Bonobos often form friendly alliances with other bonobos in different social groups, analogous to some humans – but quite unlike chimpanzees.
While chimpanzees are often so hostile towards other groups of chimpanzees that they kill each other, bonobos willingly share food with members of other groups. Martin Surbek of Harvard University says the findings challenge the idea that humans evolved from ancestral apes that were naturally violent.
“This ability to form cooperative relationships between different groups is not uniquely human and may have happened earlier than we thought,” he says. “There is potentially much more behavioral variation in our evolutionary past than we have so far estimated.”
Many animals – including various mammals, birds, and insects – cooperate with each other, but they appear to do so only with people within their close social circle or group. Hostile interactions between groups are common in animals, including chimpanzees (pan troglodytes, Because chimpanzees are humans’ closest living relatives, scientists have often assumed that violence and hostility toward other social groups are innate in humans, says Liran Samuni, also at Harvard.
However, humans also cooperate extensively with people from different social sectors – for example, by trading, sharing, and teaching others what they know – without receiving any immediate benefits from that cooperation.
bonobos (pan paniscus) are humans’ other closest living relatives. While bonobos have been significantly less studied than chimpanzees, researchers do know that they tend to be more peaceful, Surbek says — at least insofar as they don’t kill or sexually assault each other. .
To learn more about interactions between groups, Surbek and Samuni observed 31 adult bonobos from two different social groups in the Democratic Republic of the Congo over a two-year period. The pair documented 95 encounters between the two groups lasting from less than an hour to more than 14 days, representing about 20 percent of their total observation time.
As expected, bonobos prepared food, shared food, and formed alliances with group members. But unlike chimpanzees seen in previous studies, they also showed cooperation with outside group members. In fact, 10 percent of mutual sharing of equipment and 6 percent of food sharing occurred between members of different social groups.
While bonobos that groomed others usually got an immediate benefit from the recipient returning the favor, sharing food rarely yielded gifts in return. Only 14 percent of the bonobos that received food were observed to respond in the same way. This suggests that their actions “were not motivated solely by selfish interests or immediate rewards”, Surbek and Samuni report.
Caspar Otten at the Center for Research and Documentation in the Netherlands finds the study “exciting”, particularly because it “challenges the idea of human exceptionalism” with regard to cooperation outside one’s group.
Otten says that the bonobos that were most cooperative and least aggressive within their groups were also the ones that cooperated more with out-group members, which matches the findings in humans. “Scholars used to believe that ‘love’ in the group went hand in hand with ‘hate’ outside the group, but recent research shows that those who cooperate in the group are often also the ones who cooperate outside the group, ” he says.
The reason why chimpanzees and bonobos differ so much in their levels of hostility and cooperation is still uncertain. Samuni says this may be related to social structures based on male dominance in chimpanzees, whereas women have higher status in bonobos. “There are all these theories about the warrior male hypothesis in human evolution, and the differences between the sexes, and how we see things, and how competition plays out differently within the sexes,” she says.