It was a twisted scene right out of a horror movie. On a warm September day in 1989, in the Amboseli Basin of Kenya, a male came charging toward a group of females and juveniles, attacking them indiscriminately. He homed in on a pregnant female in the group, pinning her down and viciously biting her. As she screamed and tried to escape, others came to her rescue, tackling the attacking male to little avail. Not long after, the bleeding baboon lost her fetus.
The attacker was a 70-pound male baboon named Hobbes, who earned his moniker after the English philosopher who famously referred to the lives of men as “nasty, brutish, and short.” Hobbes’ name was “a humorous reference to his very aggressive behavior,” says Susan Alberts, at the time a biology grad student who was in Kenya to study the group behavior in baboons, and watched the attack unfold barely a few feet from her. Eight-year-old Hobbes had recently immigrated to this particular troop in search of a mate.
This was the first time that Alberts, now a biology professor at Duke University, witnessed what would turn out to be feticide in a baboon population. Now, she and her fellow researchers have tapped into more than four decades’ worth of data on the region’s baboon populations—collected as part of the Amboseli Baboon Research Project, one of the world’s longest-running studies of primates in the wild—to better understand this disturbing behavior. They recently published a study in the Proceedings of the Royal Society B that described infanticide in groups of baboons found at the base of Mount Kilimanjaro in grasslands that span Kenya and Tanzania.
The study, which appears to be the first to systematically document feticide in baboons, finds that feticide can be a sound evolutionary strategy. Nature is a brutal game, and individuals do what they must to survive. By targeting females that wouldn’t otherwise be ready to mate, these individuals give themselves a valuable reproductive benefit. As a result, this behavior is by no means rare in the animal kingdom: Lions and horses, for instance, are also known for killing the offspring of females they want to mate with.
Alberts and her colleagues traced a roughly 6 percent spike in feticide two weeks after a new male immigrated to a group. To document this behavior, they performed the painstaking process of examining every female’s rear end daily and assessing her reproductive state. (These are largely non-invasive observations, though, as females have several external indicators including a change in the color of their hips from black to pink when they’re pregnant.)
They also studied the data for evidence of infanticide and found similar patterns. Deaths of infant baboons increased by little over 2 percent two weeks after a male baboon immigrated to the group. Here, too, a female that wasn’t reproductively available would stop lactating when her nursing infant was killed and become fertile again—giving the infanticidal male a chance to mate with her. Meanwhile, new males didn’t target one- and two-year-old baboons that had already been weaned from their mothers.
The findings aren’t surprising, according to Dorothy Cheney, a biology professor at the University of Pennsylvania who wasn’t involved in the study. For decades Cheney has documented infanticide in baboons in Botswana, where the behavior accounts for at least 50 percent of all infant deaths. Cheney notes that, in the Botswana populations at least, a dominant male typically mates with multiple females—in the process fathering a high proportion of infants—but only retains his tenure as alpha male for a few months.
“What this means, along with this high mating skew, is that when a male achieves the alpha position, he only has a limited amount of time before he’s deposed,” Cheney says. “This is thought to increase the rate of infanticide.”
Other factors include group size and access to available females. Since female baboons are sexually receptive barely 20 percent of the time, it’s likely that immigrant males “might, by poor luck, find a vast majority of females that are currently nursing or pregnant,” says Matthew Zipple, a biology student at Duke University and a lead author on the recent study. If groups are close enough to each other, a male might venture off to a neighboring group; if not, he might stick around and destroy the female’s chances of either producing or raising another male’s child.
So what can these findings tell us about the workings of human society? “Behavioral strategies of this kind—which may appear to be maladaptive at worst and extremely puzzling at best—often have explanations whose broad, general principles apply across many species,” says Alberts. In this case, the principle is that males and females may have conflicts of interest with respect to reproduction. It’s in the male’s interest to have mating opportunities immediately, while it’s in the female’s interest to delay reproduction until her current offspring is independent.
“These conflicts of interest can give rise to behaviors that don’t look very nice, but they may take different forms in different species or social systems,” she says.
Some of these principles could apply to human society, Alberts adds. In ancient human societies, Greeks and Romans often resorted to infanticide if the child was illegitimate or born with some sort of defect. In modern society, research shows that children who live in households where the adult male is not their biological father are more likely to suffer abuse, a trend known as “the Cinderella Effect” among psychologists.
“[The behavior] is adaptive in humans and baboons,” says Kit Opie, an anthropologist at University College London. “That is the underlying evolutionary force.”
Yet researchers caution against drawing direct parallels between baboon behavior in the wild and human behavior in a complex social context. “It’s very difficult to get into the mind of the animal and ask, ‘why did you do this?’” says Cheney. Also, says Alberts, “there is a risk in the other direction of simplifying the human phenomenon and, thereby, not appreciating the societal influences that shape a behavior, as well as the unusually great flexibility of human behavior.”