Primatologists have been observing and recording chimpanzee violence for decades. Beginning in the 1960s, primatologist Jane Goodall documented extensive violence among wild chimpanzees in Tanzania. Her observations overturned earlier views that chimpanzees were uniformly peaceful animals. Shortly thereafter, a team of Japanese primatologists, led by Toshisada Nishida, confirmed Goodall's finding of lethal intergroup aggression among chimpanzee communities. Subsequent field studies have provided further insights into the nature of chimpanzee violence, which includes violence within a community (for example, infanticide, sexual coercion and killing of adults) and across communities (including lethal attacks that, over time, have culminated in the extermination of neighboring chimpanzee communities).
This violence exists side by side with rituals of cooperation and affiliation that play a role in male affiliation and coalition building. The picture of chimpanzee violence that emerges from these field studies is a complex one; it consists of conflict, cooperation and cooperation in conflict.
Primatologists have compared these findings with those of field studies of bonobos (a closely related species). This comparison shows that even small genetic differences can be associated with striking differences in behavior across species. It also shows that both violent aggression and peaceful cooperation can take place without a supporting role of complex cultures and social norms typically associated with humans.
Overview
Chimpanzee societies are characterized by male dominance hierarchies, coalition formation, territorial patrols, and occasional lethal violence between neighboring communities. Inter-group relations between chimpanzee communities are "predictably hostile." Each community has a clear alpha male and high ranking males gain clear reproductive benefits in the form of greater access to fertile females.
Males cooperate in defending the group's territory and may attack isolated members of rival groups. Attacks are always one-sided and only occur when the attacking party has clear numerical superiority. Such conflicts can result in serious injury or death. These physical struggles have clear physiological costs for the males.
Within the same community, males compete for mating opportunities and high ranking males have preferential access to fecundable females. Although most mating occurs through social interactions within the group, coercive sexual behavior has been observed. Males are described as "gregarious" and spend significant amount of time in activities that contribute to group cohesion and affiliation. They also form coalitions (often dyadic) of varying stability with other males, especially males of the same age group and rank. Building affiliations is accomplished through such behaviours as mutual grooming, meat sharing and maintaining physical proximity. Kinship is found to have only a "limited" impact on the pattern of male cooperation and affiliation.
Female chimpanzees leave their natal group at the onset of sexual maturity.Nakamura, M. (2015). Mahale chimpanzees : 50 years of research. Cambridge University Press. They are described as being "relatively asocial" and are "aggressive primarily in the context of feeding competition. Mother chimpanzees spend much of their time alone with their offspring or in nursery parties with other mothers and young." Females with young tend to use an individual foraging range which is smaller than the range defended by males. Females also have a hierarchical ranking that confers fitness benefits. Infanticides perpetrated by coalitions of females have been observed. Despite evidence that female rank has important effects on reproduction, aggression by parous females against other parous females is rare, and female dominance ranks are stable over long periods of time." Thus, the propensity for violence is primarily, but not exclusively, a characteristic of male chimpanzees.
Field research conducted over decades shows not only that chimpanzee societies engage in conspecific killings, but they do this more often than many other mammalian species. According to Aaron Sandal and David Watts, many “animals engage in aggression, but chimpanzees stand out in terms of fatal attacks on adults of their own species.” Observations from three long-term field sites — Gombe Stream National Park, Mahale Mountains National Park, and Kibale National Park — played a central role in establishing that violence between neighboring chimpanzee communities is a natural, adaptive feature of chimpanzee behavior. Specifically, a compilation of long term field studies found the following: .... we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio).
Field studies Scientific understanding of lethal intergroup aggression in chimpanzees developed gradually over several decades. When Jane Goodall first reported organized, lethal warfare among chimpanzees in the early 1970s — what became known as the Gombe Chimpanzee War — many scientists simply didn't believe her. At the time, there was a widespread view that humans had invented warfare and that our chimpanzees lived in fundamentally peaceful societies. Goodall's observations gradually convinced the scientific community that coalitionary killing was not exclusively human. Field studies have been conducted at at least 40 sites across Africa and these show conclusively that violence is a "pervasive feature of chimpanzee societies", as is cooperation.
