Young female chimps carry sticks as a form of "play-mothering," much in the same way girls cradle their dolls, scientists said Monday.
The findings, published online in the journal Current Biology, imply that gender roles might be more biologically rooted than some people think, the authors said — and that might hold for human beings, too.
Lead author Sonya Kahlenberg, a biological anthropologist at Bates College in Lewiston, Maine, looked at incidences of stick-carrying in a chimp community in Uganda's Kibale National Park over a period of 14 years. After examining more than 100 cases, she and coauthor Richard Wrangham, a biological anthropologist at Harvard University, noticed a distinct gender difference. Of the young females, 67% carried sticks, as opposed to just 31% of males.
Aside from their other stick-related activities — using them to probe holes that might hold honey or water, or brandishing them like weapons — the young chimps would also occasionally cradle a longer, thicker stick as they went about their business, almost as if it were a baby.
They would even bring the sticks into their nests, which never happened with the sticks used for honey-hunting or play-fighting. Some of the young chimps even played the "airplane" game: lying on their backs and lifting the stick in the air, much as human parents entertain their youngsters.
The authors say the stick-carrying demonstrates a type of "play-mothering." Those males who did carry sticks stopped doing so as they grew older, and the females would cease when they gave birth.
"This is an entirely new way of thinking with which I entirely agree," said Joyce Benenson, a developmental psychologist at Emmanuel College in Boston who was not involved in the research. The same tendencies can be seen in human baby girls and boys, she added, at a time too early for parents to have imposed their gender ideas on their infants.
Kahlenberg pointed to previous research showing that when presented with dolls or trucks, female chimps in captivity preferred the dolls and males went for the trucks. The males may have gravitated toward the trucks because they intuitively favor toys that allow more freedom of movement, she said.
But Kahlenberg added it was probable that the stick-carrying was a socially learned tradition subtly passed from one generation to the next, unique to this particular chimpanzee community.
"It's kind of a unique case of having nature and nurture in one community .… Everything is so intertwined," she said.
Kahlenberg herself is waiting to see which toys her 9-month-old daughter picks up. "Right now it's just blocks .… It's too early to say," she said.