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This next story is for all you parents out there who've ever wondered: Why is it that my daughter only plays with dolls, or my son just wants a toy truck?
Well, scientists reported this week that they've seen similar behavior among chimpanzees in Uganda.
NPR's Dan Charles reports.
DAN CHARLES: For 23 years now, Richard Wrangham has been part of a team of scientists studying wild chimpanzees in a national park in Uganda. They'd sometimes see young chimps pick up sticks and carry them around, but Wrangham didn't think too much about it at first.
But one day in 1993, they saw a young male chimp carry a short log into a nest. The chimp laid down, put the log on the palm of his upturned hand, and flew it back and forth, back and forth.
Dr. RICHARD WRANGHAM (Primatologist and Anthropologist): It really looked just like a chimp or a human mother playing the airplane game, you know, saying wee, wee, as it goes from side to side.
CHARLES: And every once in a while, Wrangham saw something else that made him think these young chimpanzees seem to be treating sticks and logs like imaginary infants. Wrangham had the idea if they were, he'd see young female chimps doing it more often than males, because in many primate species, it's the girls who play with real babies.
Dr. WRANGHAM: You find that young females are more interested in infants smaller than themselves than young males are. They carry them and show more interest in playing with them than young males do.
CHARLES: So he and a colleague went back through 14 years of the team's records, adding up all the times when scientists wrote down that a young chimp had done something with a piece of wood. And indeed, the girl chimps had carried sticks twice as often as boy chimps. But there's an intriguing twist: Wrangham says scientists who've been watching other chimpanzees in other places have not reported this behavior.
Dr. WRANGHAM: So it looks as though there's a social tradition, a behavior that has become part of the cultural repertoire of this particular community for some reason. And so that means that these chimps are learning the behavior from others in the community.
CHARLES: Wrangham published the observations this week in the journal Current Biology.
Now, another primate researcher, Kim Wallen at Emory University in Atlanta, would like to see more evidence. For instance, the journal includes a picture showing a young female chimp carrying a stick. But is she really cradling it like a baby?
Dr. KIM WALLEN (Department of Psychology, Emory University): This doesn't happen to look like that to me. This looks like pausing to reconnoiter the surrounding before you start shuffling off into the woods.
CHARLES: But Wallen is not surprised to hear that girl chimps and boy chimps treat sticks differently. In his experiments with a group of captive rhesus monkeys, young females were happy to play with trucks or stuffed animals; young males only liked the toys with wheels.
As for whether that difference comes from biology or culture, Wallen prefers to say that biology produces a bias, which is channeled by experience.
Dr. WALLER: So, for example, the bias could be something as simple as increased energy expenditure in males and less energy expenditure in females.
CHARLES: But environment and culture channels that difference in energy toward specific ways to play. And so we get trucks for boys and dolls for girls. Maybe even among chimpanzees.
Dan Charles, NPR News, Washington.