Warfare, and other forms of collective violence, are one of the more distinctive, if less pleasant, forms of Human behaviour. In these acts, Humans are able to sort themselves into groups defined by traits other than kinship, such as ethnicity, religion, language, or other cultural traits, in order to engage in conflict against other groups of Humans. It has been proposed that this is a result of our ability to use cultural traits to define cohesive social groups, giving us a distinct sense of who is 'us' and who is 'them', enabling closer cooperation with members of our group, while at the same time maintaining a hostility to outsiders.
However, this cannot explain how conflict can erupt within formerly cohesive groups, leading to rebellions and civil wars. This has been explained by suggesting that shifting interpersonal ties and rivalries can sometimes reach a point where internal hostilities overcome the cohesion of the group, regardless of the cultural connections which have been built up. This hypothesis has some evidential support, with observations suggesting that Humans can rapidly come to regard former members of an in-group as members of an out-group for the most arbitrary of reasons.
Non-Human Animals also engage in territorial aggression, and sometimes lethal conflict with cospecifics, despite not having the religious, cultural, or political ideologies seen in Humans. This has been observed in a variety of Animals, including Banded Mongooses, Lions, Wolves, and Primates. Non-Human Animals living in social groups also have episodes of fission, in which one group splits permanently into two or more, something which is often explained in terms of feeding competition and social pressures, although most Animal groups do not engage in lethal combat during such fission episodes.
Chimpanzees, Pan troglodytes, are among our closest relatives, and in some ways can be seen as a bridge over the gap between Human and non-Human behaviour. Male Chimpanzees stay within the group they were born in, and engage in cooperative defence of the group's territory, as well as raids on the territories of neighbouring groups. Lethal violence between males of different groups is quite common during both of these activities. This can be explained in terms of hostility towards outsiders, while remaining loyal to members of the kin group. Chimpanzee groups are known to occasionally split, a process which is thought to involve lethal violence between males familiar with one-another.
In the 1970s, a group of Chimpanzees living in the Gombe National Park in Tanzania was thought to split in two, with the males of one of the new groups subsequently killing one of the adult females and all six adult males in the other group over a period of about four years. However, this group was not under constant observation, so that much of this activity is inferred rather than having been directly witnessed, and no subsequent observations of similar splits in Chimpanzee groups were recorded in the following decades. Furthermore, a study published in 2014 which looked at the genetic structure of Chimpanzee groups suggested that such ruptures were extremely uncommon, with groups splitting on average once every 500 years.
In a paper published in the journal Science on 9 April 2026, a group of scientists led by Aaron Sandel of the Department of Anthropology at the University of Texas at Austin, present a detailed and extensively documented study of a fission event which split a group of Chimpanzees living in the Kibale National Park in Uganda, and the lethal violence associated with this split.
Sandel et al. analysed 24 years of data on the social interactions of the group, ten years of GPS data, and 30 years of demographic data on the Ngogo Chimpanzee Group, which demonstrated a three step process, in which a formerly cohesive group polarised into two clusters with little social interaction between these clusters, these clusters then actively avoided one-another for two years, before engaging in a series of lethal aggressive actions. Sandel et al. take this as evidence that Chimpanzee groups can fracture and engage in collective violence against members of the same community without any of the cultural markers seen in Humans.
The Ngogo Chimpanzees have been the subject of a continuous research project since 1995. For the first two decades of the study, they remained a single group, although they did go through a regular fission-fusion dynamic in which the larger group split into parties which changed throughout the day, though individuals frequently moved between parties and all remained within the overall territory; this is something commonly seen in Chimpanzee groups. Females typically migrated from (or to) the group at adolescence, while males born into the group remained there for life. The males formed a strict dominance hierarchy, associated in mixed-sex parties, hunted together, and cooperated in territorial patrols.
Each adult male in the group was followed for 2-3 months each year between 1998 and 2024 to see which other individuals they associated with, stayed close to, and engaged in grooming with. Despite being a single group, the Ngogo Chimpanzees typically split into two-to-four clusters over the course of a year, with two persistent and long-lived clusters, the Western and Central clusters. Membership of these clusters was fluid, with 29% of Chimpanzees switching cluster each year, and extensive ties maintained between Chimpanzees in different clusters.
