Conflict and civil war have been an ever-constant presence in humanity, but they are not uniquely human phenomena. For the first time, researchers have described in detail the way that a chimpanzee group fissioned—or split—into two groups. Notably, the members of one group then began to violently attack the other group, killing chimpanzees that had been their groupmates only a few years ago.
Some background: conflict and warfare in animals
Human warfare is driven by a variety of factors, including our ability to readily develop cultural groups that then form strong ingroup attachments and outgroup hostility (Efferson et al., 2008). In other words, we readily form groups and show favoritism towards people who we consider to be also in the same group, while we are much less favorable towards people who we view as not in the group. Cultural differences can then become a driver of conflict.
However, culture—language, social norms, ethnicity, religion, ideology, etc.—are distinctively human concepts. Nonhuman animals that also live in social groups, such as lions and wolves, can also have lethal, humanlike “wars” over issues like territory (Mosser & Packer, 2009; Cubaynes et al., 2014). Such animals notably do not have cultural markers in the same way humans do, and their conflicts are definitively driven by other factors, which can provide some insight into how group conflicts have evolved.
But what about conflict that emerges from within a group? Somehow, ingroup morphs into outgroup. This happens in humans, but social animals can also experience permanent divisions in their groups, where the original group splinters into two distinct groups. Those divisions often occur mainly because of environmental reasons, such as competition over food, and the two groups formed after the split typically do not attack and kill each other (Sandel et al., 2026).
One notable exception to this is a group of chimpanzees at Gombe, Tanzania, which permanently split into two groups in the 1970s. Males from one group, over the course of four years, killed all six males and one female in the other group. This is exceedingly rare—estimated to only occur roughly once every 500 years—and exactly how and why it happened is unclear: researchers at the time provided food to the chimpanzees, and certain details of the split are unclear due to their limited observations of the primates (Langergraber et al., 2014; Sandel et al., 2026).
Chimp civil war
Chimpanzees (Pan troglodytes), along with bonobos (Pan paniscus), are the closest animal relatives we humans have. Chimpanzees live in social groups, with males staying in the same social group for their entire lives, occasionally venturing out to attack other groups and expand their own territory (Mitani et al., 2010). Male chimpanzees mainly attack and kill male chimpanzees in other groups, but lethal violence can also erupt between chimpanzees in the same group, as seen with the Gombe group (Wilson et al., 2014). As such, studying their social behaviors may lend insight into our own.
In their study published in 2026, Aaron Sandel and colleagues analyzed data from a group of over 200 chimpanzees, known as the Ngogo chimpanzees, in Kibale National Park, Uganda. Starting from 1998, the researchers tracked the movements of the Ngogo chimpanzees by recording the interactions the of the adult male chimpanzees of the group. Initially, from 1998 until 2014, the Ngogo chimpanzees were one cohesive group, with chimpanzees living, patrolling, feeding, and mating together. Subgroups of the main group existed, with a Western cluster and a Central cluster, but on average every year, almost a third of the chimpanzees would switch between the clusters, and almost half the infants conceived were by chimpanzees from different clusters.
But this began to change in 2015, which would be the first sign of the fission of the Ngogo group. The group suddenly began to polarize. Chimpanzees in the Western cluster interacted more with each other and less with chimpanzees in the Central cluster, and vice versa. The clusters also increasingly stayed in only their region of the territory and began actively avoiding each other. After this year, infants were also only conceived by males and females in the same group.
In 2016, the two clusters began to patrol their own territories, guarding against the other group, and in 2017, the two groups had increasingly hostile and violent encounters. In 2018, Ngogo chimpanzees permanently split into two groups, the Western group and the Central group.
After this fission, despite consisting of less individuals, the Western group would regularly perform territorial patrols into the Central group’s territory, killing at least six adult males. In 2021, the Western group also started attacking infants, killing at least 14.
It’s worth mentioning again that only a few years ago, these chimpanzees were all members of one cohesive group. This is emphasized in this one example highlighted by the researchers. In 2015, a group of Ngogo males, including three future Western males and one future Central male, patrolled together and showed friendly social interactions, such as embracing each other for reassurance. In 2019, a group of Western males, including the same three males, attacked and killed the Central male that they patrolled with less than 4 years prior.
Why did this happen?
The researchers proposed a few reasons why this fission happened.
First, the original Ngogo group consisted of almost 200 individuals, and many males. With this large group, maintaining relationships with all the other chimpanzees in the group is impossible, and the large number of males in the group (30) meant that there was increased competition between the male chimpanzees.
Additionally, a new alpha male rose to power in the group in 2015, at the same time the group began to polarize. Although both the previous alpha male and the new alpha male were part of the Central group, this change could have increased the instability and tension of the group.
Lastly, the split may have been caused in part by deaths of certain chimpanzees who acted as important nodes in the network of relationships in the group. The deaths of those chimps may have weakened relationships across the group and facilitated the fission. For example, in 2014, just before the beginning of the polarization, five adult males and one adult female died. In 2017, as the group was already beginning to divide, 25 chimpanzees in the group died in an epidemic, including an adult male that was one of the last individuals that connected the two groups. The deaths of these individuals could have weakened the relationships between the two clusters, effectively helping cleave the original Ngogo group into the Central and Western group.
What does this mean?
According to the researchers, this violence is notable because it shows us how aggression and group identity forms in chimpanzees and how group conflicts develop in the absence of cultural factors.
