Do young female chimps play with sticks as dolls?

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In Kibale National Park, Uganda, female chimps have taken to carrying sticks around with them. There’s nothing obviously unusual about that – chimps are clever tool-users, who use sticks as probes, projectiles and spears. But these chimps aren’t doing very much with their sticks – they simply hold and cradle them while they go about their usual business.

Sonya Kahlenberg and Richard Wrangham think they know why. They suggest that the stick-carrying chimps are playing at being mothers. Their sticks are the chimp equivalents of human dolls and the chimps treat them like pretend infants.

It might seem like a far-fetched idea, but the duo make their case strongly. For a start, the sticks have no other obvious use. Kahlenberg and Wrangham spent 14 years watching the chimps of Kibale’s Kanyawara community. In that time, they’ve seen the apes using sticks in all sorts of ways but around 40% of the time, they just carry sticks (or pieces of bark, logs or vines) to no obvious end. These sticks tend to be twice as thick and long as those that they use as probing tools and the chimps often carry them when they aren’t doing very much. Some even hold the sticks while they sleep.

On top of that, females carry sticks more often than males (even though they’re not more likely to use sticks in general). It’s also the young females who carry sticks. Adults only did so if they didn’t have any children of their own. Mothers never carried sticks with purpose, although they did certainly use them as tools. Without any form of teaching from the adults, it’s likely that the youngsters are picking up the behaviour from each other. (That’s fascinating in itself because play traditions among children, such as nursery rhymes and games, have only been seen in humans.)

Kahlenberg, Wrangham and others have even noted several instances of chimps treating sticks in a motherly way. One (a male) went as far as making a separate nest for his stick. Another (a female) started patting her log while her mother did the same to her sick sibling.

All in all, it seems that the sticks are indeed standing in for future babies. Richard Byrne, who studies chimp culture at the University of St Andrews, says, “I think that it is quite hard to explain it in other (more obvious) ways.” For example, female chimps use sticks more often to fish for termites. “But the objects selected for this “stick carrying” are clearly nothing like insect-fishing tools,” says Byrne. Likewise, the sticks are certainly heavy enough to be thrown as weapons, but that’s an adult male behaviour, not a young female one.

However, Byrne wonders, “I wonder why the juvenile females choose objects with such un-infant properties. Little girls, I take it, prefer dolls that are soft and rounded, like babies. Is there really nothing in the environment of wild chimpanzees that is more doll-like? Moss bundles? Dead animals?” He even mentions a case where young chimps (mostly female) showed care towards a dying baby leopard that had been severely beaten by adult male chimps.

If Kahlenberg and Wrangham’s interpretation is right, this is the first evidence that wild chimps play with objects in different ways, depending on their gender. They’re not the only ones. Human boys and girls tend to play with different types of toys from an early age, with boys preferring vehicles and weaponry and girls preferring dolls.

Some scientists think that these differences are social. Parents push specific types of toys onto their babies according to social norms. That’s been documented across many studies but it’s clearly not the case for the Kanyawara chimps, since their mothers never carry sticks. Others think that human toy preferences reflect some basic biological differences between the sexes. Indeed, these differences show up even when babies are just a few months old, and they appear even in countries like Sweden where gender equality is strong emphasised.

To top it all off, two species of baby monkeys show the same gender differences in their bias towards human toys, even though these are clearly not part of their natural environment. It’s not clear why. Perhaps female infants are more interested in other infants, while males are more keen on rough-and-tumble play.  Maybe it’s a case of texture: male infants are drawn to the hard angles and surfaces of vehicles and weapons, while females prefer soft toys.

And that’s if the sex differences are as clear as suggested. Some studies find that boys and girls have their own biases for toys. Others (including one of the monkey studies) have found that males are drawn to traditionally “male” toys, but females will happily play with anything. In the chimps, young females have a penchant for stick-carrying, but it’s not clear if young male chimps have a bias towards another type of plaything.

These conflicting results make it difficult to assess Kahlenberg and Wrangham’s final claim: that stick-carrying might represent ancient sex differences which arose before our split with chimps, and that pre-date the social pressures of modern humans. And if that’s the case, it’s hard to understand why stick-carrying has never been seen in any other population of chimps. In terms of sex differences in toy choice, the chimps seem to throw up more questions than answers.

Update: Brandom Keim at Wired has a good take on this story. This is smart, analytical journalism that evaluates as well as reports.

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, dolls are 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.

Reference: Current Biology; citation to be confirmed

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