Researchers have recently discovered that male chimps routinely physically abuse female chimps, sometimes even using branches as weapons. But, why would they do that?
Chimps don’t form monogamous pairs; rather, mating is dictated by the females’ estrus cycles. During estrus, the competition for access to these few fertile females is intense. The leading theory, albeit a shaky one, is that the physical abuse is punishment for female chimps’ promiscuity. By bullying them, they are discouraged from seeking other males, making it more likely that resulting offspring is his. Another explanation is simply that the violence is the result of disputes over food resources.
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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.
However they also found high levels of cortisol–a stress hormone–in the urine of beaten-up females. At persistent high levels, cortisol can result in gastric ulcers and other physical problems, which would be decidedly counter-productive to reproduction.
Muller et al. 2007. Male coercion and the costs of promiscuous mating for female chimpanzees. Proc. R. Soc. B. doi:10.1098/rspb.2006.0206