Bonobos win females through friendship

Relationship affects hormone levels of the highest-ranked males

Bonobos at grooming in Lui Kotale, Salonga National Park, Democratic Republic of the Congo © Caroline Deimel / Lui Kotale Bonobo Project
Read out

In the bonobos, friendship between the sexes is trump: The highest-ranking males do not win their partners by aggressive advertising and showmanship, but by making friends with them. German researchers have found this out in observations of these great apes in captivity and in freedom.

As the scientists observed, the friendly relationships between the sexes even change the hormonal balance of the high-ranking males: Although they have to defend their position among their peers through aggression and dominance, their testosterone levels are lower than those of lower-ranked males.

Friendship influences the hormonal balance of bonobo males

This was unusual and probably a consequence of the soothing friendship with the females. This relationship apparently influenced the hormonal balance of the bonobo males similarly as one knows from human relationships, the researchers report in the journal "Animal Behavior".

Such hormonal influences of relationships have been known so far mainly of species in which males actively participate in the rearing of the young or where the sexes enter into permanent pair bonds, says first author Martin Surbeck from the Max Planck Institute for Evolutionary Anthropology in Leipzig.

Males with pronounced ranking

Bonobos are among the closest relatives of humans. Like other apes they live in groups with several males and females. Among the males, there is a pronounced ranking in which the most aggressive and dominant males stand up. display

In other species, high-ranking males in such hierarchical groups usually have higher levels of the sex hormone testosterone in their blood. Among other things, this messenger substance increases the aggressiveness and ensures a particularly masculine appearance. Males with these attributes usually also compete with female partners.

However, the bonobos have a complicating factor: unlike other ape species, males are not generally superior to female group members. As it turns out, these balanced gender relationships cause the bonobo males to act differently when searching for mates than other great apes.

More friendships and lower testosterone levels

In the presence of fertile females, the aggressiveness of all males has increased, the researchers report. But only at the lower rankings had the testosterone levels also increased in parallel. High-ranking males, on the other hand, had lower testosterone levels. They were more of a friend of unrelated females, write Surbeck and his colleagues.

Unlike other species where men compete heavily for access to females, bonobos have no relationship between dominance status, testosterone and aggression. (Animal Behavior, 2012; doi: 10.1016 / j.anbehav.2011.12.010)

(Max Planck Institute for Evolutionary Anthropology / Animal Behavior / dapd, 18.01.2012 - NPO)