Gombe Stream National Park The first documented evidence of sustained lethal aggression and, in particular, of intergroup aggression among wild chimpanzees came from Gombe Stream National Park in Tanzania, where primatologist Jane Goodall began field studies in 1960.
Between 1974 and 1978, researchers observed a prolonged conflict between two communities that had formed following the division of a previously unified group. During what later became known as the Gombe Chimpanzee War, adult males from the larger Kasakela community systematically attacked members of the smaller Kahama community. Several males were killed, females disappeared or joined the victorious community, and the Kahama community eventually ceased to exist.
Goodall's observations challenged the prevailing view that chimpanzees were fundamentally peaceful animals and suggested that organized coalitionary violence was not unique to humans. Initially, many primatologists questioned her finding that the violence reflected natural chimpanzee behavior. Some suggested that provisioning—the practice of supplying food to habituate chimpanzees to human observers—may have increased competition and artificially promoted aggression. However, subsequent field studies’ findings confirmed Goodall's results. By the late twentieth century, evidence from multiple field sites had persuaded many researchers that coalitionary aggression between neighboring communities formed part of the normal behavioral repertoire of wild chimpanzees, although the frequency and intensity of such aggression varied among sub-populations.
Mahale Mountains National Park In 1966, a team of Japanese primatologists, led by Toshisada Nishida, began a field study of chimpanzees in Mahale Mountains National Park, also in Tanzania. The research project provided important confirmation that lethal intergroup aggression was not confined to Gombe. The researchers' observations documented territorial patrols, coalitionary attacks, and occasional lethal encounters between neighboring chimpanzee communities under ecological conditions that differed from those at Gombe.
Over a long period of time, the study documented the disappearance of what researchers named the K-group of chimpanzees, which numbered about 30 individuals in 1974. The last member died in 1982. Researchers attributed this decline to "attacks by the larger M-group, studied from 1968 to the present." These findings weakened the argument that the Gombe conflict resulted primarily from food provisioning by the research group or other unusual local circumstances.
Ngogo, Kibale National Park
Another long term field study of wild chimpanzees not only supported earlier findings, but also provided evidence for the adaptive significance of inter-community violence. This study observed the Ngogo chimpanzee community in Kibale National Park, Uganda, one of the largest and most-studied chimpanzee populations. Since 2015, the Ngogo chimpanzee community has fractured into rival factions which have engaged in a protacted violent conflict, known as the Ngogo chimpanzee war.
The Ngogo observations differed from those at Gombe in several important respects. They were recorded by multiple researchers using systematic behavioral observations over many years and involved an unusually large chimpanzee community. These observations showed measurable ecological benefits in the form of increased access to food resources and territory, a finding that the authors interpret as supporting the adaptive value of inter-community violence.
These findings have been interpreted as supporting the imbalance-of-power hypothesis, which proposes that chimpanzees preferentially attack neighboring individuals when numerical superiority minimizes the risk of injury to the attackers. Under this hypothesis, coalitionary killing can increase reproductive success by expanding territory and improving access to resources at relatively low cost for the attacker.
Some researchers caution that the exceptional size of the Ngogo community and the favorable ecological conditions at Kibale National Park may limit the extent to which these observations can be generalized to all chimpanzee populations. Nevertheless, the Ngogo findings offer evidence that lethal coalitionary aggression can produce long-term fitness advantages (that is, greater food availability).
Types of aggression
Within-community aggression
Dominance aggression. This type an aggression in pursuit of and maintenance of hierarchical rank is part of the everyday “politics” of chimpanzee society. Adult males compete for status and, relatedly, for access to fertile females. They do this by charging, biting, dragging and intimidating rivals. Displays include branch shaking, throwing rocks or sticks, piloerection (hair standing on end) and loud vocalisations. Serious injuries are relatively uncommon during this type of aggression because opponents usually submit before the fights escalate. Though this competition for rank inflicts high physiological costs, the dominant males gain clear reproductive benefits by monopolizing access to fertile females. In contrast, females are aggressive mainly in the context of feeding competition. High ranking females clearly gain important fitness benefits from their rank, but dominance aggression among females is rare and female dominance ranks are stable over long periods of time.