Observations of chimpanzees and bonobos can provide insight into how our early human ancestors lived together. A study of wild bonobos now shows that they often cooperate across groups – in contrast to chimpanzees, which are usually hostile towards individuals from other groups. The social structures of bonobos could provide a model for how the first cooperative relationships beyond the family group emerged in early human societies.
For us humans, cross-group cooperation is an important success factor. We pass on knowledge across space and time and have built global trade networks. In the search for the evolutionary origins of our behavior, researchers often observe non-human primates, especially chimpanzees, which are among our closest relatives. However, interactions with other groups are usually hostile. Deadly fights are not uncommon on the hard-defended borders of these great apes’ territories, and researchers have observed wars between hostile groups on several occasions. So is our evolutionary heritage warlike and only our culture allows us to enter into strategic cooperation?
Neighboring groups observed
A new study of bonobos now paints a different picture. For two years, Liran Samuni from the German Primate Center in Göttingen and Martin Surbeck from Harvard University in Cambridge observed wild bonobos in the Kokolopori Bonobo Reserve in the Democratic Republic of Congo. They recorded the interactions between two neighboring social groups, the Ekalakala group, consisting of three adult males and eight adult females, and the Kokoalongo group, which includes seven adult males and 13 adult females.
“We documented 95 encounters between the two groups, which together accounted for around 20 percent of the total observation time,” report Samuni and Surbeck. The duration of the individual encounters ranged from less than an hour to 14 consecutive days. “When bonobos from different groups met each other, they showed similar behaviors to those within the group: we observed that they groomed each other, helped each other and shared food with each other.” Aggressive arguments also occurred, but not more frequently than within the group. Unlike chimpanzees, none of these conflicts ended in death.
Selective interactions
“The remarkable tolerance between bonobo groups paves the way for pro-social, cooperative behaviors and is in stark contrast to what we observe in chimpanzees,” says Samuni. However, family relationships apparently do not play a significant role in the social behavior of bonobos across groups, as the team reports. It is known that sexually mature females of these great apes leave their group and join another one. However, such an exchange has not occurred between the Ekalakala and Kokoalongo groups since at least 2016 and genetic analyzes showed that there are no close relationships between the two groups.
However, the bonobos do not enter into a cooperative relationship with everyone at random. “They prefer to interact with certain members of other groups who are more likely to return the favor, which leads to strong bonds between prosocial individuals,” explains Surbeck. “Such connections are also a key aspect of cooperation in human societies.” The observations suggest that bonobos are able to remember in the long term which member of the other group has already helped them. In addition, Samuni and Surbeck found that individuals who were particularly cooperative within their own group were also more likely to interact positively with members of the other group.
Basis of human social evolution?
“Our results show that cross-group cooperation between unrelated individuals without immediate reciprocation does not only occur in humans,” the researchers write. “This suggests that such cooperation can arise even without social norms or strong cultural dispositions.” In today’s complex human societies, social norms and culture are important mechanisms for enabling large-scale cooperation. “However, in our evolutionary past, when we lived in smaller societies, one can assume that reciprocity played a more important role,” say the researchers.
In a commentary accompanying the study, also published in the journal Science, Joan Silk of Arizona State University writes: “If the ancestors of modern humans treated members of other groups as they treated the bonobos, this could be the first step in the evolution of “This does not mean that reconstructions of early human social systems should rely exclusively on bonobos. In many other respects, such as the use of tools, chimpanzees are much more similar to humans. In addition, the social networks within the groups of chimpanzees and bonobos are different. “Understanding the selective forces that led to these differences could help clarify how and why humans evolved into such an unusual ape,” Silk said.
Source: Liran Samuni (German Primate Center Göttingen) et al., Science, doi: 10.1126/science.adg0844