As well as clusters, Sandel et al. identified a number of 'cliques' of males that consistently stayed together, even when switching clusters. One of these cliques comprised a group of three males that would go on to form the core of the post-fission Western Group. These three males remained together consistently, even when forming clusters with males that would go on to be in the post-fission Central Group. A cluster comprising exclusively males that would go on to form the post-fission Western Group first appeared in 2014.
In 2015, Ngogo Chimpanzees at the edge of their territory hear calls from neighbouring Chimpanzees; adult males embrace each other in an act of reassurance before moving toward the calls and engaging in an intergroup encounter. 'C' and 'W' designations refer to males who would later become members of the Central and Western groups, respectively. Sandal et al. (2026).
Despite the different social clusters, all of the Ngogo Chimpanzees, including all of the males, had overlapping space use patterns, and all males shared the same set of reproductive partners. All Chimpanzees born within the Ngogo Group between 2004 and 2014 that it was possible to genetically sample had both parents from within the group, though 44% had parents from two different clusters.
The first sign of a split between the clusters was observed on 24 June 2015, when members of the Western and Central clusters were seen to approach one-another near the centre of the territory. Unexpectedly, rather than the two groups merging as usually happened on such occasions, the Chimpanzees of the Western Cluster ran away, with the Chimpanzees of the Central Cluster chasing them. The two clusters then avoided one-another for six weeks, something which had never been observed before.
The events of 2015 precipitated the greatest change of social change seen throughout the decades-long study of the group. What had been a single large group of Chimpanzees split into a number of smaller units, stabilising as two new groups by 2018.
The first patrol by one group against another happened in 2016, when males of the Western Group, accompanied by two of the Central Group males, staged a patrol against the Central Group. All subsequent patrols by the Western males contained only members of that group. In 2017, the Central Group males staged their first patrol against the Western Group, with aggressive interactions between the two groups escalating rapidly from that point. During one encounter in 2017 the males of the Western Cluster attacked the alpha male of the Central Cluster (who had been part of the Western Cluster before 2014), severely injuring him. Both groups subsequently increased the number of patrols against the other group.
By 2017 the two groups were using largely distinct territories, with the overlap between the two groups being similar in size to that seen between unrelated groups. The centre of the shared territory had become a border. This was accompanied by reproductive isolation between the two emerging groups, with the last infant with parents from different groups being conceived in March 2015. All subsequent births had both parents from the same new group.
By 2018, the original group appeared to have split into two new groups, entirely separated from one another. At this time the Western Group included 10 males and 22 females over the age of 12, and the Central Group comprised 30 males and 39 females over the age of 12. Until 2018, a few of the females and infants from the Central Group would occasionally join the Western Group when foraging from Fig trees. After 2018 all such activity ceased.
Following this complete split, the Western Group initiated a series of lethal attacks against members of the Central Group. On all occasions members of the Central Group were attacked by multiple members of the Western Group, during a patrol by members of the Western Group into the territory of the Central Group. Six lethal attacks on males belonging to the Central Group were observed between 2018 and 2024, with a seventh attributed with a high degree of confidence. From 2021 these attacks were also carried out on infants from the Central group, with fourteen infanticides observed, and another three inferred.
Sandal et al. note that this is a conservative estimate. Between 2021 and 2024 another 14 adult and adolescent males from the Central Group disappeared, and are thought likely to have been killed. None of these individuals showed signs of any illness when they were last observed.
In 2019, the Western Chimpanzees, including W1, W2, and W3, attack and kill the adult male Central Chimpanzee C1, who they had associated with before the fission, marking a notable shift in relationships. Sandal et al. (2026).
By using decades of data gathered on the Ngogo Chimpanzees in Kibale National Park, Sandel et al. have been able to demonstrate the occurrence of a fission event within a wild group of Chimpanzees, something predicted to happen only once every 500 years. This was followed by a series of lethal aggressive interactions, with targeted violence continuing years after the split, something not observed in any other non-Human Primate. The rate at which killings occurred was far higher than that seen in small Human societies. This demonstrates that Chimpanzees can develop new group boundaries and defend them violently, despite not having any concept of ethnicity, religion, or political ideology.
