In the early 1970s, several members of the Kasekela chimpanzee community at Gombe National Park, Tanzania – made famous by Jane Goodall’s pioneering study of wild chimpanzee behavior – split off and founded a new community referred to as the Kahama community (Goodall 1986). From 1974 to 1977, the larger Kasekela community repeatedly raided the Kahama territory, and by 1977, all six of the adult Kahama males and two adult females had been brutally murdered or had disappeared. The group’s remaining females joined other neighboring groups.
Similarly, between 1970 and 1982 at the Mahale National Park – a lesser known site started by another true pioneer of primatology, Toshisada Nishida, who passed away this last summer – the K-group lost all six of its adult males and one adult female, though there was evidence that one of the adolescent males at the time survived into the 1990s (Nishida et al. 1985). None of the attacks were observed by researchers, nor were the remains ever found, but it was suspected that the disappearances were due to attacks by the larger neighboring M-group. In the process, the K-group territory was gradually taken over by the M-group, and most of the K-group females eventually joined the M-group.
These are extreme examples in which entire groups were eliminated – not unlike genocide, though on a scale appropriate chimpanzee society. The methods by which they’re carried out, however, are all too common in the relations between chimpanzee groups. One of the leading causes of death among chimpanzees is death at the hand of another, both between and within groups. In 1986, Goodall reported that 30% of all of the deaths at Gombe were from fights between groups, and in a recent study of the causes of more recent chimpanzee deaths at Gombe, 36% of deaths were attributable to violence – within and between groups – which was the greater than any other factor (Terio et al. 2012). Infanticide is also common, generally by males of other groups, but within-group coalitionary infanticide by females has also been reported (Muller 2007).
Although chimpanzee communities defend large territories, they spend the majority of their time in the center, where they are less likely to encounter members of neighboring groups than on the peripheries which overlap with those of their neighbors. Communities in the Taï forest of Côte d’Ivoire spend 75% of their time in the central 35% of their territories, and the Kanyawara community at Kibale National Park in Uganda spend 90% of their time in the central 40% of their territory, although food is equally abundant in the periphery (Wilson & Wrangham 2003). Meat is thought to be even more abundant in the outlying areas, where the red colobus monkeys that chimpanzees hunt would be wise to take refuge. Nevertheless, chimpanzees appear agitated and nervous when they go into the outer regions of their territory, and often will only do so when they are in a group that includes several adult males.
The reason for this is clear. Chimpanzees vigorously defend their territories by patrolling the periphery where the territories of different groups overlap. They form a quiet line and stalk along their border, listening closely for sounds from neighboring chimpanzees. They also search for physical signs of their presence, such as discarded food, termite sticks, and feces. If other chimpanzees are found, they are often viciously attacked by the patrol, often fatally. Similar confrontations occur when chimpanzees from different groups unintentionally encounter each other in the peripheries while foraging, which explains why they prefer to travel in groups that include several adult males.
Occasionally, a group of adult males, and occasionally an accompanying female or two, will form a coalition and head out towards a rival group. As they enter the neighboring territory, they fall into line and proceed with with a deep silence, periodically stopping to listen for cues of rival chimpanzees nearby. Chimpanzees in a community split off and forage either independently or in small groups that change members often; entire communities – which may contain well over 100 individuals – virtually never congregate as a whole. When encroaching chimpanzees succeed in locating a members of the rival group, they will often commence violent attacks that appear to have murderous intent. Most encounters, however, both chance meetings in the periphery and raids, end only in loud and violent displays from both sides until one side retreats.
During their incursions into the territory of a rival group, chimpanzees do not consume the rival’s resources or actively avoid neighboring individuals, as do other mammals that carry out similar territorial incursions, such as wolves in North America. The sole intent seems to be to kill neighboring group members. After an attack – successful or not – the raiding chimpanzees return to their own territory, performing loud displays.
Whether a group of chimpanzees attack those that they encounter, either in the periphery or in a raid, largely depends on the relative number of adult males in each party. If a group of several males encounters a lone rival male, they will be very likely to attack, with success, whereas if they encounter a group of similar number, the conflict will most likely end in after only some shouting. Chimpanzees will typically only initiate a conflict if there is virually no chance that they will be injured, and this can only be assured if their own group greatly outnumbers the other. If they find a lone individual, for example, it only takes two adult males to hold another down, allowing the others to do whatever harm they choose at significat risk to any of the attackers.
The ultimate cause of violence between chimpanzee groups has been a controversial topic in behavioral biology since the 1970’s. One early hypothesis was that chimpanzee violence was a byproduct of human influence (Power 1991; Sussman 1999). To quicken the pace in which the researchers at Gombe and Mahale could learn to identify each individual and begin to study their behavior, they provisioned the chimpanzees with fruits like bananas and studied their behavior at the provisioning stations. Some have proposed that the competition between groups for that resource led to the violence, which would never have occurred in the absence of human influence.
Primatologists have now observed similar raids between chimpanzee communities from other parts Africa that had never been provisioned by human researchers. Chimpanzees are also no longer provisioned at Gombe or Mahale, and the violence has continued to the present (see Wilson et al. 2004; Mitani et al. 2010). This suggests that inter-group violence is a normal trait of chimpanzee communities – as it seems to be among humans – and that scientists should be able to find an explanation for its occurrence based on cost/benefit analyses and natural selection.
One such hypothesis, presented by Richard Wrangham, a primatologist at Harvard University, is the imbalance-of-power hypothesis. Its claim is that intergroup aggression between chimpanzee groups is an expression of a sort of struggle for dominance between groups. Wrangham (1999:11) explains that “[b]y wounding or killing members of the neighboring community, males from one community increase their relative dominance over their neighbors.”
This has certain benefits, particularly if males are killed in these inter-group conflicts. By reducing the neighboring male populations, they reduce the probability that they will be raided or that they will encounter larger groups of rival males while foraging, patrolling, or raiding. It also makes it possible to extend the borders of ones own group, which has recently been reported at Kibale (Mitani et al. 2010). By 2009, the Ngogo community at Kibale had expanded their territory by 22.3% into the north after killing 21 members of a rival community to their north between 1999 and 2008.
Reducing neighboring populations will also lead to an increase of immigrant females to ones own group that may have otherwise emigrated elsewhere. Females differ from males in that they leave their natal group at adolescence to join a neighboring group. Females tend to join communities with a greater number of adult males, so by reducing neighboring male populations, a group can increase its own population both (1) relatively, by decreasing neighboring populations, and (2) absolutely, by attracting emigrating females. Females also tend to leave groups if the male population becomes too small, as did the remaining Kahama females at Gombe in 1977 after all six males were killed. These influxes of females into a group increase the number of females with whom males can mate.
Many have argued that these are the same pressures that lead to inter-group conflict between human hunter-gatherer groups, which have a similar form in which attacks are carried out in low cost situations, such as with ambushes and night raids. Given that coalitionary violence is so rare among mammals and humans and chimpanzees share a recent common ancestor, many evolutionary anthropologists have suggested that this form of coalitionary violence may have originated in our last common ancestor. This is an interesting issue, but I’ll save it for a later post.
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