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.

Are chimpanzees going biblical on us?

Well, not quite, thought the headline of this PLoS One article makes it sound that way.

Chimpanzees Share Forbidden Fruit by Kimberley J. Hockings, Tatyana Humle, James R. Anderson, Dora Biro, Claudia Sousa, Gaku Ohashi, and Tetsuro Matsuzawa.

However, there are no snakes involved. Rather, we are talking about high-risk food here, such as meat, or cultivated plant foods. Cultivated by Humans, that is, not Chimpanzees. It is the later food type that Hockings et al focus on.

The sharing of wild plant foods is infrequent in chimpanzees, but in chimpanzee communities that engage in hunting, meat is frequently used as a ‘social tool’ for nurturing alliances and social bonds. Here we report the only recorded example of regular sharing of plant foods by unrelated, non-provisioned wild chimpanzees, and the contexts in which these sharing behaviours occur. From direct observations, adult chimpanzees at Bossou (Republic of Guinea, West Africa) very rarely transferred wild plant foods. In contrast, they shared cultivated plant foods much more frequently (58 out of 59 food sharing events). Sharing primarily consists of adult males allowing reproductively cycling females to take food that they possess. We propose that hypotheses focussing on ‘food-for-sex and -grooming’ and ‘showing-off’ strategies plausibly account for observed sharing behaviours. A changing human-dominated landscape presents chimpanzees with fresh challenges, and our observations suggest that crop-raiding provides adult male chimpanzees at Bossou with highly desirable food commodities that may be traded for other currencies.

Quite interesting. And I must admit it's a bit surprising to me that such things haven't been well documented in the past - either with sharing of crops or with meat sharing, though there is some knowledge about the later. From the article, it would seem that the behavior differs quite a bit from chimpanzee group to chimpanzee group, at least when it comes to meat sharing, but there seems to be a common tendency for males to share meat with females.

Food sharing is observed throughout the animal kingdom, albeit at varying levels and complexities. Hypotheses proposed to explain food sharing behaviours in chimpanzees [for reviews], [see 1,2] range from cognitively simple explanations, such as begging intensity [3], to more complex sharing strategies, such as reciprocity [4]. Within chimpanzee communities that engage in hunting, meat is reportedly used as a ‘social tool’ [5]; alliances and affiliative relationships are cemented by gifts of meat. Long-term data from Mahale in Tanzania suggest that alpha males use meat sharing as a coalition strategy, never sharing with potential rivals such as beta or younger adult males [6]. However in Taï, Ivory Coast, hunters receive a share of meat if they participated in the hunt, regardless of the identity of the possessor [7]. The ‘meat-for-sex’ hypothesis suggests that males share meat with females either to gain immediate access to swollen females [8], [9] or to establish or strengthen an affiliative relationship and thus increase future mating opportunities [10]. Additionally, as meat is typically energetically costly and risky to acquire for chimpanzees, sharing with others may advertise an individual's strength and prowess [11]; simply possessing a desirable item may draw positive attention to an individual, enhancing the latter's social status [12], [13].

The article suggests that if we learn to understand the behaviors and motivations for chimpanzees, we could also learn something of our own kind's behavior.