Young female chimps that live in a Ugandan park sometimes treat sticks in the same ways a little girl might treat her dolly, according to a new study in the journal Current Biology.
Studies have shown that human girls tend to play more with dolls and boys with toy vehicles and fake weapons. Captive monkeys also show a tendency to split along gender lines when they play with sex-stereotyped toys, but there has been no evidence that any young wild animals that play with toys play differently depending on whether they or male or female.
Scientists have been watching and recording the activities of the Kanyawara chimpanzee community in Uganda's Kibale National Park for 14 years. These chimps use sticks in four different ways: as probes in holes that might contain honey or water; as weapons; during play; or in a behavior the researchers have named "stick-carrying":
Stick-carrying consisted of holding or cradling detached sticks. The juveniles carried pieces of bark, small logs or woody vine, with their hand or mouth, underarm or, most commonly, tucked between the abdomen and thigh. Individuals carried sticks for periods of one minute to more than four hours during which they rested, walked, climbed, slept and fed as usual.
The researchers say that the behavior is "suggestive of rudimentary doll play" and, as with humans, more common among young females than young males. They think that with stick-carrying, the young chimps are imitating their mothers. And unlike other behaviors that employ sticks, stick-carrying always ceased when a young female had a baby of her own.
Stick-carrying is rare among the Kanyawara chimps and has never been reported elsewhere. If the behavior is unique to this population, says study co-author Richard Wrangham of Harvard University, "it will be the first case of a tradition maintained just among the young, like nursery rhymes and some games in human children."