"Here we present the first evidence of sex differences in use of play objects in a wild primate," chimpanzees, says the study. "We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males."
Boy chimps tended to use sticks as toy weapons, but girl chimps tended to carry them around instead, "holding or cradling detached sticks". Statistically, the study authors could not find an explanation in the stick use in termite mound probing or other tool use in chimps.
We suggest instead that sex differences in stick-carrying are related to a greater female interest in infant care, with stick-carrying being a form of play-mothering. Several lines of evidence support this hypothesis. First, in the few instances when we observed adult females carrying sticks, the behavior always occurred prior to the female‟s first birth. Thus, unlike probing and other object use, stick-carrying ceased with motherhood. Second, unlike other types of stick use, carried sticks were regularly taken into day-nests where individuals rested and were sometimes seen to play casually with the stick in a manner that evoked maternal play. Third, the capacity for young chimpanzees to direct care towards objects has been reported in apes raised by humans and is indicated by two detailed reports from the wild of chimpanzees treating sticks like dolls. At Kanyawara, an 8-year-old male carried and played with a small log for four hours and also made a 4 separate nest for it; and while her mother was carrying her sick infant sibling, an 8- year-old female at Bossou (Guinea) carried a log, including patting it like 'slapping the back of an infant'.
The young chimps apparently learned their play styles from each other, as their parents did not play with sticks, say the authors. "Such juvenile traditions have previously been described only in humans."
What does it mean for people? "Our findings suggest that a similar sex difference could have occurred in the human and prehuman lineage at least since our common ancestry with chimpanzees, well before direct socialization became an important influence," conclude the authors.
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Tuesday, December 21, 2010
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"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."
ReplyDeleteYoung 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 if the colony is unique until researchers studying other communities report the behavior's absence.
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."