Bonobos and chimpanzees use sex to ease social tensions, too

Just like us, our closest living ape relatives do not just have sex to reproduce. A study by a team of researchers, including Utrecht University behavioural biologist Edwin van Leeuwen and late Frans de Waal, finds that bonobos and chimpanzees use sex to ease tension and reaffirm social bonds. These findings support the idea that sex was already being used for social purposes by the last common ancestor of humans, bonobos, and chimpanzees, over six million years ago.

Bonobos are famous for regularly using sex to resolve disputes and repair social bonds. By comparison, the role of sex in chimpanzees, who are sometimes viewed as being more aggressive, has been less well understood. That is why the team set out to make a direct comparison of sexual behaviour in the two species.

Stressful periods

The study took place at two African great ape sanctuaries: Lola ya Bonobo Sanctuary in the Democratic Republic of Congo, and Chimfunshi Wildlife Orphanage Trust in Zambia. In total over 1,400 hours of observations were taken involving 53 bonobos and 75 chimpanzees, across seven months in 2019.

The researchers observed sexual contact during two stressful periods; following naturally-occurring social conflicts and prior to feeding. By comparing both apes living in similar environments, the researchers aimed to determine whether using sex to ease social tension is a common behavioural trait between the two sister species.

Similarities and differences

The findings showed that both bonobos and chimpanzees used sex in similar ways to ease tension and reaffirm social bonds prior to feeding, when competition and power differences can provoke disagreement. However, bonobos also often had sex after fights to repair social relations, whilst in these circumstances, chimpanzees were more likely to use other social behaviours to provide comfort and restore bonds.

Chimpanzees appear to be more efficient at resolving conflicts, whereas bonobos tend to engage in quick sexual encounters that seem to provide only short-term stress relief.

Lead author Jake Brooker, postdoctoral researcher at Durham University and Utrecht University: “Chimpanzees are known to have a wider repertoire of reassurance behaviours, including body kissing, but genital sex still constituted a sizeable portion of how they reconciled with each other and their stress-management behaviour.”

At the same time, the observed difference between bonobos and chimpanzees inspires future research, according to Van Leeuwen: “Chimpanzees appear to be more efficient at resolving conflicts, whereas bonobos tend to engage in quick sexual encounters that seem to provide only short-term stress relief.”

Learned behaviour

While age did not influence the use of sex to reconcile after fights amongst either bonobos or chimpanzees, the researchers saw that prior to feeding it was the older apes in both species who were more likely to initiate sex, indicating that this could be a learned behaviour, passed down over time.

The researchers also noticed that female bonobos and male chimpanzees were more likely to initiate sex prior to feeding, but in both species, sex occurred in both hetero and homosexual combinations.

Inherited

The findings shed new light on the evolution of sex for social reasons in our own species. Brooker: “The fact that both species use sex in this way provides a fascinating window back in time, further evidencing that for humans, bonobos and chimpanzees, our use of sex for social reasons is something we have inherited from our common ancestor.”