Sorry ladies….it is biological.

Deep in a Ugandan forest, Betsy Wetsy has gone wild.

A new study finds that young females in one group of African chimpanzees use sticks as dolls more than their male peers do, often treating pieces of wood like a mother chimp caring for an infant. In human cultures around the world, girls play with dolls and pretend that the toys are babies far more than boys do.

Ape observations, collected over 14 years of field work with the Kanyawara chimp community in Kibale National Park, provide the first evidence of a nonhuman animal in the wild that exhibits sex differences in how it plays, two primatologists report in the Dec. 21 Current Biology. This finding supports a controversial view that biology as well as society underlies boys’ and girls’ contrasting toy preferences.

Young male Kanyawara chimps occasionally used sticks to mimic child care. Far more often, they fought with sticks, an infrequent behavior among females, say Sonya Kahlenberg of Bates College in Lewiston, Maine, and Richard Wrangham of Harvard University.

“Although play choices of young chimps showed no evidence of being directly influenced by older chimps, young females tended to carry sticks in a manner suggestive of doll use and play-mothering,” Wrangham says.

Consistent with reported cultural traditions among adult chimps (SN: 11/21/09, p. 24), Kanyawara youngsters learned from each other to play with sticks as if caring for infants, the researchers propose. Child-bearing females never played with sticks and thus didn’t model such behavior for younger chimps.

Biological differences between the sexes make female chimps more receptive to stick-mothering than males, Wrangham hypothesizes.

A related 2008 investigation of captive rhesus monkeys found that females played more with human dolls and other typically girls’ toys, whereas males usually opted for trucks and other boys’ toys.

Scientists have long argued about whether cross-cultural contrasts in boys’ and girls’ play styles reflect, at least partly, social influences such as adults buying dolls and not trucks for girls, comments anthropologist Elizabeth Lonsdorf, director of the Lester E. Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago.

“These new data suggest that sex differences in how children play may go way back in our evolutionary lineage and predate socialization in human cultures,” Lonsdorf says. If so, then Betsy Wetsy has a family tree with some unexpected branches — make that sticks.

Kahlenberg and Wrangham defined stick play among chimps as holding or cradling sticks, bark, small logs or vines, often tucking a piece of wood between the stomach and the thigh. Individuals carried sticks for anywhere from a few minutes to several hours. They often rested in nests with their sticks, sometimes playing with them much as chimp mothers play with their infants.

Stick play occurred most commonly between ages 3 and 9. Females spent substantially more time carrying sticks than males did. Among chimps age 10 or less, 10 of 15 females and five of 16 males engaged in stick play at least once over the 14-year study.

One 8-year-old male lugged around small logs and made a nest for them while his mother was pregnant. “It would be fascinating to know if his mother’s pregnancy stimulated an interest in that behavior,” Wrangham says.

Researchers have not reported regular stick play in any other chimp community. “If these behaviors are socially transmitted between playmates, then it’s not unlikely that one could find local traditions of toy use in different chimp groups,” remarks anthropologist Crickette Sanz of Washington University in St. Louis.

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