Do young female chimps play with sticks as dolls?

By Ed Yong | December 20, 2010 12:00 pm

Chimp_stick

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

More on chimps:

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Comments (8)

  1. First, I’ll say that I think this material is really interesting and I trust the study co-authors have observed a robust finding. However, I’m hesitant to link this to an ancient sex difference. We’ve seen cultural differences among different populations of chimps, related to how much hunting they do, how territorial they are, and the types of tools they use. This could have been transmitted within this population over the years because one female chimp did it. Seeing female chimps do something more than male chimps that is tool-related is also not surprising: my understanding (though I’m not the flavor of biological anthropologist who does behavior) is that female chimps in general use tools more than male chimps, and that young female chimps seem much more interested than their brothers in copying their mothers. (Please correct me if I’m wrong, but I am fairly certain I’ve both read this and heard this from my colleagues.) So, if female chimps are more likely to manipulate objects and use them as tools, it makes sense they would also sometimes use them as toys or use them to imitate their others.

    To me, the stronger sex difference that is re-affirmed by this data is that females use objects as tools more often than males, within chimps. I would still hesitate to say that this difference is meaningful beyond the environmental/social factors that produce it: chimps have female philopatry, males are related and bonded, so males probably put more stock into building and maintaining their relationships and hierarchies. Females might spend more time figuring out how to get as many calories as possible, and spend time learning from their mothers so that they can do the whole mothering thing later in life. I don’t think an “ancient sex difference” is necessary to explain this.

  2. @KHBC – Great comment. The authors do mention that females reportedly use objects more often than males but they note that this doesn’t occur across the board. Males are more likely to use weapons, and to use leaves for wiping themselves. So one can’t say that females use all objects more frequently.

    Like you, I have no reason to doubt the interpretation of stick-carrying as analogous to doll-play, but I don’t quite buy the “ancient sex differences” suggestion.

  3. Thanks, Ed! That’s interesting about wiping — I do remember the weapon stuff but otherwise thought females were the ones who used tools a lot more.

    Either way, it’s certainly a very cool data set, from some successful and smart scholars.

  4. Nice write-up, Ed. This is tricky territory indeed. When you mentioned the studies showing that boys and girls play differently even at a few weeks or months old, I was reminded of a study that infant-cognition researcher Liz Spelke (who once debated Steven Pinker on the issue of possibly inherent gender differences in behavior) told me about. I don’t recall the study itself so can’t give a reference. But as she described it, the study observed people, both men and women, as they were given a baby to play with — and the way that people played with this very young infant would vary sharply and predictably depending on whether the infant was dressed as a boy or as a girl: if as a boy, they would be much more physical with the baby, bouncing the baby more, etc., and if it was a girl, they would more gentle physically, with more cuddling and a higher level of baby talk. This was seen in both women and men, and the actual gender of the infant (for the study used both boys and girls) didn’t matter.

    Spelke told this story to make the point that we treat babies very differently depending on gender from a very early age, and that this enough might be enough to generate different behaviors in the two genders. I don’t know the rest of the literature all that well. But this story seems to speak to the point about the early differences in play preferences.

    Ah: I easily found the debate Spelke had with Pinker. It’s at http://www.edge.org/3rd_culture/debate05/debate05_index.html. I’m not sure if she mentions the study in it but I suspect she might.

    Readers who find Spelke intriguing — which she very much is — might be interested in either my profile of her (http://daviddobbs.net/page2/page3/page3.html) or one that appeared in the New Yorker not long after (http://www.newyorker.com/archive/2006/09/04/060904fa_fact_talbot).

  5. Excellent write-up, Ed.

    While this probably cannot be done for fear of ‘contaminating’ the Kanyawara chimp’s culture, it would be interesting to replicate the monkey study you reference by providing the chimps with human toys. Perhaps this kind of thing COULD be done with captive chimps.

    @David: awesome comment. That study Spelke described is incredibly interesting indeed. Reminds me of the following John Stuart Mill quote (from The Subjection of Women) :

    “I deny that anyone knows, or can know, the nature of the two sexes, as long as they have only been seen in their present relation to one another. If men had ever been found in society without women, or women without men, or if there had been a society of men and women in which the women were not under the control of the men, something might have been positively known about the mental and moral differences which may be inherent in the nature of each. What is now called the nature of women is an eminently artificial thing — the result of forced repression in some directions, unnatural stimulation in others.”

  6. Oh, and David, is this the study Spelke was talking about? (There are a couple of similar looking papers to be found here).

  7. EMJ

    Great review of the paper Ed!

    This behavior would need to be found in multiple other colonies for it to be considered species-typical behavior. However, in our species girls playing with dolls as a form of maternal practice is the norm in many different societies (perhaps even most) and there continues to be disagreement over whether it represents innate sex differences.

  8. praisegod barebones

    I’m a bi9t disappointed that the thing that’s attracting all the attention here is the gender difference stuff. From my perspective (one of the things I work on is Theory of Mind) the fact that we’ve got something that looks like pretend-play in a non-human species is the really stunning thing about this.

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