In a fashion similar to human girls, some young chimpanzees seem to play with sticks as if they were dolls.

The findings, reported in the Dec. 21 Current Biology, are the first documented evidence of boy and girl primates in the wild playing differently with their toys. Though these patterns' origins will surely be argued, they add to the constellation of behaviors shared by humans with our closest living relative.

"We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males," wrote anthropologists Richard Wrangham of Harvard University and Sonya Kahlenberg of Bates College.

Wrangham's group has studied chimpanzees in Uganda's Kibale National Park since the late 1980s, following in the methodological steps of his mentor, Jane Goodall, whose exhaustive, patient fieldwork first revealed that chimpanzees use tools and are more like humans than once thought.

The Current Biology paper is based on observations made between 1993 and 2006, and represents hundreds of thousands of hours spent trailing individual chimpanzees from dawn to dusk, recording their use of sticks.

Kibale's chimps used sticks to probe holes containing water and honey. They used sticks to hit and threaten each other. They played with them. And, finally, they carried sticks – holding them under their arms or in their laps, for hours at a time, even while walking and climbing and feeding and resting.

"Regular stick-carrying has no discernible function," wrote Wrangham and Kahlenberg, yet it accounted for more than one-third of all stick use.

Carried sticks were shaped differently than sticks used as weapons or probes, and "unlike other types of stick use, carried sticks were regularly taken into day-nests ... where individuals rested and were sometimes seen to play casually with the stick in a manner that evoked maternal play," wrote the researchers.

Stick-carrying was also most frequent in juveniles, particularly juvenile females. With parenthood it invariably ceased.

Wrangham, best known for proposing that cooking meat jump-started hominid evolution, thinks the stick-carrying habits resemble how human children play: Regardless of culture, girls seem to play more with dolls.

Explanations for this invoke both sociology and biology. Wrangham sees the latest findings favoring the biological. "We suggest that sex differences in stick-carrying are related to a greater female interest in infant care, with stick-carrying being a form of play-mothering," they wrote.

The study's implications may, however, defy easy analysis. Though a few anecdotal reports exist of captive chimpanzees treating sticks like dolls, the behavior has never before been reported in the wild. For now, Kibale's chimps are unique in their invention and culture.

It's also tempting to think of chimpanzees as snapshots of an earlier stage in human development. But chimps have also evolved, culturally and biologically, in the 3.7 million years since our branch of the primate tree split.

Maybe the Kibale chimp dolls don't represent an echo of ourselves, but an example of cultural convergence, with two species separately developing the same behavior, just as biological features like wings and eyes have evolved in similar but independent ways.

Whatever the origins of playing with dolls, it seems to be – along with tools, grief, love and warfare – one more thing that humans and chimps have in common.

Images: 1) Female chimpanzee carrying stick./Sonya Kahlenberg. 2) Comparison of sticks chosen by chimpanzees for stick-carrying (left) and probes (right)./Sonya Kahlenberg.

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Citation: "Sex differences in chimpanzees' use of sticks as play objects resemble those of children." By Sonya M Kahlenberg and Richard W Wrangham. Current Biology, Vol. 20 Issue 24, Dec. 21, 2010.