When animals kill babies of their own species, it's brutal and shocking. Infanticide goes against everything we think adults should be in terms of looking after younger members of the species. Surprisingly, however, it happens fairly frequently in the wild. What's going on?

Primatologist Yukimaru Sugiyama of Kyoto University was one of the first scientists to document infanticide in the wild. Studying gray langurs in India in the 1960s, he found that monkey troops generally consist of a male and several females. Sometimes, however, a troop's male is driven out by a stronger "intruder" male. When this happens, the new male often kills the troop's infants.

Sugiyama suggested that the killings were often due to stress, perhaps brought on by overcrowding. Indeed, infanticide was long thought to be pathological. In other words, the only explanation for infanticide was that the animal behind the act was either stressed or sick.