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.

These days, however, we have a better understanding of infanticide in the wild ¡ª males have no genetic interest in babies that are fathered by someone else. Their evolutionary concern is to mate with females and produce their own offspring.

It¡¯s abhorrent to us but, as far as natural selection goes, it makes sense for animals to kill babies that aren¡¯t their own. It¡¯s a simple waste of resources to raise another male¡¯s offspring.

Many different animals commit infanticide, most famously lions. Prides consist of several females and a dominant male, but, inevitably, a younger and stronger male will challenge and defeat the resident male. If there are lion cubs around at that time, well, that¡¯s bad luck for them.

However, infanticide also occurs with gorillas ¡ª otherwise believed to be relatively peaceful apes ¡ª and certain monkey species, as well as mice.

If that all seems grotesque to us, it¡¯s perhaps more monstrous to find that males who kills babies can easily ¡ª and naturally ¡ª become caring parents themselves when they father their own children. It¡¯s hard to believe that male killers can transform into a doting parent, and yet that¡¯s just one of those wonderful things about science.

Kumi Kuroda is a scientist who is fascinated by the parent-infant relationship. Based at the RIKEN Brain Science Institute, her past work has included a study showing why babies ¡ª both human and nonhuman ¡ª calm down when they are picked up and carried.

¡°The ultimate goal of our research is to understand and assist the parent-infant relationship in humans,¡± Kuroda says.

Kuroda was a medical psychiatrist before she became a neuroscientist, and many people she saw were psychologically damaged by the parent-infant relationship they experienced in childhood.

¡°So I joined the neuroscientific research field to really...