A new look into the private lives of chimpanzees has found that the primates settle disagreements with close friends by rubbing genitals together, a behavior previously that's commonly seen in their closely related bonobos. This study sheds light on where human sexual behavior originated, some six million years ago.
In the first comparative study of sexual behavior among chimpanzees and bonobos, a team of researchers led by Durham University has observed how in times of social stress, both species will use sex to "make up" to restore social cohesion. Like humans, this behavior is more about social bonds than something just reserved for courtship and reproduction – rare in the animal kingdom.
"Given similar evidence in humans, our results support the notion that this was a trait probably also present in our last common ancestor," the researchers noted in the new paper.
“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," said lead author Jake Brooker, a postdoctoral researcher in the Department of Psychology, Durham University. “Chimpanzees are known to have a wider repertoire of reassurance behaviors, including body kissing, but sex still constituted a sizeable portion of how they reconciled with each other and their stress-management behavior."
The researchers spent more than 1,400 hours in the field over seven months, observing 53 bonobos (in three separate groups) and 75 chimpanzees (two groups) living at the Lola ya Bonobo Sanctuary in the Democratic Republic of Congo and the Chimfunshi Wildlife Orphanage Trust in Zambia.
They observed a range of behaviors: Genital touch, either by hand or foot; genito-genital contact, which saw two apes – most commonly female bonobos – engage in mutual genital rubbing; mounting; testicle shaking among chimpanzees, where male apes would shake their genitals towards another male; and oral-genital contact, which was seen across both species.
They found that female bonobos and male chimpanzees, respectively, were more likely to engage in these behaviors within their social group prior to feeding. And both same-sex and male-female social-sexual contact was frequently observed.
Age was not a factor in which apes were more likely to settle conflict with sex, but the older individuals of both species were more likely to exhibit this behavior prior to feeding. And non-kin pairs were more likely to engage in this genital-to-genital contact than kin. The scientists believe both these factors hint that this form of conflict resolution is a learned one, which is then adopted by younger apes.
Interestingly, this challenges common thought that chimpanzees settle disputes with aggression and hierarchical dominance, and instead display social bonding behavior through acts like grooming and hand-holding.
“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," said senior author Professor Zanna Clay, from the Department of Psychology at Durham University. "This study highlights that beyond reproduction, sex plays an important role in their societies, and most likely for our early ancestors too."
“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," said Clay. “Bonobos are famous for using sex to navigate social challenges but much less is known about the role of sex in chimpanzee society."
The new observations about chimpanzees now suggests that sexual behavior as a social tool predates the point where humans, bonobos and chimpanzees branched off from each other on the phylogenetic tree.
“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," said Brooker.
The research is published in the journal Royal Society Open Science.
Source: Durham University via EurekAlert