Sandal et al. speculate that a number of factors might have contributed towards the division of the Ngogo Chimpanzee Group. Firstly, the group was unusually large, with almost 200 individuals, and almost 30 males, much larger than other known Chimpanzee groups, which may have strained the males ability to maintain good relations with all other members of the group. Secondly, feeding competition has been shown to play a role in group fission in other Primate species. The area occupied by the Ngogo Group had abundant food, but the large size of the group may have caused strain at some times of the year. Thirdly, the two groups became reproductively isolated before finally separating into two groups, something which may have increased male-male aggression as they had to compete for a smaller number of mates.
Other factors which may have played a role are also observed by Sandal et al.. Firstly, six adult members of the group, five males and a female, died in 2014. The cause of these deaths is unknown, but two showed signs of illness before their deaths, making it possible that the group was hit by a disease. The loss of more than 10% of the male Chimpanzees in the group may have weakened the groups network of social relationships, leading to the group beginning to break up in 2015.
Next, the dominant male in the group changed in 2015, immediately before the fission of the group. Such changeovers are known to raise tensions among male Chimpanzees, leading to increases in behaviours such as aggression and avoidance. The former dominant male belonged to the Central Group, whereas the new dominant male came from the Western Group, but moved to the Central Group when he ascended to the top of the hierarchy, which Sandal et al. suspect may have increased tensions between the two groups.
Finally, in January 2017, the group was hit by a respiratory epidemic which killed 25 Chimpanzees, including four adult males and ten adult females. Two of the males that died were from the Western Group, including one of the last males in that group to be maintaining relations with the Central Group. Thus, even though this event happened after the groups had started to split, it may have contributed to the final breakdown in relations between the two.
Chimpanzees are known to have a strong sense of who is in their group, and who is not. Female Chimpanzees leave the group they were born into as adolescents and look for a new group to join, but males remain in the group they were born into their entire lives. Thus any unfamiliar male Chimpanzee is treated as a stranger, with no recorded instances of wild male Chimpanzees forming relationships or cooperating with Chimpanzees in other groups. Under these circumstances, intense hostility towards outside males appears to be an adaptive trait in male Chimpanzees, leading to potential territorial extensions for the group, which in turn leads to more food and other resources, increased female fertility, and a greater survival rate among juveniles.
This cannot, however, explain the lethal aggression sometimes displayed to members, or former members, of the same group. During the fission of the Ngogo Group, males which had lived, fed, groomed, and patrolled together for years became targets of lethal attacks on the basis of their new group membership. This leads Sandal et al. to conclude that Chimpanzees have a sense of who belongs to their group which is based upon more than simple familiarity, and which can be updated to reflect changes in circumstances.
These results challenge previous assumptions about intergroup conflict in Chimpanzees in a number of ways. All observed attacks were initiated by the numerically smaller Western Group, which contradicts the assumption that larger groups have an advantage in such conflicts. The emergence of greater social cohesion in the smaller group appears to have given them an advantage in conflict which more than made up for the greater numbers of their opponents. These closer bonds appear to have developed before the onset of aggressive activity, suggesting that an external threat is not needed to forge these bonds. Conversely, the original Ngogo Group underwent a territorial expansion in 2009, which appears to have reduced the threat that it faced from rival groups, something which may have contributed to the long-term decline in social cohesion within the group. If this is the case, than an external threat may not be needed to forge close bonds, but it may be needed to maintain them.
Sandal et al.'s findings also have implications for Human behaviour. If Chimpanzee groups can split in this way without any of the cultural markers associated with Human conflict, then these may be less important to Humans than we generally assume, masking the actual roots of aggression, which may have more to do with interpersonal relationships. Humans tend to attribute the conflict to ethnic, religious, or political divisions, but this may be misleading, covering the actual causes of conflict. If this is the case, then it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.
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