Food sharing may be an altruistic act, but there is often something in it for the sharer, as well. It can be a way to gain favours, to pursue the opposite sex or even to show off, saying, "Look what I've got."

This is true for people, and it is also true for chimpanzees. Only the main thing that chimps share is not a nice big box of chocolates. It's meat.

Now, though, researchers have found evidence of a population of chimps that shares something else: plant foods. And not just any plant foods, but foods from farms and orchards that the chimps raid.

Kimberly J. Hockings of the University of Stirling in Scotland and colleagues observed the behaviour of male adult chimps in a village, Bossou, Guinea, in West Africa, over more than two years. (A video of a male taking a papaya is at nytimes.com/science.)

Prey animals are rare in the area, so these chimps almost never share meat. And as with other chimp populations, they do not share wild plant foods. In 58 of 59 instances of sharing, Hockings reports in the open-access online journal PLoS ONE, they shared papayas, oranges, cassava and other foods they stole from farms.

Most of the sharing involved a male's allowing a female of reproductive age to take some of the food. So as with instances of meat sharing among other chimps, the aim of most crop sharing by the Bossou chimps appears to be related to attracting the opposite sex.

But crop raiding is often accompanied by scratching, an indication that the activity makes the chimpanzee nervous. The researchers say males who raid crops may be showing off, advertising that they are bold and willing to take risks. So in addition to attracting females, crop raiding could serve to intimidate other males.

The researchers say further study of this raiding and sharing may help to understand the evolution of food sharing among humans, how we got to the point where divvying up a big box of chocolates helps build social relationships.