Murder Comes Naturally

A major study suggests that killing among chimpanzees results from normal competition, not human interference.

Apart from humans, chimpanzees are the only primates known to gang up on their neighbors with lethal results - but Primatologists have long disagreed about the underlying reasons.

One proposal was that human activity, including destroying habitats and providing food, increased aggression.

But the new findings, published in Nature, suggest this is not the case.

Instead, murder rates in different chimp communities simply reflect the numerical make-up of the local population.

The international study was co-written by more than 30 scientists and gathers data from some 426 combined years of observation, across 18 different chimp communities.

A total of 152 killings were reported. This includes 58 that were directly observed by researchers; the rest were counted based on detective work - tell-tale injuries or other circumstances surrounding an animal's death or disappearance.

Interestingly, the team also compiled the figures for bonobos, with strikingly different results: just a single suspected killing from 92 combined years of observation at four different sites. 

This is consistent with the established view of bonobos as a less violent species of ape.

The researchers' global compilation of chimp violent crime statistics allowed them to consider what conditions in a community produce a higher murder rate.

Chimpanzees live in well-defined colonies, and groups of males patrol the borders of each colony's territory. This is where violent conflicts are known to arise, particularly if a patrol encounters a single chimp from a neighboring community - but never before has this much data on the lethal nature of those interactions been combined in a single study.

When the scientists compared the figures across chimpanzee research sites, they found that the level of human interference (e.g. whether the chimps had been fed, or their habitat restricted) had little effect on the number of killings.

Instead, it was basic characteristics of each community that made the biggest difference: the number of males within it, and the overall population density of the area.