After at least 5 million years of separate evolutionary history, chimpanzees and humans still have one thing in common: Males of both species kill each other over territory. Male chimpanzees in Uganda launched coordinated surprise attacks on their neighbors, invading their homelands and killing 21 of them in attacks witnessed by primatologists over the past decade, according to a study published  today in Current Biology.
Researchers have known since the mid-1980s that chimpanzees kill their neighbors, thanks to observations by primatologists Jane Goodall and her colleagues. But the reason for the violence was unclear. At Gombe Stream National Park in Tanzania, for example, one group took over the territory of its neighbors after a series of lethal attacks, suggesting that they killed to expand their territory. But both groups at Gombe originally had been part of one large group, and researchers had provisioned them with food, so it was unclear whether male chimpanzees naturally kill each other in the wild—or if the Gombe chimpanzees were acting strangely because they had been tainted by contact with researchers who fed them and may have disrupted their normal behavior.
From 1999 to 2008, a team led by primatologist John Mitani of the University of Michigan, Ann Arbor, and David Watts of Yale University followed male chimpanzees in the Ngogo group in Uganda's Kibale National Park whenever the chimpanzees would stop their usual noisy social behavior to form patrols  of several males, about once every 2 weeks or so. When the chimpanzees heard a call from a solitary chimpanzee in a nearby territory, these patrols would move silently and rapidly through the forest, toward the sound. In 18 cases, the researchers observed the Ngogo chimpanzees ambush a solitary male—often an infant—and kill it by pummeling it and biting it, says Mitani. In three other cases, the primatologists did not witness the attacks but found the beaten, torn bodies of chimpanzees after they heard an attack, or they observed the Ngogo males eating a freshly killed infant chimpanzee.
The researchers also noticed that 13 of the 21 cases occurred in the same neighboring territory, a region now occupied solely by the Ngogo chimpanzees. Thus, the Ngogo males have apparently successfully invaded and occupied another group's territory, expanding their own foraging terrain. "The take-home message is quite clear and simple," says Mitani: "Chimpanzees kill their neighbors to get more land."
Others find the evidence convincing. "These data indicate that benefits come not merely from eliminating neighboring groups but also from land grabs," says Harvard University primatologist Richard Wrangham, whose team has also observed killings apparently over land between two other groups at Kibale. Experts have speculated that chimpanzees take over these territories to access more fruit trees or acquire new females that live there.
Still, it's unclear why the Ngogo group moved into the new terrain, as its homeland is an unusually high-quality habitat that supports 150 individuals, and no females from the vanquished group have joined the Ngogo group so far. It might be as simple as the fact that the Ngogo group has many males—and that they have enough strength in numbers to successfully expand their range. Thus, "the Ngogo data suggest that chimpanzees are not aggressive when they are weak, pressured, disturbed, stressed, or starving, but when they are strong enough to assert themselves on vulnerable neighbors," says primatologist Martin Muller of the University of New Mexico in Albuquerque. "By annexing new feeding areas, the Ngogo males will likely continue to increase the size and strength of their community. If I were a chimpanzee, I wouldn't want to share a border with them."