No one has ever witnessed a male baboon forcing a female to have sex with him.
Now a French-English research team thinks they have found the reason: raping her is simply unnecessary. The males use a much more subtle and longer-term strategy to get the amount of sex they want.
"When I was in the field and observing the baboons, I often noticed that males were directing unprovoked attacks or chases toward females [in heat]," says Alice Baniel, an evolutionary biologist at the Institute for Advanced Study in Toulouse, France, and lead author of the study.
Such attacks started weeks before females reached ovulation and were able to make babies.
Repeated aggression without cause appears to put females under constant pressure, to make them compliant when the male feels it's time for sexual intercourse.
These recent findings "question the extent of sexual freedom left for females in such societies," Baniel suggests.
The study was published today in "Current Biology".
Violence pays off
The researchers investigated two wild large chacma baboon groups at Tsaobis Nature Park in Namibia over a period of four years.
They observed males chasing females around for half a minute or more, biting them, shaking them or pushing them strongly onto the ground.
A male might also chase a female up a tree and push her onto thin branches, co-author Elise Huchard of the University of Montpellier, France, tells DW.
The female gets stuck there, where the male continues to harrass her for several minutes. "And she is screaming and screaming and screaming." Sometimes the male forces her to jump from the tree, Huchard says "and when it is very high up, she might hurt herself. It is very nasty."
Male aggression is even a major source of injury for fertile females.
Yet, the researchers found that males who were more aggressive toward a certain female had a better chance to mate with her later on.
Observations ruled out that these copulations were motivated by a general preference of females for aggressive males - something which does occur in many other species.
Link to humans
"Because sexual intimidation - where aggression and matings are not clustered in time - is discreet, it may easily go unnoticed," Baniel says.
This behavior is already known to occur in chimpanzee groups.
With baboons showing the same behavior, the researchers suggest sexual intimidation might be a common trait in primates living in large groups - particularly when males are typically larger than females.
That would include humans.
Huchard suggests that their recent findings "open the possibility that sexual intimidation has an evolutionary basis."
It would even explain sexual violence of men towards women.
"The forms of sexual intimidation in baboon societies are the same as in human societies," Elise Huchard says. "They are expressed in the context along stable bonds between one male and one female. That resembles a bit what happens with domestic violence in humans."
But Huchard stresses that this hypothesis would have to be confirmed in further studies. "It is too early to say."