Is toy use sex-specific?: gender differences in use of sticks by chimpanzees

We all know that little girls tend to play more with dolls and little boys with trucks and soldiers.  “Blank-slaters” have long attributed this difference in behavior to socialization rather than innate genetic preferences (or a combination of the two).  A new paper in Current Biology by Sonya Kahlenberg and Richard Wrangham (Sex differences in chimpanzees’ use of sticks as play objects resemble those of children), suggests, at least according to the authors, that the behavior could be partially hard-wired with some evolutionary roots.  This is based on the observation that, at Kibale National Park in Uganda, female chimps play with sticks more often than do males, and in a way that suggests what the authors call “play-mothering”: the sticks are surrogate children in whom females take a greater interest.

Over 14 years of observation at the park, the authors observed juvenile chimps using sticks in several ways.  They’re used as “weapons” in aggressive encounters, as solitary play objects, and during the well-known phenomenon of “probing”: chimps stick twigs into holes in search of either water or honey. But the authors were more interested in “stick-carrying” behavior, which the authors describe thusly:

Stick-carrying consisted of holding or cradling detached sticks (median length, 36 cm; median weight, 112 g; n = 6 recovered sticks). The juveniles carried pieces of bark, small logs or woody vine, with their hand or mouth, underarm or, most commonly, tucked between the abdomen and thigh.

Here’s a photo, by Sonya Kahlenberg, of a 9-year-old female carrying a big stick in the “groin pocket”:

This behavior was observed significantly more often in females than in males, peaking at about 5 years of age in males and 7 in females, and dropping off to nearly zero behaviors observed after age 11.  This isn’t explained by a generally higher use of object by females, since weapon use and the use of leaves to wipe one’s body are in fact male-biased behaviors.  Probing, however, is seen more often in females than in males.  But the authors claim that probing isn’t correlated with stick-carrying, because females that probe more often don’t show a correlated increase in stick-carrying, nor was there an overlap between the size of probe sticks versus “carried” sticks.

I’m not fully convinced by this special pleading.  First, the sample size was small (10 individuals), and unless the correlation is really high you wouldn’t observe it in such a small sample.  More important, perhaps females just tend to have a penchant for carrying sticks in general, and simply differ in their preference for sticks of different sizes.  If males like candy more than females, but some males prefer Skittles over Twizzlers, and vice versa, then it’s possible that overall you’d see males eating candy more than females, but individual males who ate more Skittles wouldn’t necessarily also eat more Twizzlers.

Nevertheless, the authors suggest that this is a sex-specific difference in “play mothering” for several reasons:

  1. Females carry sticks only before they give birth; the behavior isn’t seen after motherhood.
  2. Unlike other sticks, carried sticks are taken into “day nests” and fondled in a way that suggests “maternal play” to the authors.
  3. Young chimps reared by humans also show similar behaviors that sometimes strongly resemble the behaviors of mothers carrying infants.

The authors suggest, then, that this “maternal” stick-carrying is hard-wired in chimps, so that females are genetically predisposed to treat sticks like infants.  If this is true, it could either be directly adaptive (practice for motherhood), or simply an early and nonadaptive expression of behaviors that become adaptive after motherhood.

The behavior can’t be completely hard-wired, though, because stick-carrying isn’t seen in chimps outside of the Kanyawara park.  It appears to be socially learned from other juveniles.  The idea, then, is that individuals learn to carry sticks in a specific “maternal” way, but that females have a greater propensity to imitate this behavior.  It’s a behavior that involves gene/environment interaction.

The authors conclude that differences in male and female humans in their propensity to use toys may also have this genetic/evolutionary component:

The sex difference in stick-carrying in juvenile Kanyawara chimpanzees arises without any teaching by adults and is consistent with practice for adult roles. Our findings suggest that a similar sex difference could have occurred in the human and pre-human lineage at least since our common ancestry with chimpanzees, well before direct socialization became an important influence.

