Chimps are known to live in a violent society; they are renown to be the most aggressive apes.

Unlike their cousins, bonobos, which solve their problems through sex, chimps fix almost everything through violence. Even when it's about sex.

Male chimps are often seen using violence against females, beating them brutally, even with branches.

A new research suggests that many times it could be jealousy, linked to chimps' sexual behavior.

Chimps are highly promiscuous.

Everybody is free to have sex with everybody; males do not guard a harem like in gorillas, and can't fight for females because they form "brotherhoods", a kind of gangs that defend the group's territory, attacking and even killing and sometimes eating an intruder chimpanzee.

The gangs also organize monkey hunting parties, so the links between the males cannot be destroyed by fighting for females.

But in a group, only very few females are in estrus at a given moment; the others are suckling infants.

As males can not inflict coercion on other males, they will apply sexual coercion in females: punishing a female's promiscuity, a male increases the chance of her offspring being fathered by him.

This was till now just a hypothesis, as male-on-female violence could have been generated by competition for food, or just a spillover from male-male aggression.

But a 7 years study led by Martin Muller, a biological anthropologist at Boston University in Massachusetts, made on wild chimps in Uganda, biased towards the sexual hypothesis.

The team meticulously recorded every push and slap, along with every sexual contact and pregnancy.

They collected urine from leaves to check glucocorticoid ("stress") hormones.

The research showed that those receiving the worst beatings not only had far more sex with the males that beat them, but were also the most fertile, with twice the average odds of a coitus resulting in pregnancy.

"Males are basically trying to force females into exclusive mating relationships," says Muller.

But the violence has a price: high cortisol amounts in the urine of persecuted females meant intense stress, which can have harmful effects: higher incidence of gastric ulcers and immune suppression.

"This is obviously not a good thing for females who are victims of aggression," notes John Mitani, a biological anthropologist at the University of Michigan, Ann Arbor.

The team wants now to see if females developed strategies against male abuse.