Food for Sex!

. . . among chimpanzees, at least.

Although chimps share meat (such as small monkeys) that they have hunted, mostly to cement alliances, they almost never share the plants they have foraged. The reason seems to be that hunting is risky and strenuous, so sharing says to potential allies, "Lookit this monkey I just killed with my bare hands! I am strong and fit, buddy; wanna join forces?" Sharing some figs you’ve pulled down from a branch falls a bit short in the self-advertising department.

Now scientists are reporting the first-ever observation of wild chimpanzees sharing plant food they have raided from farmers’ fields. And since the lucky recipients of the largesse were sexually-receptive females living in the troop near the village of Boussou in the west African Republic of Guinea, one conclusion leaps to the fore: “We believe the males may be using crop-raids as a way to advertise their prowess to other group-members, especially the opposite sex,” said Kimberley Hockings of the University of Stirling, Scotland. “Such daring behavior 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."

Adult male chimps raided nearby fields about 22 times a month, pinching papayas, bananas, oranges, rice, maize, cassava and other goodies, the scientists reported Tuesday evening in the online journal PloS ONE. Females and juveniles almost never engaged in agricultural shop-lifting. At least the males had a guilty conscience about their theft: before and during the raids, the chimps exhibited classic signs of nervousness, including a characteristic scratching (OK, if not guilt, maybe the anxiety reflected worries about being caught by the farmers). And they always carried their ill-gotten gains back to the safety of the forest before sharing, the better to escape the wrath of the farmers.

Males almost never shared with other males, in contrast to their behaviour with meat. That suggests that sharing plant food, even that obtained at some risk, was not enough to advertise a male’s desirability as an alliance partner. Instead, the males shared the crops with females, especially one who was at the peak of her reproductive cycle and who took part in the most “consortships” (in which an adult female and an adult male slip away from others in the community to mate). Interestingly, it was the second-ranking adult male who shared the most with her—accounting for 43 percent of all sharing compared to the alpha male’s 13 percent. That suggests that the beta-male was trying to achieve through food-sharing what he might not have managed through rank.

It worked. Says Hockings, “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, that is, ‘food-for-sex and –grooming’.”

As to why the female went along with this, a clue comes from the circumstances under which raided food was shared most often: it was fruit and other crops obtained when the locals were around, and the crops were in exposed locations. By exhibiting such derring-do, the lower-ranking males were able to show off their courage and machismo, which promises to pay off reproductively. The scientists filmed the chimps in action, including footage of an adult male raiding papaya and then sharing it with his would-be conquest.