Researchers have reported some of the first evidence that chimpanzee youngsters in the wild may tend to play differently depending on their sex, just as human children around the world do. Although both young male and female chimpanzees play with sticks, females do so more often, and they occasionally treat them like mother chimpanzees caring for their infants, according to a study in the December 21st issue of Current Biology, a Cell Press publication.
The findings suggest that the consistently greater tendency, across all cultures, for girls to play more with dolls than boys do is not just a result of sex-stereotyped socialization, the researchers say, but rather comes partly from "biological predilections."
"This is the first evidence of an animal species in the wild in which object play differs between males and females," said Richard Wrangham of Harvard University.
Earlier studies of captive monkeys had also suggested a biological influence on toy choice. When juvenile monkeys are offered sex-stereotyped human toys, females gravitate toward dolls, whereas males are more apt to play with "boys' toys" such as trucks.
The new observations come from 14 years of observation of the Kanyawara chimpanzee community in Kibale National Park, Uganda. Wrangham and coauthor Sonya Kahlenberg of Bates College in Maine found that chimpanzees use sticks in four main ways: as probes to investigate holes potentially containing water or honey, as props or weapons in aggressive encounters, during solitary or social play, and in a behavior the researchers refer to as stick-carrying.
Wrangham said they had seen stick-carrying from time to time over the years and suspected that females were doing it more than males. Their detailed behavioral investigation has now confirmed that suspicion.
"We thought that if the sticks are being treated like dolls, females would carry sticks more than males do and should stop carrying sticks when they have their own babies," Wrangham said. "We now know that both of these points are correct."
Young females sometimes took their sticks into day-nests where they rested and sometimes played with them casually in a manner that evoked maternal play, the researchers report.
It's not yet clear whether this form of play is common in chimpanzees, the researchers say. In fact, no one has previously reported stick-carrying as a form of play, despite considerable interest among chimpanzee researchers in describing object use. "This makes us suspect that stick-carrying is a social tradition that has sprung up in our community and not others," Wrangham said.
Because stick-carrying is relatively rare even in the Kanyawara chimps that Wrangham and Kahlenberg studied, they won't be sure until researchers studying other communities report its absence. They note that chimp play is generally poorly described because chimp communities are usually small with few youngsters at any one time.
If it turns out that stick-carrying is unique to the Kanyawara chimps, "it will be the first case of a tradition maintained just among the young, like nursery rhymes and some games in human children," Wrangham said. "This would suggest that chimpanzee behavioral traditions are even more like those in humans than previously thought."
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