Typically, as male chimpanzees stay in the same group their entire lives, they behave aggressively towards strangers. However, as the attacks described were towards chimpanzees they already knew, it shows that group identity in chimpanzees can be flexible and is controlled by other factors, beyond just familiarity.
This study also describes how relationship dynamics can play a major role in group conflicts. Although not to the scale of human wars, this “civil war” between the Ngogo chimpanzees shows how a major group conflict can stem from the dissolution of relationships between individuals. If the right (or rather wrong) relationships are broken, the unity of a group may also soon be in jeopardy, even without cultural factors like ethnicity and religion.
The fission of the Ngogo chimpanzees can feel like an omen of our future as humans, forever violent and marked by splintering factions. But it’s worth remembering that we humans are capable of violence but also of empathy. Even comparatively violent, conflict-prone chimpanzees show empathetic actions. Chimpanzee bystanders of a fight will go to comfort the losing chimp afterwards, and chimps can also show reconciliatory behaviors after a fight, such as holding hands, embracing, or kissing (De Waal, 2012; De Waal, 2000).
As James Brooks notes in his 2026 perspective article on the study, we can also turn to look at our other, equally close great ape relative, the bonobo. Once thought to be a subspecies of chimpanzees, bonobos are much more peaceful: even though bonobos live in social groups, the groups do not violently interact. Bonobos not only form cooperative relationships within their own groups but also with individuals from other groups (Samuni & Surbeck, 2023). Lastly, even though bonobo groups can permanently split, it doesn’t result in the violence seen in the Ngogo fission. A group of bonobos in Wamba, Democratic Republic of the Congo permanently split almost 50 years ago, but continue to peacefully coexist (Brooks, 2026; Kuroda, 2023).
Overall, this study shows that conflict emerges even in the absence of cultural markers—instead, a key factor of group conflict might lie in the way social relationships form, are maintained, and break down. As such, this can also be reversed, as those basic social processes might also be a way to approach conflict resolution. By better understanding the way in which a group splits into warring factions, we can also make headway into understanding how to prevent and resolve such instances from occurring in ourselves.
Indeed, as the researchers state in the closing of their paper, “it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.”
Writing by Joyce Ma
Picture by © Giles Laurent, gileslaurent.com, License CC BY-SA, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=134376811
References
Brooks, J. (2026). Civil war among wild chimpanzees. Science, 392(6794), 146–147. https://doi.org/10.1126/science.aeg6719
Cubaynes, S., MacNulty, D. R., Stahler, D. R., Quimby, K. A., Smith, D. W., & Coulson, T. (2014). Density‐dependent intraspecific aggression regulates survival in northern Yellowstone wolves (Canis lupus). Journal of Animal Ecology, 83(6), 1344–1356. https://doi.org/10.1111/1365-2656.12238
De Waal, F. B. M. (2000). Primates–A natural heritage of conflict resolution. Science, 289(5479), 586–590. https://doi.org/10.1126/science.289.5479.586
De Waal, F. B. M. (2012). The antiquity of empathy. Science, 336(6083), 874–876. https://doi.org/10.1126/science.1220999
Efferson, C., Lalive, R., & Fehr, E. (2008). The coevolution of cultural groups and ingroup favoritism. Science, 321(5897), 1844–1849. https://doi.org/10.1126/science.1155805
Kuroda, S. (2023). From the Early Days of Bonobo Research. In Bonobos and People at Wamba: 50 Years of Research (pp. 25–37). https://doi.org/10.1007/978-981-99-4788-1_2
Langergraber, K. E., Rowney, C., Schubert, G., Crockford, C., Hobaiter, C., Wittig, R., Wrangham, R. W., Zuberbühler, K., & Vigilant, L. (2014). How old are chimpanzee communities? Time to the most recent common ancestor of the Y-chromosome in highly patrilocal societies. Journal of Human Evolution, 69, 1–7. https://doi.org/10.1016/j.jhevol.2013.12.005
Mitani, J. C., Watts, D. P., & Amsler, S. J. (2010). Lethal intergroup aggression leads to territorial expansion in wild chimpanzees. Current Biology, 20(12), R507–R508. https://doi.org/10.1016/j.cub.2010.04.021
Mosser, A., & Packer, C. (2009). Group territoriality and the benefits of sociality in the African lion, Panthera leo. Animal Behaviour, 78(2), 359–370. https://doi.org/10.1016/j.anbehav.2009.04.024
Samuni, L., & Surbeck, M. (2023). Cooperation across social borders in bonobos. Science, 382(6672), 805–809. https://doi.org/10.1126/science.adg0844
Sandel, A. A., He, Y., Ren, J., Kei, Y. L., Lee, K. C., Clark, I. R., Reddy, R. B., Negrey, J. D., Birungi, C., Apamaku, B. A., Kanweri, D., Kalunga, D., Aliganyira, C., Ramírez-Amaya, S., Nakayima, P., Katumba, R., Kamugyisha, B., Acosta-Florez, D., Van Boekholt, B., . . . Mitani, J. C. (2026). Lethal conflict after group fission in wild chimpanzees. Science, 392(6794), 216–220. https://doi.org/10.1126/science.adz4944
Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., Hobaiter, C. L., Hohmann, G., Itoh, N., Koops, K., Lloyd, J. N., Matsuzawa, T., Mitani, J. C., Mjungu, D. C., Morgan, D., Muller, M. N., Mundry, R., Nakamura, M., Pruetz, J., . . . Wrangham, R. W. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414–417. https://doi.org/10.1038/nature13727
