Sex differences in object manipulation in wild immature chimpanzees and bonobos: Preparation for tool use?

Sex differences in immatures predict behavioral differences in adulthood in many mammal species. Because most studies have focused on sex differences in social interactions, little is known about possible sex differences in ‘preparation’ for adult life with regards to tool use skills. We investigated sex and age differences in object manipulation in immature apes. Chimpanzees (Pan troglodytes) use a variety of tools across numerous contexts, whereas bonobos (Pan paniscus) use few tools and none in foraging. In both species, a female bias in adult tool use has been reported. We studied object manipulation in immature chimpanzees at Kalinzu (Uganda) and bonobos at Wamba (Democratic Republic of Congo). We confirmed that chimpanzees showed higher rates and more diverse types of object manipulation than bonobos. Against expectation, male chimpanzees showed higher object manipulation rates than females, whereas in bonobos no sex difference was found. However, object manipulation by male chimpanzees was play-domi nated, whereas manipulation types of female chimpanzees were more diverse. Manipulation by young immatures (<3 years old) of both species was similarly dominated by play, but only in chimpanzees did it become more diverse with age. In chimpanzees, object types became more tool-like (i.e. sticks) with age, further suggesting preparation for tool use in adulthood (i.e. ant-dipping). The male bias in object manipulation in immature chimpanzees, along with the late onset of tool-like object manipulation, indicates that not all (early) object manipulation in immatures prepares for subsistence tool use. Object play may also function in motor skill practice for male-specific behaviors like dominance displays.