How a chimpanzee views a video of an infant chimp from another group being killed gives a sense of how human morality and social norms might have evolved. So says Claudia Rudolf von Rohr of the University of Zurich in Switzerland, lead author of a paper in Springer's journal Human Nature. It provides the first evidence that chimpanzees, like humans, are sensitive to the appropriateness of behaviors, especially those directed toward infants. It also shows that these primates might only take action when a member of their own group is being harmed.

The researchers filmed two social groups of chimpanzees living in two Swiss zoological gardens while the animals repeatedly viewed film clips. The films portrayed the actions of other chimps unknown to them. The control clip showed chimps doing neutral activities such as walking or cracking nuts. The experimental clips included aggressive scenes, such as an infant chimpanzee being killed by its own kind, a small colobus monkey being hunted and killed by chimps, and socially aggressive behavior between chimpanzee adults. The taped sessions were analyzed to find out how long the chimps in the experiment looked at the screen, whether they were aroused or affected by what they saw, and how they behaved while doing so.

The zoo animals looked up to four times longer at the infanticide scenes than at any of the other films. The chimps did not merely respond to the infant screams they heard, they paid preferential attention to these scenes as a whole. This shows that chimpanzees can distinguish severe aggression against infants from other forms of aggression and harmful behavior. It also indicates that such incidents do not match the social expectations of tolerance normally granted toward infants. This is a form of so-called proto social norms at work, where individuals react as bystanders to a violation of a certain expectation of how others should behave.

Interestingly, although the chimps viewed the infanticide scenes much longer, the research team found only limited evidence that what they saw caused the viewing chimps to be more aroused or to react to them.

"The results suggest that chimpanzees detect norm violations both within their group as well as in a group of unfamiliar individuals, but that they will only respond emotionally to such norm violations within their own group," says Rudolf von Rohr.

The Swiss study adds to the growing body of evidence that identifies the building blocks of human morality in our closest living relatives. These precursors include consolation and policing behavior. Rudolf von Rohr believes that future research might provide important insights into the evolution of specific social norms in humans and why some of them are widely accepted and others are more difficult to establish.