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 at the University of Zurich 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.