This is intriguing, and goes along with some other evidence for a genetic component in humans.  For example, the authors note that girls who have been exposed to high levels of androgen in utero make more masculine toy choices (I’m not familiar enough with this work to know if there’s potential socialization bias based on appearance).  But, though intriguing, the data are too scanty to make a really convincing case.  This is not because of a lack of effort, for these data were collected over 14 years of observation in Uganda.  It’s just tough to collect a huge number of rare behaviors in a wild, free-roaming primate.

______________

Kahlenberg, S. M. and R. W. Wrangham. 2010.  Sex differences in chimpanzees’ use of sticks as play objects resemble those of children. Current Biology 20:R1067-R1068.  (doi:10.1016/j.cub.2010.11.024)

24 Comments

  1. Kit Blumenstein
    Posted December 22, 2010 at 8:23 am | Permalink

    Then there are the girls that prefer trucks and boys that like dolls and both grow into hetero-sexual adults. I have long believed too much emphasis is placed on singular behaviors. Indeed some may be innate, while some is social, it’s a slightly interesting study, nothing more.

    • Dominic
      Posted December 22, 2010 at 9:07 am | Permalink

      True, but whereas dolls represent humans, trucks are not a part of our prehistory. Is the question what children do with toys once they have them? With no toys children will happily play with stones, sticks or whatever comes to hand.

      • Kit Blumenstein
        Posted December 22, 2010 at 10:25 am | Permalink

        Very true. Play is mostly about learning the limits and possibilities of the world. Items that are a normal part of the household are those most children find independently and enjoy. Save money on toys, and allow a child to play with a cardboard box or Tupperware and a wooden spoon. Allow them some opportunity for creativity.

      • MadScientist
        Posted December 22, 2010 at 11:33 am | Permalink

        No, no, no. The question is do female chimps prefer sticks while male chimps prefer milk bottles?

        • Dominic
          Posted December 23, 2010 at 3:28 am | Permalink

          Moooo.

          That’s me being a cow.

    • Posted December 22, 2010 at 9:19 am | Permalink

      Boys love playing with dolls. They just call them “action figures.”

      And G.I. Joe is hardly a recent phenomenon; toy soldiers have been around longer than tinsmiths.

      Lots of grownups like dolls, too, and have for millennia. Rodin mastered the craft, but people were making dolls before they were making beer or bread.

      Cheers,

      b&

  2. daveau
    Posted December 22, 2010 at 10:01 am | Permalink

    Nature vs nurture; the eternal question. Yes, I had dolls, but I played war and my sister played house. If you had completely ambiguous toys, what would you have done and why? Excellent line of inquiry.

  3. Sal Bro
    Posted December 22, 2010 at 10:06 am | Permalink

    The authors suggest, then, that this “maternal” stick-carrying is hard-wired in chimps, so that females are genetically predisposed to treat sticks like infants.

    I’m not seeing where the authors claim that their observations support hard-wiring. In the last paragraph they acknowledge that learning likely explains this behavior:

    Given that regular stick-carrying has not been reported outside Kanyawara, a social learning component appears important…[S]ex differences in juvenile stick-carrying did not result from females observing their mothers carrying sticks, since mothers never carried sticks. Instead, youngsters apparently learned socially from each other.

    Although, they don’t exclude genetic predisposition, either (last sentence)–

    “[A] similar sex difference could have occurred in the human and pre-human lineage at least since our common ancestry with chimpanzees, well before direct socialization became an important influence”

    –as well they shouldn’t, since their study wasn’t equipped to address underlying causes.

  4. K.D. O'Brien
    Posted December 22, 2010 at 10:20 am | Permalink

    Reminds me of an ex-boyfriend’s mother who told me, after seeing one of my boyfriend’s nieces cradling a doll, how natural it was for girls to be nurturing. The implication was there was something wrong with me because I didn’t want children. She failed to see that about 5 minutes later, the little girl was violently beating the doll’s head on a stair. I think sometimes we read into behavior what we want to see.

