Evolution of Sexual Intimidation: Male Baboons Beat up Females to Increase Mating Success

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Female baboon with her newborn baby. Alice Baniel

Male baboons have been observed carrying out long-term abuse of their female partners as a means of control and to increase mating success.

The discovery—the result of a four-year research project—provides more evidence to support the idea that sexual intimidation among humans has evolutionary roots, potentially helping explain why domestic abuse is so frequent in humans today.

Researchers from the Zoological Society of London, U.K., and CNRS in France monitored a population of chacma baboons in Namibia to find out whether male aggression towards females was a type of sexual coercion, where females were intimidated into mating rather than being directly forced to.

"When I was in the field and observing the baboons, I often noticed that males were directing unprovoked attacks or chases toward females in oestrus [in heat]," study author Alice Baniel said in a statement. "They also maintained close proximity and formed a strong social bond with one particular cycling female, from the beginning of their cycle until the end.”

Researchers monitored the baboons for attacks and sexual activity in the 20 minutes that followed and found there was no increase in mating directly after violent attacks, but further analysis revealed another trend. Their findings are published in the journal Current Biology.

baboon A male baboon attacking a female. Scientists found males use long-term sexual intimidation to increase their mating success. Alecia Carter

Over four years, researchers found fertile females suffered more aggression from males than those that were pregnant or lactating. Male aggression was a major source of injury to fertile females. Males that were more aggressive towards one particular female were found to have had more mating success than those that were less aggressive.

Instead of forcing the females to mate after violence, the males appear to be using the attacks as a means of long-term sexual intimidation that, over time, encourages the female to stick with the male aggressor.

Elise Huchard, another author on the study, tells Newsweek the patterns seen appear to work as a mating strategy in two ways—it discourages the female from leaving the proximity of the male, while also “inciting her to accept his mating facilitation.”

Similar long-term sexual intimidation has previously been observed in chimpanzees and may well be present in other primates. "Because sexual intimidation—where aggression and matings are not clustered in time—is discreet, it may easily go unnoticed," Baniel said. "It may therefore be more common than previously appreciated in mammalian societies, and constrain female sexuality even in some species where they seem to enjoy relative freedom."

Because both chimpanzees and baboons are relatives of humans, this behavior being present in all three could indicate it has “a long evolutionary history,” Baniel said.

“Sexual intimidation was first described in chimpanzees a few years ago and now we’ve got evidence of sexual intimidation in baboons,” Huchard says: “This suggests sexual intimidation might be widespread in social primates, so it opens the possibility for an evolutionary origin of human sexual intimidation.

“But it’s just a possibility. It doesn’t mean it has an evolutionary basis. All we can say at the moment is that it’s now well documented in animals—animals that are closely related, so it’s not impossible to think that human sexual intimidation has a long evolutionary history.”

She says they will next need to find more evidence of this behavior in other mammals to pinpoint the systems involved. “That would shed more light on human sexual intimidation and whether it’s an evolutionary trait,” she says.