Female chimpanzees treat sticks and small logs as dolls by cuddling them, creating games and even putting them to bed, new research finds.
Since young male chimps were less inclined to play dollies, the authors say their study presents the first evidence of an animal species in the wild in which play differs between males and females.
"Our data fit with previous studies of humans and other primates to suggest that there is something innate that predisposes girls and boys to react differently to the same objects," co-author Sonya Kahlenberg of Bates College told Discovery News.
The new observations, published in the latest Current Biology, come from Kahlenberg and colleague Richard Wrangham's 14 years of studying the Kanyawara chimpanzee community in Kibale National Park, Uganda. The researchers determined both male and female chimps use sticks in four primary ways: as probes when trying to find honey or water, as props or weapons in aggressive encounters, during solitary or social play, and as doll-like objects.
Males of all ages were more likely to use the sticks as weapons, while females were more inclined to treat sticks as dolls. There were some notable exceptions, however.
"The most striking example was actually an 8-year-old male who took a log into a nest and played the 'airplane game,'" Wrangham of Harvard University told Discovery News. "He laid on his back holding the log above him on the palms of his feet and hands, and moved it from side to side."
"This airplane game is something that humans do with their infants, and chimpanzee mothers do also," he added. "Later this male, Kakama, made a small nest and put the log in it, before going back to this own nest."
When presented with what the researchers call "sex-stereotyped" toys, monkeys also display clear preferences, according to the scientists.
Wrangham explained prior research determined that when young vervet monkeys were presented with toy cars, balls, cooking pots and dolls, the females mostly went for the pots and dolls while the males gravitated toward the cars and balls.
A separate study on rhesus monkeys found that males preferred wheeled toys while females went for plush ones. When given picture books and toy dogs, no such sexual preferences were detected.
Joyce Benenson, associate professor of psychology at Emmanuel College, has performed related research.
Benenson told Discovery News that the new "findings illuminate the biological mechanisms underlying children's toy preferences" since "chimpanzees are not socialized to play with sticks in different ways, just because they happen to be male or female."
She added, "My own research supports the findings and suggests additionally a biological basis for human sex differences."
Wrangham and Kahlenberg agree that "biological predilection" appears to be involved in both toy selection and forms of play among human and non-human primate males and females.
It even could be the case that such sex differences, along with doll games, are more common in the animal kingdom than previously thought.
"I once watched a young killer whale playing with a stick for a long time, and earlier this year I watched a lion cub playing with a stick in Ngorongoro Crater," Kahlenberg said.
"Maybe there are some other species in which object play is more human-like than we are used to thinking."