    • Wayne Robinson
      Posted December 22, 2010 at 4:15 pm | Permalink

      You owe me a new computer screen. I was drinking a cup of coffee at the time I read your comment, and now the screen’s ruined …

      • K.D. O'Brien
        Posted December 23, 2010 at 10:39 am | Permalink

        Well, gosh, then how’d you write the reply…

        I remember popping the heads off of dolls a lot when I was a kid. I wanted to see how the goggly eyes worked. Ended up with a few dolls I couldn’t get the heads back on.

  5. MadScientist
    Posted December 22, 2010 at 11:32 am | Permalink

    Could this be a case of monkey see monkey do? (Or should I say chimps aping.) It may be a case of socialization even if it is not overt socialization. Appropriate tests would be to observe captive individuals of the species who were never exposed to this (but I wouldn’t advocate isolating individuals just to do such an experiment; that’d be cruel). I’m also extremely skeptical of any claims of an “evolutionary link to behavior”. Such claims are extremely difficult to support.

  6. GhostOfColemanYoung
    Posted December 22, 2010 at 11:47 am | Permalink

    If stick-carrying itself is thought to be socially learned, why would they assume that the observed sex difference is hard-wired rather than also socially learned? Once you establish that they do have socially learned behaviors, it seems plausible that (just like humans) sex differences in exhibiting the behavior can also be learned.

    I am not a blank-slate guy myself, but I don’t see much here that indicates anything one way or the other.

    • theshortearedowl
      Posted December 22, 2010 at 3:08 pm | Permalink

      Exactly – why would you assume that, just because a behaviour is seen in chimps, it is “hard-wired” rather than socialized? Chimps have pretty complex social behaviour too.

  7. אביגיל
    Posted December 22, 2010 at 1:15 pm | Permalink

    “Blank-slaters” have long attributed this difference in behavior to socialization rather than innate genetic preferences (or a combination of the two).

    You don’t have to be a “blank-slater” to assert that.

    What, exactly, is the real difference between a “doll” and a “toy soldier”? Semantics, really. Boys and girls play with dolls, boys just call them action figures.

    Until we can raise some kids in controlled, culture-free laboratories, I don’t really give much creed to what boys and girls supposedly “prefer” to play with.

  8. miko
    Posted December 22, 2010 at 2:18 pm | Permalink

    “Blank-slaters” have long attributed this difference in behavior to socialization rather than innate genetic preferences (or a combination of the two).

    Why are people who believe in a complex interaction between genes and environment in producing behavior (i.e. a “combination of the two”) “blank slaters”? Interestingly, in other company this makes me a “genetic determinist.”

    And who says chimps don’t have “gendered” socialization?

    • Posted December 22, 2010 at 4:37 pm | Permalink

      Interestingly, in other company this makes me a “genetic determinist.”

      :0-)

  9. stvs
    Posted December 22, 2010 at 3:09 pm | Permalink

    “Blank-slaters” have long attributed this difference in behavior to socialization rather than innate genetic preferences

    This must be before they have children themselves.

    I read Pinker’s book not long before becoming a parent, and partly because of this adopted a purposefully laissez faire approach to my daughter’s interests simply out of curiosity to see what they might be without parental influence. She hordes dolls and pretend dolls and nurtures just about everything. In no way do I attribute her interests to socialization.

    But the incident that really revealed innate behavior to me wasn’t about dolls or trucks: it was a disgusting infestation of writhing flesh-colored grubs. I would carry my daughter on walks through the woods and show her whatever we found. One day, when she was around 8 months old, we found a mushroom and I was pushing it with a stick to look underneath. The mushroom fell right over because of the grub infestation. It was very gross, but I said nothing and kept a poker face because I wanted to see how my baby daughter would react. She recoiled and was visibly disgusted.

    Shouldn’t it be a lot more controversial if things like repulsion to bugs and sex-specific behavior weren’t innate?

    • CW
      Posted December 22, 2010 at 5:28 pm | Permalink

      There are some problems with your grubs anecdote. For starters I would have serious questions about just how effective your poker face actually is. Even if we assume that you managed to convey absolutely no reaction to your daughter (rather unlikely considering you were unprepared) then all we have to do is try the same scenario with a male child and then we would have a very compelling study with deeply suspect methodology and a sample size of 2.

