Scientists at Harvard University and Bates College find female chimpanzees   appear to treat sticks as dolls, carrying them around until they have children of their own. Young males engage in such behavior much less frequently. The new work by Sonya M. Kahlenberg and Richard W. Wrangham, described this week in the journal Current Biology, provides the first evidence of a wild nonhuman species playing with dolls, as well as the first known sex difference in a wild animal's choice of playthings. The two researchers say their work adds to a growing body of evidence that human children are probably born with their own ideas of how they want to behave, rather than simply mirroring other girls who play with dolls and boys who play with trucks. Doll play among humans could have its origins in object--carrying by earlier apes  , they say, suggesting that toy selection is probably not due entirely to socialization. "In humans, there are obvious sex differences in children's toy play, and these are remarkably similar across cultures," says Kahlenberg. "While socialization by elders and peers has been the primary explanation, our work suggests that biology may also have an important role to play in activity preferences." In 14 years of data on chimpanzee behavior at the Kibale National Park in Uganda, Kahlenberg and Wrangham counted more than 100 examples of stickcarrying. Some young chimpanzees carried sticks into the nest to sleep with them and on one occasion built a separate nest for the stick. "We have seen juveniles occasionally carrying sticks for many years, and because they sometimes treated them rather like dolls, we wanted to know if in general this behavior tended to represent something like playing with dolls," says Wrangham, a Professor at Harvard. "If the doll hypothesis   was right, we thought that females should carry sticks more than males do, and that the chimpanzees should stop carrying sticks when they had their first child. We have now watched enough young chimpanzees to prove both points." It can be concluded from the passage that   _  .
both humans and chimpanzees choose their playthings due to sex difference different factors cause humans and chimpanzees to choose different playthings only female chimpanzees have playthings chimpanzees usually choose playthings for their children