For male chimpanzees, long-term bullying of females, including physical assaults, apparently offers the evolutionary benefit of improving their chances of fathering babies with them, a study suggests.

That's one result of research involving chimps in Tanzania over 17 years that recorded long-term aggressive behavior by males, researchers say.

"It is certainly not a happy message," says Arizona State University evolutionary anthropologist Ian Gilby. "Males who directed aggression toward females at high rates were more likely to sire those females' offspring than less violent males were."

The effect of nature rewarding what in humans would be considered inappropriate behavior was particularly noted among the highest-ranking males within the chimpanzee community, Gilby added.

In the study of chimpanzees Tanzania's Gombe National Park, the researchers recorded aggressive behavior by males and then categorized it on a rising scale, from males charging at females to frighten them to physical attacks involving biting or striking that left females injured.

They compared the number of such behaviors to the number of babies fathered by each male, confirming paternity with genetic tests.

Males who consistently bullied females as a form of sexual coercion had the edge in successful mating and paternity, the researchers found.

"This indicates that males, particularly those of high rank, successfully employ a strategy of long-term sexual intimidation," says Gilby, an assistant professor in the School of Human Evolution and Social Change.

Mating did not always take place immediately after the bullying behavior, the researchers observed; rather, it was ongoing aggressive behavior stretching over 2 or 3 years that was most effective in securing successful fatherhood.

"The hypothesis is that females are intimidated by long-term aggression from the male so that they acquiesce or even solicit mating from the male when they are fertile, and avoid mating with other males in his presence for fear of further aggression from the male," says study participant Anne Pusey, an evolutionary anthropologist at Duke University.

While the study's findings could be considered as clues to the origins of human sexual violence, Gilby cautions against jumping to conclusions.

While chimpanzees are one of our closest living relatives, they and we are separated by 7 million years of evolution and our mating systems are quite different, he emphasizes.

"Nevertheless, recognizing the adaptive value of male-female aggression in chimpanzees may ultimately help us to understand, and hopefully prevent, similar behavior among humans," he says.

The study of chimpanzee aggression and mating was published in the journal Current Biology.

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