Chimpanzees may have a sense of right and wrong that echoes human concepts of morality, a study has found.
Two groups of the apes paid more attention to film clips of an infant chimp being killed by its own kind than those showing other acts of violence.
The fact that they singled out this attack suggests that the animals viewed it as being outside the boundaries of normal behaviour.
Scientists believe the research may help to shed light on how human morality and social norms evolved.
The study involved 17 chimps housed at two Swiss zoos in Gossau and Basel.
They were observed as they watched film clips of chimpanzees unknown to them either engaged in neutral activities such as walking or cracking nuts, or displaying aggressive and violent behaviour.
One of the movies showed chimpanzees killing one of their own infants. The zoo animals spent four times longer looking at this clip than any of the other films, including those showing a colobus monkey being hunted and killed by chimps, and socially aggressive behaviour between chimpanzee adults.
The scientists ruled out the infanticide scene attracting more attention because of "striking" features such as screaming, which could also be heard on other videos.
Nor was there any difference between the extra attention paid by male and female viewers.
The researchers, led by Dr Claudia Rudolf von Rohr, from the University of Zurich, wrote in the journal Human Nature: "We found that chimpanzees discriminated between a video clip depicting severe aggression against an infant and video clips depicting other forms of social aggression or neutral behaviour.
"Specifically, they showed significantly longer looking times in the infanticide condition than in the control conditions.
"This result is consistent with the idea that severe aggression against infants did not match chimpanzees' social expectations of a certain tolerance normally afforded to infants."
The team found that despite viewing the infanticide scene for longer, on the whole the chimpanzees did not become more emotionally aroused by what they saw.
This could be because as uninvolved "bystanders" they failed to empathise strongly with a victim not from their group, said the scientists.
"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," said Dr Rudolph von Rohr.
The researchers concluded: "This fruitful topic for future research might provide us with important insights into the evolution of specific social norms in humans and why some of them are widely accepted and others more difficult to establish."