We don't just have sex to reproduce - new research suggests that using sex to manage social tension could be a trait that existed in the common ancestor of humans and apes six million years ago.
Humans share this behavioural strategy with our closest living ape relatives – bonobos and chimpanzees.
Now researchers, led by Durham University, UK, have undertaken what is thought to be one of the first direct comparisons of sexual behaviour amongst bonobos and chimpanzees during periods of social stress.
Their findings, published in the journal Royal Society Open Science, could give us new insights into humans' own behavioural origins.
The least studied of the apes, the bonobos, are known to regularly use sex to resolve disputes and repair social bonds. By comparison, the role of sex in chimpanzee society has been less well understood.
The team observed sexual contact within sanctuary-living bonobos and chimpanzees during two stressful periods; following naturally-occurring social conflicts and prior to feeding.
By comparing both apes living in similar environments, the researchers could understand if using sex to ease social tension is a common behavioural trait between the two sister species.
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 more after fights to repair social relations, whilst in these circumstances, chimpanzees were more likely to use other social behaviours, with clearer signals, to provide comfort and restore relations.
The researchers say the fact that both bonobos and chimpanzees have sex to ease social tension supports the idea that using sex for social purposes was already present in the last common ancestor they share with humans, dating back over six million years. Human sex is not only for reproduction, and the same seems to be the case for our relatives too, as well as other animals.
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.
Lead author Jake Brooker, a postdoctoral researcher in the Department of Psychology, Durham University, said: "This is a first direct comparison of sexual behaviour in bonobos and chimpanzees, two of our closest cousins.
"Whilst bonobos, who are famously hypersexual, were more likely to engage in so-called 'make up sex' after periods of conflict, we found that chimpanzees, who some view as our more aggressive cousin, also use sex to ease tensions in many circumstances.
"Chimpanzees are known to have a wider repertoire of reassurance behaviours, including body kissing, but sex still constituted a sizeable portion of how they reconciled with each other and their stress-management behaviour.
"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."
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.
Amongst the observations made by the researchers it was noted 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.
Research senior author Professor Zanna Clay, Department of Psychology, Durham University, said: "Bonobos are famous for using sex to navigate social challenges but much less is known about the role of sex in chimpanzee society.
"By directly comparing the two species in similar environments during two key situations, we can test the social role of sex in our closest ape relatives, and gain deeper understanding about how it may have evolved in our own species too.
"Against the assumptions of pacifist sex-mad bonobos and aggressive chimpanzees, we found that both species used sex in similar ways during tense situations, including same-sex pairings. This study highlights that beyond reproduction, sex plays an important role in their societies, and most likely for our early ancestors too.
"An exciting next step would be to test the functions of sex further, in different contexts, and to see how much overlap we see between chimpanzee and bonobo sexuality in the wild."
The research was funded by the Templeton World Charity Foundation: Diverse Intelligences Framework and included researchers from Harvard University and Emory University in the USA, Utrecht University in The Netherlands and Max Plank Institute for Evolutionary Anthropology in Germany.
The research was conducted with full approval from the Chimfunshi Research Advisory Board (CRAB), the Senior Veterinary Advisory Team of Lola ya Bonobo Sanctuary, and the Animal Welfare Ethics and Research Board (AWERB) of Durham University. Data collection consisted of naturalistic observations and experimental procedures that adhered to the legal requirements of the Democratic Republic of the Congo and Zambia, as well as the International Primatological Society's Principles for the Ethical Treatment of Nonhuman Primates.
