both female sexuality and male mating strategies are involved in the shifting dominance relationships between the sexes.