A bloody 10-year dispute in the Ugandan jungle ended in mid-2009 with the victors seizing territory held by the vanquished. The episode represents the first solid evidence that chimpanzees kill their rivals to acquire land, and could help explain the evolutionary origins of some aspects of belligerent as well as cooperative behaviour in humans.
John Mitani, a primatologist at the University of Michigan in Ann Arbor, and his team have observed the Ngogo chimpanzee troop in Uganda's Kibale National Park for over a decade. Between 1999 and 2009, they witnessed 18 lethal attacks led by Ngogo males on another, smaller group of chimps. They also found indirect evidence of another three lethal attacks, making the Ngogo troop one of the most violent groups of chimpanzees so far studied.
With more than 150 individuals, the troop is two or three times as large as other well-studied groups. Superiority in numbers allows it to patrol its territory's hinterlands, where members are likely to encounter smaller, neighbouring troops. "Attacks are made when there's more of us than them," says Mitani.
In mid-2009, his team noticed that the Ngogo chimps had finally seized part of the home range of their rivals, so increasing the size of their territory by 6.4 square kilometres, or 22 per cent. Where only adult males on patrol had previously visited this area, now the team saw them "going in there with females and children and acting and shouting like they would if they were in the middle of their territory", Mitani says. The Ngogo chimps were probably drawn by food: black mulberry trees had begun fruiting in the area around the time of the takeover.
The territorial gain is likely to bring about other advantages. Chimps belonging to troops with large home ranges tend to weigh more than those with less land and their females tend to have more offspring. What's more, territorial gains could draw in females from neighbouring troops, offering more mating opportunities to the males.
Similar changes have been seen in human hunter-gatherer communities, but Mitani cautions against drawing too many parallels between our own battles and those of chimps. Humans go to war for a variety of reasons ranging from disputes over resources to religion, and such conflicts can often be settled by negotiation. "We might be comparing apples and oranges," he says.
In fact, rather than explaining the evolutionary origins of war, chimpanzee disputes could help explain the evolution of human cooperation. Samuel Bowles at the Santa Fe Institute in New Mexico has used evolutionary models and archaeological evidence to argue that altruism emerged in humans as a result of violent conflicts between groups of people who were willing to die for their comrades and more selfish, individualistic populations ? with the altruistic warriors winning out.
Likewise, says Mitani, the Ngogo chimps worked as tightly knit coalitions to kill their opponents, with benefits for the victorious troop.
"There's definitely an interesting relationship between cooperation and competition in both chimpanzees and in humans," says Michael Wilson, a primatologist at the University of Minnesota in St Paul, who was not involved in the study.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2010.04.021
Original Article
