Sitting amongst wild chimpanzees, watching two youngsters dangle from branches above their mothers' heads while the adults lie sleeping amid the buzzing of cicadas in the cool of the afternoon, it is easy to believe that these animals represent some lost Eden, a salvation from the human condition.
To watch a full-grown adult male play-wrestle and chase an juvenile or adolescent, it is hard to imagine the callous brutality with which the same male can seize an infant from its mother's breast, tear into its abdomen with its canines and start feeding on the infant's intestines while it still lives. This is the contradiction in the behaviour of chimpanzees, one that brings them all too close to us.
Mitani, Watts and Amsler report on lethal aggression in the Kibale forest, Uganda, but these are not unique to that forest, or that community of chimpanzees. I have witnessed similar attacks in the Budongo forest, to the north of Kibale, and this duality in chimpanzee behaviour is found wherever chimpanzees have been studied.
Males habitually patrol the borders of their territory, moving silently, in single file, pausing, gesturing, moving on, for all the world looking like a foot patrol of infantryman from human conflict. Jane Goodall described these decades ago.
On occasion, such patrols will penetrate deep into the territory of a neighbouring community. If they encounter chimpanzees of that community, often the patrolling chimpanzees steal away, quietly, to the security of their own territory. But if they have weight of numbers they will attack. A lone chimpanzee caught by one of these attacks must make a quick escape or face almost certain death.
The way that males co-operate in this aggression is all the more remarkable given that they endlessly compete with one another for social rank, and for mates – not that you would know it, watching them snooze together in the afternoon sun.
The writer is a primatologist and behavioural ecologist at the University of Kent