Aggressive male chimpanzees who consistently bully females tend to father more babies with their victims, according to a new study.
The findings are based on a long-term study of interactions between chimpanzees in the famous Gombe National Park in Tanzania.
"Sexual coercion works for chimpanzees because females mate promiscuously with most of the males in their group during each cycle, leaving males with an incentive to try to constrain female choice," said Joseph Feldblum, the first author on the study.
"But the system that favours male coercion in chimpanzees is less prevalent in humans," said Feldblum.
"We tested whether both short-term aggression (when females were swollen) and long-term aggression (when females were not swollen) influenced mating rates and likelihood of paternity, while controlling for other independent variables that might influence paternity patterns," Feldblum said.
Aggression in chimps can range from chasing and "directed displays," in which a male charges and thrashes vegetation, to serious physical attack including biting and hitting.
"Sometimes they really pound the females, slapping or stamping on them and cause wounds," said Anne Pusey, chair of evolutionary anthropology at Duke University.
The researchers found that short-term aggression during a female's swollen, sexually receptive period increased a male's chances of copulation. But surprisingly, that aggression was not a predictor of paternity.
What did predict paternity was aggression, particularly by high-ranking males, during the females' non-swollen periods. Long-term aggression in the two- or three-year period leading up to conception resulted in fatherhood.
"Long-term aggression increased paternity success," Feldblum said.
"Males that showed more aggression towards females during non-swollen periods were more likely to achieve paternity of those females' infants, and this held most true for the highest-ranking males," said Feldblum.
Independent of aggression rates, rank had a positive effect on the likelihood of paternity, Feldblum said.
"High-ranking males who were highly aggressive towards particular females were much more likely to sire offspring by those females, but rank and male-female aggression had independent effects here," Feldblum said.
The study appears in the journal Current Biology.
