Male chimpanzees will risk serious injury to provide females with the "forbidden fruit" that they crave, reveals a study of chimps in western Africa.

The males advertise their prowess and impress potential mates by stealing papaya from local farms, researchers found.

Kim Hockings at the University of Stirling, UK and colleagues spent two years observing the behaviour of a chimpanzee group living in wild forest surrounding a farming village in the Republic of Guinea. They noticed several of the male chimps in the group ventured repeatedly into the crop fields, even though farmers tried beating them away with sticks.

The males sought a special treat that they could not find in the forest – papaya fruit. And before scrambling to take this forbidden fruit, the animals showed signs of nervousness, such as scratching their bodies, which indicates they understood the dangers of getting caught. "They won't get killed, but they're clearly nervous," explains Jim Anderson at the University of Stirling, a study co-author.

The risky theft of papaya, however, appears to have a sweet reward. Researchers found this out when they followed three adult male chimps that had stolen papaya and watched what happened when the animals returned to join their group.

Currying favour
It turns out that the males often offered up their booty to females – and when they did, they gave it to females of reproductive age about 90% of the time. One particular female that was extremely willing to mate with the males after receiving papaya got more than 50% of the stolen fruit offerings.