Sharing bonobos bond with strangers

Bonobos – great apes from the region south of the Congo River in central Africa – are the closest living relative of Homo sapiens. The findings of a study by researchers from the Department of Evolutionary Anthropology and the Center for Cognitive Neuroscience at Duke University published in the open access journal PLOS ONE this week suggest that bonobos may share a surprising trait with humans – the propensity to share with complete strangers. The evidence suggests that, although bonobos will share for unselfish reasons, it is more likely that they donate food to individuals from outside their group in order to expand their social network.

Based on a set of four experiments testing the willingness of bonobos to share food under various circumstances and with individuals previously known or unknown to them, the results showed that bonobos will give up food for the chance to interact with a stranger. The bonobos involved in the experiments – all orphans of the bushmeat trade living in the Lola ya Bonobo sanctuary in Kinshasa – were observed to aid strangers in gaining access to food even when there was no opportunity for social interaction. However, they will not give away their food when there is no chance of a social interaction.

Graduate assistant Jingzhi Tan, who authored the paper along with Associate Professor Brian Hare, took the time to answer ScienceOmega.com’s questions on the research and it’s implications for our understanding of behavioural evolution in humans and chimpanzees as well as bonobos…

What prompted you to carry out this research?
Humans are believed by many scientists to be unique in their willingness to share. They even share with strangers who cannot pay them back, demonstrating a genuine concern for the well-being of others. This leads to a very influential hypothesis in economics and evolutionary anthropology that only humans are capable of this kind of ‘kindness’ or ‘altruism’ and that this is what forms the foundations of human societies.

This seems to be backed up by looking at nonhuman primates, because almost none of these species interact peacefully with strangers. Bonobos, however, have been known to sometimes have peaceful interactions with neighbouring groups in the wild and even though there is occasional tension, it rarely escalates. So we decided to examine whether bonobos share with strangers.

What did your experiments reveal about the motivations for xenophilia and prosociality in bonobos? How significant was their tendency to exchange food for social interaction?
I think the results of our experiments are twofold. First, we show in experiment 3 of the article that bonobos would be nice to a stranger even if there was no immediate benefit, therefore revealing that bonobos are capable of being unselfishly motivated to help strangers.

Secondly, we show that there is a second (and overlooked) motivation to be nice to stranger: social reward. In experiments 1, 2 and 4, we showed that bonobos would voluntarily let strangers (and only strangers) into the room with them to share food. That pile of food was highly desirable and they could have easily monopolised it first, but they chose not to. However, when we took away the social reward, blocking potential social interactions by separating the subject and the recipient, sharing disappeared. These experiments reveal that sharing with strangers does not have to be unselfish; we think their selfish motivation was to start a new friendship.

How does bonobo behaviour compare to chimpanzee and human behaviour in this area?
Chimpanzees in the wild are known to show strong hostility towards strangers including neighbours and new immigrants. Sometimes they will cooperate to kill strangers. It is extremely difficult to integrate a captive chimp into a new social group, and they won't approach a stranger unless they outnumber the opponent by at least three to one.

Not only did bonobos not avoid the strangers, they even chose to forgo highly valuable resources to start a relationship with a stranger. What's more, in experiment 1, we let one bonobo choose to release either a stranger or a groupmate to eat with. Once a recipient was released, there was still the option to release the third bonobo. It turned out the just-released stranger was the one to let the third bonobo into the room; the original stranger chose to let herself be outnumbered by two groupmates who were unfamiliar to her!

In comparison with humans, there are similarities and differences. Even when there is no selfish benefit, bonobos are capable of helping strangers when the cost is low, as was shown in experiment 3. This is evidence that the unselfish motivation to benefit strangers is not unique to humans. In terms of difference, this unselfish motivation is limited in bonobos. When the cost was raised but the selfish benefits were kept to zero in experiment 4 (a situation similar to a dictator game in which humans usually share), bonobo sharing disappeared.

Why do you think bonobos preferred strangers even to their own groupmates? Does this limit the potential to draw parallels between the evolution of prosociality in humans and bonobos?
This does seem to be a difference between bonobos and humans. Humans are known to show ingroup-love and outgroup-hate. One possibility is that bonobos went further to the extreme. Under this scenario, it is still possible that humans and bonobos share similar ‘xenophilic’ motivation, with their difference being a matter of degree. Humans will be nice to strangers, but this kindness may not exceed their preference for groupmates. 

What do you hope to find with further research on this topic?
One question we are trying to answer is exactly the last question: whether humans share similar preferences. We are also hoping to develop a paradigm that allows us to compare chimpanzees and bonobos directly. The current task would be unethical on chimpanzees given their strong xenophobia.


To read the full text of the research paper, click here.