Meat for sex among Chimpanzees

PLoS One has published an interesting new study on primate behavior.

Wild Chimpanzees Exchange Meat for Sex on a Long-Term Basis by Cristina M. Gomes and Christophe Boesch

It's long been known that chimpanzees share meat with each others, and that there is a tendency for male chimpanzees to share with female chimpanzees. This has lead to a hypothesis of an exchange of meat for sex, but so far there was no evidence to back this up.

Gomes and Boesch, however, observed a group of wild chimpanzees, and found that over a 22 month period, female chimpanzees more frequently had sex with male chimpanzees which had shared meat with them, than with other similar chimpanzees.

Humans and chimpanzees are unusual among primates in that they frequently perform group hunts of mammalian prey and share meat with conspecifics. Especially interesting are cases in which males give meat to unrelated females. The meat-for-sex hypothesis aims at explaining these cases by proposing that males and females exchange meat for sex, which would result in males increasing their mating success and females increasing their caloric intake without suffering the energetic costs and potential risk of injury related to hunting. Although chimpanzees have been shown to share meat extensively with females, there has not been much direct evidence in this species to support the meat-for-sex hypothesis. Here we show that female wild chimpanzees copulate more frequently with those males who, over a period of 22 months, share meat with them. We excluded other alternative hypotheses to exchanging meat for sex, by statistically controlling for rank of the male, age, rank and gregariousness of the female, association patterns of each male-female dyad and meat begging frequency of each female. Although males were more likely to share meat with estrous than anestrous females given their proportional representation in hunting parties, the relationship between mating success and sharing meat remained significant after excluding from the analysis sharing episodes with estrous females. These results strongly suggest that wild chimpanzees exchange meat for sex, and do so on a long-term basis. Similar studies on humans will determine if the direct nutritional benefits that women receive from hunters in foraging societies could also be driving the relationship between reproductive success and good hunting skills.

Given the fact that earlier research has shown that chimpanzees can understand the concept of bartering, I think the findings sounds plausible, but nevertheless I can't help pointing out that the meat sharing and the more frequent sex might both be symptoms of an already existing bond between the chimpanzees.