There's nothing like a prime rib dinner to boost a guy's chances of getting lucky. At least, the strategy seems to work for male chimpanzees. Wild female chimps mate much more frequently with males who share their meat--even if the gift was given several months prior--according to a new study. The discovery explains a long-standing puzzle in chimpanzee behavior and may also shed light on gender relations of early humans.

Chimpanzees, like humans, hunt cooperatively and occasionally share their kills with each other. For male chimps, sharing meat with another male often wins him an ally when a fight breaks out. Males also share meat with females, which researchers initially assumed to be a kind of courtship. In fact, males will make a point of sharing many types of food with females (ScienceNOW, 12 September 2007). However, in addition to giving the meat to sexually receptive females, male chimps give a portion of their meat to nonreceptive females. The motive behind this seemingly unselfish act has stumped scientists. Why bother giving up a meal if you get nothing in return?

Cristina Gomes and Christophe Boesch, primatologists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, weren't ready to call the chimps selfless. They suspected that a meat-for-sex agreement was in play but that it was longer term. To check the idea, the team observed a group of five male and 14 female chimpanzees living in Côte d'Ivoire's Taï National Park and kept track of all meat-sharing exchanges and copulations for 3 years. The team observed 90 successful hunts, in which the chimps typically preyed on monkeys.

On average, males shared with about six of the eight females that were sexually receptive--and, as expected, they got sex in return. But some males also shared with sexually unreceptive females. And these females eventually repaid the generosity. The pattern was consistent: During the 3-year study, males that shared their meat with both receptive and nonreceptive females were twice as likely to get sex than were males that had been stingy, the team reports this week in PLoS ONE.

The authors say the meat-for-sex hypothesis may be applicable to our own species. Successful hunters in ancestral and modern human tribes have more wives and more children--and it could be meat sharing that drives women's preference for them over unsuccessful hunters.

Regardless of the human connection, experts agree that the study offers strong proof of the meat-for-sex theory, which had previously been highly debated. Anne Pusey, director of the Jane Goodall Institute's Center for Primate Studies at the University of Minnesota, St. Paul, notes that it's the first study to show such a strong statistical correlation, for example. She adds that it's not just the females that are thinking long-term; males may be investing in future offspring by making sure their mates are well-fed.

Still, Craig Stanford, an anthropologist at the University of Southern California in Los Angeles, says there's more than nutrition at play, noting that meat is a valued delicacy for chimps. "They're not really eating the meat in quantity to get calories per se," he says. "If a guy takes a girl out for a meal, the issue is about the quality of the meal, not the quantity."