It's almost Christmas, and, as the song goes, Barney and Ben hope for Hopalong boots and a pistol that shoots, while Janice and Jen would like dolls that will talk and go for a walk.
Now new research suggests that such gender-driven desires are also seen in young female chimpanzees in the wild—a behavior that possibly evolved to make the animals better mothers, experts say.
Young females of the Kanyawara chimpanzee community in Kibale National Park, Uganda, use sticks as rudimentary dolls and care for them like the group's mother chimps tend to their real offspring. The behavior, which was very rarely observed in males, has been witnessed more than a hundred times over 14 years of study.
"The stick serves no immediate function, they just carry it—sometimes for a few minutes, other times for hours," study leader Richard Wrangham, a biological anthropologist at Harvard University, said via email.
"Carriers regularly take sticks into the nests they rest in during the day, something that isn't done with other objects. Individuals are [also] known to play with their sticks while in their nests."
Love of Dolls Learned or Inherited?
The research represents the first time wild animals of different sexes have been observed playing differently with objects, a practice well known among human children and previously observed in captive vervet and rhesus monkeys, Wrangham said.
For instance, young female monkeys in captivity have been known to favor dolls as playthings, while their male counterparts prefer "boy" toys like trucks.
"The fact that captive monkey males and females prefer typically masculine and feminine human toys, respectively, suggests that there is something biologically different about the sexes that makes them gravitate to different types of toys," co-author Sonya Kahlenberg, a biologist at Bates College in Maine, said by email.
"What is unknown is how this relates to primates in the wild."
The study now gives Kahlenberg some insight into that question—and supports at least some biological origin for different play between the sexes.
That's because, in part, young wild chimps never see their parents using objects like sticks as playthings. (See chimpanzee pictures.)
Nature-Nurture Combination the "Real Magic"
But "biology and 'nurture' of course aren't mutually exclusive," study leader Wrangham added.
It's possible that biological factors influencing gender-specific play confer some evolutionary advantage, such as preparing future mothers to carry their infants later in life while climbing or eating. But the practice may not be common enough to add much advantage.
It's also possible, Wrangham said, that stick-carrying is just a playful expression of cognitive abilities found in chimps and humans but few other animals.
"This ability would be a capacity for imagination, or forming a mental image that is not real but nevertheless represents reality."
Nature and nurture may also work in concert, added Walter Gilliam, director of the Edward Zigler Center in Child Development and Social Policy at the Yale School of Medicine.
"This interesting study seems to support the notion that [doll-playing is] probably in part because of biology, in part because of socialization, and probably in the biggest part because of the interaction of the two—that may be where the real magic is.
"It's definitely very useful that what we see not only in humans, and in primates raised by humans, is also in primates that are raised by primates," said Gilliam, who was not involved in the study.
"So whatever the mix is of genes and environment, it seems to be in play in all three of those cases."
Young Chimps Ape Each Other
Some chimp behaviors unique to some local communities—such as nut-cracking or using sticks for termite "fishing"—appear to be passed down socially from adults to youth.
But the stick-carrying behavior has not been seen in mothers—"instead this is a behavior of chimpanzee 'childhood,'" said co-author Kahlenberg, whose study will appear December 21 in the journal Current Biology.
"Thus, it is likely that the young ones learn it from watching each other. This sort of 'juvenile tradition' has not been described in chimpanzees before but is known, of course, in humans. Think of playground games like hopscotch that kids learn from each other."
The doll-like stick play in the Kanyawara chimps has not yet been seen in any other populations, so it's possibly unique to the population. If so, the practice likely has cultural roots as well, Harvard's Wrangham noted.
"That is, all female chimpanzees are probably more oriented toward infants than are males, but only in a few places such as Kanyawara did the juvenile females get the bright idea to use sticks as 'dolls,'" he said.
Chimp Play Has Human Parallels?
The doll play or "stick carrying" peaked among five- to eight-year-old chimps, which are roughly equal to six- to nine-year-old humans in terms of development, according to Wrangham.
Indeed, the study brings up some interesting parallels with human behavior, according to Yale's Gilliam. Chimps and humans are believed to have evolved from the same common ancestor."
"Human children love to explore adult roles they witness around them, and often do so with symbolic objects," Gilliam said.
"While the [young] chimps didn't see mothers carrying around sticks, they saw them carrying around babies.
"It certainly seems within the realm of possibility that they might be replicating that socially learned behavior though this symbolic behavior with the stick."