FILES - A picture taken on April 26, 2005 shows three female chimpanzees nodding-off as they sit on rocks in a family group, with the sun on their backs in their open air enclosure at the Taronga Zoo in Sydney. The way to a chimp's heart is through her stomach, German researchers revealed on April 8, 2009. A study by scientists from the Max Planck Institute has shown that female chimpanzees are more likely to mate with males who share food with them, findings that could shed light on relationships in primitive human societies. AFP PHOTO/Rob ELLIOTT