A connection between threats from outside groups and the unity of the group itself has been recognized as an evolutionary adaptation for competition among groups since Darwin’s time. Over the years, various studies conducted on animals such as chimpanzees, cichlid fish, and mongooses have supported this idea. However, an important question has lingered: How do species without strong competition between groups fit into this view?
When facing threats from external groups, species like humans and chimpanzees, along with some others, tend to bond more closely with their own group. A new study led by researchers from Kyoto University has discovered that even bonobos—species known for their peaceful behavior and never having been observed to harm outsiders—display a slight version of this behavior. This suggests that such social bonding may have originated millions of years ago, before the evolutionary paths of these species diverged.
A connection between outgroup threats and ingroup cohesion has been understood since Darwin’s era as an adaptation to facilitate competition among groups. Various studies spanning different species—from chimpanzees to cichlid fish to mongooses—have provided evidence supporting this idea. Nonetheless, a significant inquiry remains: what about species that experience little inter-group competition?
To investigate this, the Kyoto University research team designed an experiment that closely mirrored a previous study conducted with chimpanzees, which involved playing vocalizations from other groups for the bonobos. The study looked at eight bonobo groups across five locations in four countries.
“We were uncertain about the outcomes of this study,” noted lead author James Brooks.
“In the absence of lethal competition between groups, you might expect that the connection between ingroup bonding and outgroup threats would be less beneficial. However, if this trait has been present since before the split between humans, chimpanzees, and bonobos, we could observe remnants of it in today’s bonobos.”
The results, published in the journal PLOS ONE, show that the bonobos responded to the calls from other groups with heightened alertness, although their increase in bonding with their own group was relatively modest when contrasted with chimpanzees. The bonobos displayed a tendency to sit more upright and rest less, along with a slight rise in social grooming activities—essential for strengthening their social connections.
The researchers suggest that our common ancestor, who lived approximately 5-6 million years ago, might have experienced some form of group conflict. However, as bonobos evolved and the climate of conflict lessened, the effects associated with group bonding also diminished.
“Our research highlights deep-seated roots of group conflict in our species, but the key insight is that such patterns can be overcome,” Brooks emphasized. “This applies not just in isolated cases, but at a broad species level.”
Other ape species, including gorillas, orangutans, chimpanzees, gibbons, and humans, have been observed engaging in lethal acts against one another in the wild. Bonobos may have managed to break this cycle, primarily because they neither display fatal aggression today nor have they done so at several points in their evolutionary journey.
“Humans possess the capacity for both extremes: we can commit terrible acts against those we perceive as outsiders, yet we are also capable of cooperation and collaboration across divides,” said senior author Shinya Yamamoto.
“Bonobos remind us that the behaviors of our ancestors towards other groups do not dictate our fate. Our species embodies traits of both chimpanzee and bonobo social dynamics, making it essential to comprehend the evolution of both.”
