In a paper published this week in the journal iScience, an international team of researchers documented chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) purposefully using signals to start and then end social activities, a behavior not seen outside of the human species until now.
Joint action structure of chimpanzees and bonobos resembles that of humans. Image credit: Emilie Genty.
“We were able to launch rockets and land on the Moon because we have the ability to share our intentions, which allows us to achieve things so much bigger than a single individual can achieve alone,” said Dr. Raphaela Heesen, a postdoctoral researcher at Durham University.
“This ability has been suggested to be at the heart of human nature.”
Sharing intentions and working together on a common goal leads to a mutual sense of obligation otherwise known as joint commitment — and now, the study authors are seeing evidence in great apes that might challenge the long-held claim that joint commitment is unique to humans.
In previous experiments of joint commitment, human children protested when an experimenter abruptly stopped playing with them. Offering toys or vocalizing, the children tried to re-engage the experimenter in their previously agreed-upon play.
After witnessing a similar situation between two bonobos, who were interrupted while grooming but then used gestures to resume the interaction with each other, Dr. Heesen and her colleagues became curious to learn more about how and when joint commitment first emerged in the human lineage.
But unlike previous scientists, the researchers proposed that joint commitment isn’t solely based on the feeling of obligation between two participants to fulfill a shared promise.
Instead, it also involves the process of setting up the agreement and mutually deciding afterward that the agreement has been fulfilled.
That means something as simple as entering a conversational commitment with eye contact and a ‘hello’ and then signaling that a conversation is wrapping up with repeating ‘okay, sounds good’ or a ‘goodbye’ could be an example of this process.
So the scientists set out to see if great apes had a similar interaction entry and exit process, which they argued would demonstrate the process of joint commitment.
They analyzed 1,242 natural play and grooming interactions of captive chimpanzees and bonobos.
They found that the apes did in fact frequently gaze at and communicate with each other to start and end interactions.
Bonobos exchanged entry signals and mutual gaze prior to playing 90% of the time and chimps 69% of the time.
Exit phases were even more common, with 92% of bonobo and 86% of chimpanzee interactions involving exits.
The signals included gestures like touching each other, holding hands or butting heads, or gazing at each other, before and after encounters like grooming or play.
The team also considered factors like how close the apes were to each other socially or who had more power over the other.
Interestingly, the closer bonobos were to each other, the shorter the duration of their entry and exit phases, if they existed after at all. This pattern is similar to how we, as humans, communicate with others, too.
“When you’re interacting with a good friend, you’re less likely to put in a lot of effort in communicating politely,” Dr. Heesen said.
“However, the level of friendship and strength of social bonds didn’t seem to affect the chimpanzees’ entries and exits at all.”
“This could be because in comparison to chimps’ despotic power hierarchies, bonobo societies in general are documented to be more egalitarian, with emphasis on friendships and alliances between females and close mother-son relationships.”
Raphaela Heesen et al. Assessing joint commitment as a process in great apes. iScience, published online August 11, 2021; doi: 10.1016/j.isci.2021.102872