Richard Musgrove
There’s more to sex than reproduction — new research lead by the UK’s Durham University suggests that using sex to manage social tension is a trait our distant ancestors shared with all their hominoid descendants — humans, chimpanzees, bonobos, orangutans and gorillas.
The study, one of the first direct comparisons of sexual behaviour amongst bonobos and chimpanzees during periods of social stress, published in Royal Society Open Science, provides new insights into our own behavioural origins, says lead author, Dr Jake Brooker.
The researchers make a further distinction — this was a study of non-conceptive (no babies) sexual behaviour — called ‘sociosexual behaviour’. Such behaviours have been linked to psychological and emotional well-being, the researchers say, reducing stress and enhancing interpersonal connection, and are widespread throughout the animal kingdom, particularly among non-human primates.
Sex releases endorphins, oxytocin and dopamine, and all apes share the same hormones and similar genital anatomy and physiology. Evolution has favoured their use, and conservation of the responsible genes, as far back as, and probably further than, our last common African ancestor, 6-7 million years ago.
Researchers documented sexual contact in two separate groups of apes, noting behaviours after social conflicts and before feeding. Bonobos are known to regularly use sex to resolve disputes and repair social bonds; chimpanzee behaviours are less well known.
Coauthor Professor Zanna Clay of 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.
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.
Both sanctuaries are swathed in primary forest — Lola ya Bonobo Sanctuary covers about 60 hectares, and supports about 60 bonobos, and Chimfunshi covers 10,000 hectares and contains about 150 chimpanzees.
More than 1,400 hours of observations were taken involving 53 bonobos and 75 chimpanzees, across seven months in 2019.
“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,” says Clay.
Sociosexual behaviours included friendly contact with distressed victims after fights, says Brooker, and before feeding, in the context of potential competition risk, and ranged from genital touching to genital-genital contacts and mounting (with and without intromission).
“Genital contacts were offered by and received by individuals of all ages and sexes in both species.”
Clay says: “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.
Read more about the weird sex lives of animals
There were differences — chimpanzees showed a wider variety of reassurance behaviours than bonobos, says Brooker, including several vulnerable mouth-to-body behaviours, such as body kissing and finger/hand in mouth. Genital contacts may be the dominant form of reassurance in bonobos, he adds.
“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.”
Neither Lola ya Bonobo Sanctuary nor Chimfunshi are completely wild — humans are around, feeding and caring for these apes. But the same behaviours have been seen in wild populations says Dr Sophie Berdugo of the University of Oxford.
Berdugo has worked with the Bossou chimpanzee community in Guinea, West Africa for 25 years and confirms “that wild chimps there use sex to calm down after stressful situations,” per New Scientist.
“I saw many cases of the males standing off against each other and then mounting each other at the end of the interaction. And as found with Chimfunshi chimps, she says, “the frequency of the sex in these competitive situations were more common among males and the chimps and more common in the females in the bonobos”
Such research would also shed light on the overall significance of sexual behaviour in regulating social relationships through hominid evolutionary history, concludes Brooker.
