Infanticide – the killing of the very young – has been observed in many species, from lions and dolphins to ratsbats and especially primates. But baboons take things a step further. Not only will male baboons kill the progeny of rivals, but they're also known to attack pregnant females in order to cause miscarriages. This behaviour, known as feticide, is rarely observed and not very well understood.

A young baboon in Amboseli National Park, Kenya. © Steve Garvie

To get a better grasp on why male baboons do this, a group of researchers from the United States and Kenya looked to troops living around Amboseli National Park. Across decades of records, they found a pattern to the violence: youngsters often fell victim to new males who took over a group, and the cull freed up the females for mating.

When a male baboon grows up, he leaves his troop and goes out in search of a new one, a pattern also seen in male lions. Baboon troops can be quite large, and a male may join and leave multiple groups. That means a troop is likely to have one or two new males at any given time. But just being part of the family isn't enough: males seek social dominance and the mating rights that go with it. So, also like lions, newly dominant baboons have a habit of removing the offspring of previous rulers.

The new study found that infant deaths and miscarriages were especially common within a couple of weeks after a new male joined a troop. In fact, the researchers estimated that one out of every 50 infant deaths (and three in 50 lost pregnancies) were caused by male newcomers. But only baboons under a year old were affected. Any older than that, and the aggressive males seemed to have little interest in dealing with them.

The explanation is almost disturbingly simple. In nature, success means survival and reproduction. When a female baboon is nursing and caring for a very young infant, she doesn't ovulate, which means she can't mate. A male entering a new group may have to wait many months for a mother to be free of her dependent young, and even longer for pregnant females. But if the baby is lost, the female's reproductive cycle will resume within several weeks.

'They've got a pretty short window," explains study co-author Susan Alberts of Duke University in a press release. Even a male who rises up the social ladder quickly will last only about a year on average before being edged out by new competition.

"In situations where males have few opportunities, they resort to violence to achieve what’s necessary to survive and reproduce," adds lead author Matthew Zipple, also of Duke University.

Zipple and colleagues found that infanticide and feticide were more common when resources like food were in short supply, and in troops with many pregnant or nursing females. Males that rose to dominance quickly were also more successful at removing the existing young. And typically, the offending males ended up mating with the females not long afterward.

Understanding the biology behind this behaviour reveals that baboons aren't just cruel murdering monkeys – their circumstances play a big role in the actions they take to survive. This can help shed light on similar cases across the animal world, and on how environmental conditions can hugely impact animal social behaviour.

Header image: Peter Stewart