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Peer-reviewed: Animals
Physical touch may be an important evolutionary tool to help manage social tension and keep the peace.
That is according to a new study led by Durham University in the UK, analysing behaviours of our closest ape relatives, the bonobos and chimpanzees.
The study, published today in Proceedings of the Royal Society B: Biological Sciences, tested whether reassuring body contact, such as embracing, touching and kissing, seen in bonobos and chimpanzees alike, worked to improve social tolerance.
The results show that individuals who engaged in more reassuring touch before a socially-tense events like feeding, subsequently spent more time co-feeding alongside others – indicating a direct link between pre-competition reassurance and peaceful resource sharing.
The researchers believe this shows that friendly touch may function as a flexible, short-term tool to manage social tension and competition, to maintain social relationships and group harmony.
The close similarity in the types of reassurance behaviours between both bonobos and chimpanzees suggests the practice has a common evolutionary origin, dating back at least six million years to humans' closest ancestor, shared with our ape relatives.
To carry out the study, a team led by Durham University's Department of Psychology, analysed reassuring body contact before competitive feeding events amongst five semi-wild groups of bonobos and chimpanzees.
In total, 116 individuals were filmed during a controlled five-minute window before food was released into a shared, restricted space – a setup designed to mimic the social tension of natural resource competition, such as fruiting trees.
The researchers then painstakingly recorded who touched whom, in what way and how much time individuals then spent co-feeding alongside others. They also recorded aggression events before, during and afterwards.
For both bonobos and chimpanzees, those who engaged in more pre-feeding reassurance, subsequently spent more time co-feeding.
Within bonobos, the strongest effect was seen amongst female-female pairs, consistent with the central role female alliances play in bonobo society.
In chimpanzees, the effect was strongest amongst male-male pairs, reflecting the importance of male bonds in chimpanzee social structure.
However, in both species, these effects were clearest in the more socially tolerant groups, suggesting that alliance relationships reinforced by reassurance prior to feeding may function to maintain a tolerant social climate.
Researchers also noted that chimpanzees, particularly males, engaged in high rates of risky intimate behaviours such as placing fingers or hands in each other's mouths.
These vulnerable acts were seen considerably more often in the more tolerant group, suggesting the behaviour may function as a signal of trust between individuals confident the other won't cause harm.
Lead researcher, Dr Jake Brooker, who was based at Durham University's Department of Psychology at the time of the study, said: "Comparing bonobos and chimpanzees – our two closest relatives – offers fascinating insight into the evolutionary roots of social behaviour and how they, and we, manage tension, competition and cooperation.
"Social touch is a powerful communicative tool, present long before language and likely one of the earliest channels through which social bonds form. Its use in these contexts seems to be something we share with our closest relatives.
"The riskier mouth-to-body contacts among chimpanzees, such as placing fingers or hands in another's mouth, were particularly striking.
"Chimpanzees are often associated with aggression, but these behaviours reveal a deep level of trust and a willingness to be vulnerable in order to navigate tense moments.
"We were partly inspired to conduct this study by an observation from our most tolerant chimpanzee group: one male embracing another from behind, then a second joining in, then a third, one by one, until the whole chain settled — what my colleagues and I started calling a 'reassurance train'."
Research senior author, Professor Zanna Clay, based in Durham University's Department of Psychology adds: "Our study highlights the ancient and deeply powerful role that touch plays in regulating our social relationships, particularly during tense times when emotions run high. "
"Although much attention has been placed on the role of aggression and competition in human evolution, our study shows that our lineage has also evolved rich and flexible behavioural tool-kits to manage social tension to protect social relationships and promote social harmony."
"It is striking that many of the reassurance behaviours that we observed in our closest ape cousins mirror those we also see in humans today. This highlights how much we still depend on preserved behavioural tools that hark back millions of years to navigate our social worlds, particularly in the most challenging of moments"
In total 60 cofeeding sessions totalling over 10 hours of observation were analysed.
The researchers hope to continue this work by examining the physiological basis of reassurance – specifically whether touch in these contexts reduces stress hormones in the actor and receiver – and extend the analysis across other competitive and socially tense contexts.
They also hope to test whether pre-feeding reassurance between specific pairs predicts their later co-feeding tolerance and risk of conflict, to see if this is relationship specific.
The research took place at two African great ape sanctuaries: Lola ya Bonobo Sanctuary in the Democratic Republic of Congo, and the Chimfunshi Wildlife Orphanage Trust in Zambia.
The team included academics from The Max Planck Institute of Animal Behaviour, The Max Planck Institute for Evolutionary Anthropology and and the University of Konstanz in Germany, Utrecht University in The Netherlands and New York University in the USA.
This project was supported by funding from the Templeton World Charity Foundation.
