Lead researcher, Dr Kimberley Hockings from the University of Stirling's Department of Psychology said: "We believe the males may be using crop-raids as a way to advertise their prowess to other group-members, especially the opposite sex. Such daring behaviour certainly seems to be an attractive trait and possessing a sought-after food item, such as papaya, appears to draw even more positive attention from the females."

The study, which took place in the West African village of Bossou in the Republic of Guinea, is the only recorded example of regular sharing of plant foods by unrelated, non-provisioned wild chimpanzees.

Dr Hockings explained: "It is unusual behaviour as even though the major part of chimpanzees' diets consists of plant foods, wild plant food sharing (defined as an individual holding a food item but allowing another individual to consume part of that item) occurs infrequently. However, in chimpanzee communities that engage in hunting, meat is frequently used as a 'social tool' for nurturing alliances and social bonds.

This research shows that chimpanzees at Bossou use crop-raiding as an opportunity to obtain and share desirable foods, providing further insights into the evolutionary basis of human food sharing. In humans, the pursuit of certain foods is also strongly sex-biased; for example, it has been proposed that men in hunter-gatherer societies acquire large and risky-to-obtain food packages for social strategising and to garner attention."

The researchers found that adult males mainly shared the spoils of their crop-raids with females of reproductive age; particularly with a female within the group who took part in most consortships (where an adult female and an adult male chimpanzee move to the periphery of their community so that the male gains exclusive mating access).

Dr Hockings said: "The male who shared the most food with this female engaged in more consortships with her and received more grooming from her than the other males, even the alpha male. Therefore the male chimpanzees appear to be 'showing off' and trading their forbidden fruit for other currencies, i.e. 'food-for-sex and --grooming'."

 The study is published in the September 12 issue of the online, open-access journal PLoS One.

The study was carried out by the following researchers: Kimberley J. Hockings (University of Stirling), Tatyana Humle (University of Wisconsin-Madison), James R. Anderson (University of Stirling), Dora Biro (University Of Oxford), Claudia Sousa (New University of Lisbon), Gaku Ohashi (Kyoto University) and Tetsuro Matsuzawa (Kyoto University).

About wild chimpanzees

Wild chimpanzees have declined by more than 66% over the last 30 years, to a mere 200,000. Although the chimpanzees of Bossou are fortunate enough to be afforded a degree of protection and tolerance by local Manon people, other chimpanzee communities throughout Africa are not so privileged. Chimpanzees and other non-human primates are threatened by an intricate web of factors including unrelenting deforestation and fragmentation, poaching, disease, and capture for the pet trade, all of which threaten their long-term survival in the wild. These are human problems, the solutions to which will benefit both people and chimpanzees.

Cultivated plant foods are shared much more frequently than wild plant foods at Bossou, even though adult male chimpanzees often appear nervous when raiding crops (rough scratching, a self-directed behavioural pattern shown in response to anxiety, was used to quantify levels of anxiety). The shared cultivated fruits are usually large and easily divisible, and adult males are most likely to share such foods obtained in exposed locations and in the presence of local people (which is also associated with increased levels of anxiety).

Citation: Hockings KJ, Humle T, Anderson JR, Biro D, Sousa C, et al (2007) Chimpanzees Share Forbidden Fruit. PLoS ONE 2(9): e886. doi:10.1371/journal.pone.0000886

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