Why Male Baboons Kill Babies of Their Own Kind

Duke University researchers dig into the reasons why the primates sometimes kill infants and attack pregnant females.

Male baboons, under certain circumstances, will turn to a grisly practice in order to increase their chances of siring offspring: They'll attack pregnant females in order to kill infants in utero.

Researchers from Duke University, in a study published in the journal Proceedings of the Royal Society B, documented that behavior as well as infanticide in a group of baboons in Kenya's Amboseli basin and explained why such attacks happen and under what circumstances.

Infanticide - killing infants of their own kind - is seen in multiple animal species, including baboons: Lions willdo it, and so will dolphins. Feticide – killing an unborn infant by attacking its pregnant mother – is less well studied, say the researchers, who took a closer look at baboon life to measure how often both killings happen and the conditions present when they do.

The Duke team studied a baboon population in Amboseli National Park, using findings from a long-term study of the animals running since 1971. They found that male baboons new to the troop were behind 2 percent of infant deaths and 6 percent of miscarriages.

However, in times where fertile females were in short supply, the death rates were more than three times as high.

"In situations where males have few opportunities, they resort to violence to achieve what's necessary to survive and reproduce," lead author Matthew Zipple confirmed in Duke Today. "When reproductive opportunities abound, this behavior is less frequent."

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It starts with new members of a population. Baboon troops usually have a couple of males that simply show up and join - called "immigrants" in the research - after having left their initial populations in a quest for mates.

Within two weeks of immigrant males arriving, the scientists found, there were upticks in infant deaths and miscarriages. Because older - 1 to 2 years of age - baboons were not harmed, the researchers reasoned that the baboon violence was specifically aimed at pregnant or nursing females.

What's going on here? The prevailing hypothesis, say the scientists, is that infanticide and feticide both speed the availability for otherwise pregnant or lactating females to mate again. Whereas it could take up to a year before a child-rearing baboon could mate again, with the acts of domestic violence on the born and unborn the females could be ready to mate again in scarcely more than a month.

Most of the time, the team found, the males did in fact end up mating with the females whose unborn or infants they killed.

Feticide and infanticide rates were highest, the scientists found, with immigrant males that had ascended quickly to top of the social ranks, which dovetails with something study co-author Susan Alberts told Duke Today. Baboon males at the top of the social ladder have to make tracks: They usually get bounced from prominence within a year of their ascendancy. Passing on their name, as it were, becomes a top priority.

"They've got a pretty short window," Alberts said.

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