“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.”
RELATED: Human Hands More Primitive Than Chimp Hands
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.