For chimpanzees living next to the West African village of Bossou, Guinea, scoring papayas can also mean scoring a mate. That's one conclusion of a 3-year study that followed Bossou's adult males as they staged daring raids on crops and then used the plundered foods to woo females. According to the authors, the crops-for-sex strategy has never been recorded outside of Bossou, and it provides further evidence of an evolutionary basis for the seductive power of male bravado.
"We believe the males may be using crop raids as a way to advertise their prowess, especially to the opposite sex," explains Kimberley Hockings, a behavioral ecologist at the University of Stirling in the U.K. "It's not just meat that can be used as social currency but any risky or difficult-to-obtain foods."
Bossou is one of six sites in Africa where long-term chimpanzee studies are being carried out, the oldest and most famous being Tanzania's Gombe Stream National Park, where Jane Goodall first observed cooperative hunting and meat sharing in the early 1970s. Since then, studies from other sites have proposed that male chimps share the meat of colobus monkeys and other small prey to build coalitions, reward allies, and recruit receptive females. Similar status-enhancing strategies have been noted in hunter-gatherer societies as well--the Hadza of Tanzania, for example, and the Aché of eastern Paraguay.
Because prey animals are scarce in Bossou, the chimps' diet there is largely vegetarian. Native plants and fruits in their patch of forest (just 6 square kilometers) are easily foraged though rarely shared. Hockings and her colleagues tracked a trio of males who routinely went on the hunt for more desirable commodities, with papayas as their preferred grab. The more exposed and anxious the chimps were during raids, the more likely they were to share food with females of reproductive age. One female with whom they shared the most food was also their most frequent consort and grooming partner, the team reports today in PLoS One.
Zoologist Frans de Waal, who directs Emory University's Yerkes National Primate Research Center in Atlanta, Georgia, says the study reinforces his findings that zoo chimps share high-value plant foods in exchange for services such as sex or grooming. "We know very little about showing off, but ... if true, this suggests that, besides nutritional value, recipients of the food also take effort and danger into account" when choosing a mate.
Ian Gilby, a behavioral ecologist at Harvard University who studies meat sharing among chimpanzees, cautions about extrapolating the findings to chimpanzees in general, noting that too few subjects were involved in this study, actual copulation frequencies were not reported, and no alternative hypotheses were tested.
Still, as the Bossou researchers point out, the pressure to convert Africa's forests to cropland is unrelenting, and the future may hold plenty of opportunities to test the crops-for-sex theories elsewhere.