Sexual Violence Among Baboons Shows Links to Human Behavior

A new study reveals patterns of sexual intimidation in chacma baboons that mirror behavior previously reported in chimpanzees and humans.

Such is a typical story of a wild, female chacma baboon, which in many ways is a tale that mirrors those of certain chimpanzees and human domestic abuse survivors the world over, a new study published in the journal Current Biology indicates. Males of all three species may use long-term sexual intimidation to control their mates, suggesting that this mating strategy has a long history in primates, including humans.   

The behavior has often been reported in our species, and has been documented in male chimps over the past decade. The new study focuses on chacma baboons, which are among the largest of all monkeys.

“What is interesting is that the forms of sexual violence reported in chacma baboons may resemble some common patterns of sexual intimidation in humans, namely domestic violence, in the sense that they are similarly expressed in the context of long-term relationships between one male and one female, which are otherwise characterized by close spatial proximity and — sometimes — high levels of affiliation,” said senior author Elise Huchard of the University of Montpellier’s Institute of Evolutionary Sciences.

“There is nothing paradoxical in forming a strong bond with someone, and displaying aggression in the context of such relationship,” added Huchard. “Conflict is an integral part of social life in every species including humans, and it's often with those people that you often see that you may have a conflict.”

Huchard, lead author Alice Baniel, and co-author Guy Cowlishaw studied wild chacma baboons at Tsaobis Nature Park, a semi-arid environment in Namibia. The study occurred over four different periods from 2005–2014, during which time the researchers documented 222 chases or attacks led by males.

The researchers observed that males often formed social bonds with particular fertile females, which they then attacked and chased repeatedly — usually without provocation — in the weeks preceding her ovulation and prior to their mating.

“It can also be that there is an event triggering the attack, such as a rival approaching or vocalizing, or the proximity of another baboon group,” Huchard said. “The latter case is typical: males often chase and attack some females of their own group when meeting another group, and they generally target sexually receptive females in such occasions.”

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Some of the females were badly injured in the attacks, with certain individuals suffering premature deaths after repeated bouts of injuries.

The prior studies on sexual intimidation in chimps found that fertile females have higher levels of cortisol, a hormone indicative of stress. Increased stress can alter immune response. It can also disrupt reproduction and growth.

If a female baboon does give birth to offspring sired by the male, his behavior somewhat changes.

“Several studies (on baboons) have shown that it’s often the male who has been monopolizing a female during her conceptive estrus who becomes her friend when she gives birth,” Huchard explained. “The female follows the male everywhere with her newborn, and the male essentially tolerates her presence; however, studies have shown that males defend their female friend's offspring against predators or infanticidal attacks, which are not uncommon in baboons.”

She continued, “Male-female bonds progressively dissolve as infants grow towards independency, and are often finished when a female becomes fertile again, when her juvenile is fully weaned.”

A common factor among primates that practice long-term sexual intimidation is that the species tend to have males that are larger than the females. Such size differences, in turn, appear to be driven by patterns of male-to-male competition. This can happen when there are several adult males for each sexually receptive female within a population.

Since sexual coercion can stunt a victim’s growth, it might even further drive sexual size dimorphism, helping to keep the vicious cycle going.

Not all primate species have males that engage in long-term sexual intimidation, though. In lemurs, for example, it is common that females are larger than males. Even among baboons, chimps, and certainly humans, not all males practice sexual coercion.

“There is increasing research to show that animals are capable of innovations, rational decisions, self-control, empathy, strategic behavior, etc.,” Huchard said. “So, it's possible that male baboons are just driven by their sexual hormones, but it's also very possible that their actions are strategic and adjusted to the social context.”

In humans, the intensity and frequency of long-term sexual intimidation by males vary widely across cultures. This points to a strong cultural component affecting the behavior.

“It’s even possible that there is a cultural basis for baboon sexual intimidation, as for human sexual intimidation,” Huchard said. “That’s a question for future research, but it won’t be an easy one to answer.”