Social Touch in Chimpanzee Life

Rachna Reddy, Research Associate, Department of Human Evolutionary Biology, Harvard University

At the 2024 Gruter conference hosted at the Royal Society in London, with the theme “The Evolution of Kissing and Adaptive Trade-Offs in Human Contact,” Rachna Reddy presented on the topic of Social Touch in Chimpanzee Life. Touch is a core mode of social interaction in mammals including chimpanzees (Pan troglodytes), who, along with bonobos (Pan paniscus) are our closest living relatives. Chimpanzees and bonobos touch others with their lips specifically in four key contexts: grooming, food-sharing, play and as “reassurance.” Chimpanzees receive grooming from birth and begin to groom others in infancy (with partners proliferating far beyond mothers and siblings in adolescence and reducing in old age). Grooming for chimpanzees involves using fingers to gently stroke, separate, and search the hairs of other individuals while making occasional kiss-like lip touches to any part of their bodies. Notably chimpanzees and haplorhine primates broadly (monkeys, apes, tarsiers) use their mouths and tongues during grooming far less often than do strepsirrhine primates (lemurs, lorises), other mammals, and some bird species. Kiss-like behaviors and saliva exchange also occur when chimpanzee mothers chew food and transfer it from their own mouths directly into their infants’. This “kiss-feeding” behavior occurs most often in the first 6 months of infants’ lives and ceases when they are weaned (by 5 years of age on average). Chimpanzees continue to share fruit and meat foods throughout their lives (especially with bond partners) but most transfers involve hand to mouth touches, hand-to-hand touches or direct co-feeding on a large piece of meat or fruit rather than mouth-to-mouth “kiss-feeding.” Next, chimpanzees use their mouths to touch others during play; this is especially frequent during play with young infants where adult chimpanzees buzz their lips on an infant’s chest or fingers or allow the infant to mouth their own face. Finally, lip-to-lip kissing is a rare type of “reassurance” behavior (less than 1% of all reassurance behavior in my personal dataset) in chimpanzees and is potentially the type most analogous to kissing in humans. Reassurance behaviors, also called consolation or reconciliation behaviors, refer to relatively swift touches often between non-hairy skin areas (i.e. hands, mouths, genitals) that occur in moments of tension. Examples are rubbing genitals together, embracing, holding hands, and kissing. Early studies of chimpanzees by Goodall, deWaal and Sugiyama all noted key contexts in which lip-to-lip kissing reassurance specifically occurred: 1) when seeking support from someone of higher status, 2) as reconciliation (“kissing and making up”) and 3) as a reunion. Importantly, reunions feature in the lives of chimpanzees and bonobos more often than in other nonhuman primates. Both live in fission-fusion societies where they do not see all of their group members every day. They can break into subgroups of varying company at any moment—an adult male chimpanzee may spend an afternoon with just his mother for example, a week with just his best friend, or a day entirely alone. Upon hearing other group members gathering, chimpanzees must choose whether to join or avoid them. Such moments of decision as well as the moment of reunion itself appear to cause anxiety in chimpanzees as they often grimace and, if with another individual, often engage in reassurance touch with them (e.g. hold their hand). This pattern suggests that touch may be an important way to manage social anxiety that is an everyday feature of being a fission-fusion species.