Love is not a pretty thing in the chimpanzee world. Male chimps frequently and brutally beat females, sometimes using branches as weapons. According to a new study, the belligerent behavior is meant to police girls' wandering eyes.
Chimps don't believe in monogamy. Instead, they live in a free-love commune where anyone can mate with anyone else. Only a few females are in estrus and capable of conception at any given time; the rest are suckling infants. As a result, competition for the available ladies is intense. The leading explanation for male-on-female chimp aggression is that it is a form of sexual coercion: It's in a male's interest to punish female promiscuity to increase the chance that her babies will be his. But the evidence for this theory has been lacking. Male-on-female violence could simply be the result of disputes over food resources, for example, or it may just be a spillover from male-male aggression.
To get behind the bullying behavior, a team led by Martin Muller, a biological anthropologist at Boston University in Massachusetts, pooled 7 years of observations of a group of wild chimps in Uganda. The researchers meticulously recorded every push and slap, along with every tryst and pregnancy. Swabbing urine from leaves allowed them to measure glucocorticoid hormones, an indicator of stress.
Male chimps didn't just beat up on females at random, the researchers conclude. Those that bore the worst of the attacks not only had far more sex--and most often with the males that beat them--but were also the most fecund, with twice the average odds of a sexual encounter resulting in pregnancy. "Males are basically trying to force females into exclusive mating relationships," says Muller. But tough love comes at a price: High cortisol levels in the urine of persecuted females revealed intense stress, which can result in gastric ulcers and immune suppression. The study appears in the current issue of Proceedings of the Royal Society of London B.
"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. He is curious about the kinds of strategies females might use to avoid the abuse--or at least counteract its negative consequences.