Bonobo Behavior Reveals Ancient Origins of the 'Common Enemy Effect'
Insights into Group Cohesion & Conflict Resolution in Early Human Evolution
In response to external threats, species ranging from humans to chimpanzees and other animals often close ranks, reinforcing bonds within their groups. This phenomenon, known as the "common enemy effect," has long been recognized as a strategy for survival in the face of group-based competition. A recent study1 by an international team led by Kyoto University reveals that even bonobos, a species known for their peaceful nature and lack of lethal aggression, display a moderated version of this behavior. This finding suggests that the roots of group cohesion in response to external threats may date back millions of years—long before our evolutionary paths diverged.
The Evolutionary Context of Group Solidarity
The connection between outgroup threats and increased ingroup cohesion has been a subject of interest since the time of Darwin. It is considered an evolutionary adaptation that helps species survive by fostering stronger alliances within a group to fend off external dangers. While previous studies have observed this behavior in species like chimpanzees, fish, and mongooses, a critical question remained: does this pattern of behavior exist in species that do not engage in intense inter-group competition?
To explore this question, researchers from Kyoto University designed an experiment to test whether bonobos—our closest evolutionary relatives who have never been observed to kill outsiders—also exhibit this behavior. Their findings, recently published in the journal PLOS ONE, shed light on the ancient origins of group solidarity.
"We had no idea how this would turn out," said James Brooks, the study's lead author. The research team was curious to see if bonobos, who are not known for lethal inter-group conflict, would still show signs of heightened group cohesion when faced with perceived threats from outside their group. This behavior could provide clues to how early humans and their ancestors may have reacted to external threats.
Experimental Design: Testing Bonobo Responses to Outgroup Threats
The Kyoto University team set up an experiment to mirror an earlier study conducted with chimpanzees. They played vocalizations from other groups of bonobos to eight bonobo groups at five different sites across four countries. The goal was to determine whether bonobos, like chimpanzees and humans, would respond to outgroup threats by increasing cohesion within their group.
The results revealed that, while bonobos were alert and attentive to the calls of other groups, their response was more subdued compared to chimpanzees. Bonobos exhibited a minor increase in social behaviors associated with group cohesion, such as sitting upright more frequently and resting less. They also showed a subtle increase in social grooming, a key behavior for reinforcing social bonds. However, the intensity of their response was significantly less than that observed in species like chimpanzees, which are known for their aggressive inter-group conflicts. The study's authors noted,
"Our findings suggest that although bonobos display some group-based solidarity in response to outgroup threats, the effect is far less pronounced than in species with a history of lethal inter-group competition.”
Deep Roots of Group Conflict and Cohesion
The results of the study point to a shared evolutionary history of group-based conflict and cohesion among our ancestors, dating back 5–6 million years. While the common ancestor of humans, chimpanzees, and bonobos may have engaged in some form of group conflict, the intensity of these conflicts appears to have diminished in bonobos over time. As a result, the bonobos' response to outgroup threats became more subdued, even as their capacity for group cohesion remained. Brooks said,
"Although our study exposes deep roots to group conflict among our species, the real takeaway is that this can be overcome. Not merely in individual instances, but on a species level."
This finding offers hope that the patterns of group conflict observed in humans and other primates are not immutable.
Unlike other great apes, including chimpanzees, gorillas, orangutans, gibbons, and even humans, bonobos have evolved a way to end the cycle of lethal aggression. This behavior is not only a reflection of their peaceful nature today, but also a sign that their evolutionary history diverged from the more violent tendencies of their relatives at some point in the past.
Bonobos as a Model for Conflict Resolution
Senior author Shinya Yamamoto emphasized the importance of understanding the evolutionary origins of both violent and peaceful behaviors in humans. Yamamoto explained,
"Humans are capable of both: we can commit horrific acts to those we see as outside our group, but we're also capable of collaborating and working together across borders. Bonobos teach us that the ways our ancestors treated other groups does not seal our fate."
The study suggests that, while humans may inherit tendencies for both conflict and cooperation from their evolutionary ancestors, understanding the evolution of these behaviors in species like bonobos could help us shape a more peaceful future. By examining the social dynamics of bonobos, who have evolved to avoid lethal conflict, researchers hope to gain insights into how our species can foster collaboration and minimize violence.
Conclusion: Ancient Origins of Group Behavior
The findings from this study highlight the deep evolutionary roots of the "common enemy effect" and its role in fostering group cohesion. While the behavior may have originated as a survival strategy in response to external threats, the evolution of species like bonobos shows that it is possible to overcome violent tendencies and create more cooperative social structures. As researchers continue to study the social dynamics of our closest relatives, the hope is that these insights will help inform our understanding of human behavior and guide efforts to promote peace and cooperation within our own species.
Brooks, J., van Heijst, K., Epping, A., Lee, S. H., Niksarli, A., Pope, A., Clay, Z., Kret, M. E., Taglialatela, J., & Yamamoto, S. (2024). Increased alertness and moderate ingroup cohesion in bonobos’ response to outgroup cues. PloS One, 19(8), e0307975. https://doi.org/10.1371/journal.pone.0307975