<p>Most days the male chimps behave a lot like frat boys, making a lot of noise or beating each other up. But once every 10 to 14 days, they do something more adult and cooperative: they wage war.<br /><br />A band of males, up to 20 or so, will assemble in single file and move to the edge of their territory. They fall into unusual silence as they penetrate deep into the area controlled by the neighboring group. They tensely scan the treetops and startle at every noise. “It’s quite clear that they are looking for individuals of the other community,” Dr Mitani said.<br />When the enemy is encountered, the patrol’s reaction depends on its assessment of the opposing force. If they seem to be outnumbered, members of the patrol will break file and bolt back to home territory. But if a single chimp has wandered into their path, they will attack. Enemy males will be held down, then bitten and battered to death. Females are usually let go, but their babies will be eaten.<br /><br />These killings have a purpose, but one that did not emerge until after Ngogo chimps’ patrols had been tracked and catalogued for 10 years. <br /><br />The Ngogo group<br />The Ngogo group has about 150 chimps and is particularly large, about three times the usual size. And its size makes it unusually aggressive. Its males directed most of their patrols against a chimp group that lived in a region to the northeast of their territory. Last year, the Ngogo chimps stopped patrolling the region and annexed it outright, increasing their home territory by 22 percent, Mitani said . <br /><br />The objective of the 10-year campaign was clearly to capture territory, the researchers concluded. The Ngogo males could control more fruit trees, their females would have more to eat and so would reproduce faster, and the group would grow larger, stronger and more likely to survive. The chimps’ waging of war is thus “adaptive.”<br /><br />Chimpanzee warfare is of particular interest because of the possibility that both humans and chimps inherited an instinct for aggressive territoriality. Only two previous cases of chimp warfare have been recorded, neither as clear-cut as the Ngogo case.<br />In one, a chimp community first observed by Jane Goodall in Tanzania’s Gombe National Park split into two and one group then wiped out the other. But the chimps had been fed bananas, to enable them to be observed, and some primatologists blamed the war on this human intervention. In a second case, in the Mahale Mountains National Park of Tanzania, noticed that a chimp group had disappeared, presumably killed by its neighbours, but he was not able to witness the killings or find the bodies.<br /><br />The benefits of chimp warfare are clear enough, at least from the perspective of human observers. Through decades of work, primatologists have documented the links in a long causal chain, proving for instance that females with access to more fruit trees will bear children faster. But can the chimps themselves foresee the outcome of their behaviour? Do they calculate that if they pick off their neighbours one by one, they will eventually be able to annex their territory, which will raise their females’ fertility and the power of their group? <br /><br />“I find that a difficult argument to sustain because the logical chain seems too deep,” says Richard Wrangham, a chimp expert at Harvard. A simpler explanation is that the chimps are just innately aggressive toward their neighbours, and that natural selection has shaped them this way because of the survival advantage that will accrue to the winner. Wrangham argues that chimps and humans have both inherited a propensity for aggressive territoriality from a chimplike ancestor. Others argue the chimps’ peaceful cousin, the bonobo, is a model for the joint ancestor.<br /><br />Joint ancestor<br />Wrangham’s view is that since gorillas and chimps are so similar, their joint ancestor, which lived some seven million years ago, would have been chimplike and therefore so would the joint ancestor of chimps and humans Mitani, however, is reluctant to infer any genetic link between human and chimp warfare.<br /><br />Mitani invokes the idea of group-level selection, the idea that natural selection can work on groups and favor behaviours, like altruism and cooperation, that benefit the group at the expense of the individual. Chimp warfare may be constant and ferocious, fulfiling the first condition, but young females emigrate to neighboring groups to avoid inbreeding. <br />The New York Times</p>
<p>Most days the male chimps behave a lot like frat boys, making a lot of noise or beating each other up. But once every 10 to 14 days, they do something more adult and cooperative: they wage war.<br /><br />A band of males, up to 20 or so, will assemble in single file and move to the edge of their territory. They fall into unusual silence as they penetrate deep into the area controlled by the neighboring group. They tensely scan the treetops and startle at every noise. “It’s quite clear that they are looking for individuals of the other community,” Dr Mitani said.<br />When the enemy is encountered, the patrol’s reaction depends on its assessment of the opposing force. If they seem to be outnumbered, members of the patrol will break file and bolt back to home territory. But if a single chimp has wandered into their path, they will attack. Enemy males will be held down, then bitten and battered to death. Females are usually let go, but their babies will be eaten.<br /><br />These killings have a purpose, but one that did not emerge until after Ngogo chimps’ patrols had been tracked and catalogued for 10 years. <br /><br />The Ngogo group<br />The Ngogo group has about 150 chimps and is particularly large, about three times the usual size. And its size makes it unusually aggressive. Its males directed most of their patrols against a chimp group that lived in a region to the northeast of their territory. Last year, the Ngogo chimps stopped patrolling the region and annexed it outright, increasing their home territory by 22 percent, Mitani said . <br /><br />The objective of the 10-year campaign was clearly to capture territory, the researchers concluded. The Ngogo males could control more fruit trees, their females would have more to eat and so would reproduce faster, and the group would grow larger, stronger and more likely to survive. The chimps’ waging of war is thus “adaptive.”<br /><br />Chimpanzee warfare is of particular interest because of the possibility that both humans and chimps inherited an instinct for aggressive territoriality. Only two previous cases of chimp warfare have been recorded, neither as clear-cut as the Ngogo case.<br />In one, a chimp community first observed by Jane Goodall in Tanzania’s Gombe National Park split into two and one group then wiped out the other. But the chimps had been fed bananas, to enable them to be observed, and some primatologists blamed the war on this human intervention. In a second case, in the Mahale Mountains National Park of Tanzania, noticed that a chimp group had disappeared, presumably killed by its neighbours, but he was not able to witness the killings or find the bodies.<br /><br />The benefits of chimp warfare are clear enough, at least from the perspective of human observers. Through decades of work, primatologists have documented the links in a long causal chain, proving for instance that females with access to more fruit trees will bear children faster. But can the chimps themselves foresee the outcome of their behaviour? Do they calculate that if they pick off their neighbours one by one, they will eventually be able to annex their territory, which will raise their females’ fertility and the power of their group? <br /><br />“I find that a difficult argument to sustain because the logical chain seems too deep,” says Richard Wrangham, a chimp expert at Harvard. A simpler explanation is that the chimps are just innately aggressive toward their neighbours, and that natural selection has shaped them this way because of the survival advantage that will accrue to the winner. Wrangham argues that chimps and humans have both inherited a propensity for aggressive territoriality from a chimplike ancestor. Others argue the chimps’ peaceful cousin, the bonobo, is a model for the joint ancestor.<br /><br />Joint ancestor<br />Wrangham’s view is that since gorillas and chimps are so similar, their joint ancestor, which lived some seven million years ago, would have been chimplike and therefore so would the joint ancestor of chimps and humans Mitani, however, is reluctant to infer any genetic link between human and chimp warfare.<br /><br />Mitani invokes the idea of group-level selection, the idea that natural selection can work on groups and favor behaviours, like altruism and cooperation, that benefit the group at the expense of the individual. Chimp warfare may be constant and ferocious, fulfiling the first condition, but young females emigrate to neighboring groups to avoid inbreeding. <br />The New York Times</p>