      I would ask though, before you go to the effort to expand your experiment, since when is “disgusted by bugs” even hypothetically considered an inherent female trait? (And why doesn’t it apply in cultures where “bugs” are cuisine?)

      • stvs
        Posted December 22, 2010 at 7:33 pm | Permalink

        I apologize for not being clearer—the grub incident is not illustrative of sex differences, just an innate revulsion to somewithing widely regarded as very yucky. This is obviously anecdotal, and my interpretation of a baby’s reaction may be incorrect, but it was a pretty remarkable reaction to a sight that would be very difficult to socialize, and we went out of our way NOT to teach aversion to bugs.

        As for inherent female traits, the key difference between when my daughter plays with her dolls and when I played with my “action figures” is that she cares for them like babies as the central aspect of her play. Though I’m sure that there are counterexamples, I’ve never seen a little boy do that.

    • Posted December 22, 2010 at 6:48 pm | Permalink

      I concur with CW in wondering why this would be evidence of a specifically female reaction to anything, as opposed to, say, an innate disgust reflex that occurs in both boys and girls.

      As far as this comment:

      I read Pinker’s book not long before becoming a parent, and partly because of this adopted a purposefully laissez faire approach to my daughter’s interests simply out of curiosity to see what they might be without parental influence. She hordes dolls and pretend dolls and nurtures just about everything. In no way do I attribute her interests to socialization.

      This reminds me of a passage in Cordelia Fine’s Delusions of Gender in which a mother claimed to have done the same thing, raising her daughter in a neutral way without emphasizing the usual gender stereotypes, and observed that her daughter still did things like tuck her toys into bed and sing lullabies to them. The mother assumed this to be compelling evidence of an innate gender preference that couldn’t have been learned from the environment. Then the interviewer asked her whether she or her husband was the one who customarily put the girl to bed every night. Guess what the answer was.

      Even if we don’t consciously emphasize it, even if some enlightened parents seek to downplay it, our environment and our society send a lot of messages that gender is an important distinction, as well as what the expected behaviors of the different genders are. Kids are very good at picking up on subtle cues like this – no surprise, really, since childhood is the time of greatest developmental plasticity. It is possible to raise children in a truly gender-neutral way, but doing it right takes a lot more effort and planning than most people would expect.

    • MadScientist
      Posted December 23, 2010 at 12:09 pm | Permalink

      There is a reason careful experimentation is so vital to science.

  10. windy
    Posted December 22, 2010 at 3:59 pm | Permalink

    This isn’t explained by a generally higher use of object by females, since weapon use and the use of leaves to wipe one’s body are in fact male-biased behaviors.

    But isn’t tool use more common in female chimps?

  11. Gcgmd
    Posted December 25, 2010 at 7:11 am | Permalink

    Turner syndrome is a condition where girls have one X chromosome instead of two. There have been studies (originally by Skuse et al.)that reveal cognitive and social behavior differences in these girls and adult women depending on which parent they inherited the X chromosome from. Bernard Crespi has a good review. I can’t locate the reference on Pubmed but there was a study where the researchers asked caretakers (parents and teachers) of girls (toddlers?) with Turner syndrome whether they engaged in what they called stereotypic maternal behavior such as playing with dolls. The girls were then karyotyped and analyses were performed to find the origin of the one X chromosome. Girls with the stereotypic “nurturing” behavior tended to have inherited the X chromosome from their father. This would make sense in terms of selection since a female (in all mammals) would inherit one of her X chromosomes from her father and one of a possible two X chromosomes from her mother. The X chromosome from the father could be imprinted with maternal behavior by some epigenetic phenomena such as methylation differences of certain genes on the X chromosome in the sperm. The one of two X chromosomes inherited from the mother in both males and females cannot be imprinted since its “placement” in an unfertilized egg is a random result of the reduction of the karyotype from 46 chromosomes to 23 during meiosis. The results of these and other studies are evidence that maternal behavior is inherited from a paternal grandmother through her son! I have read that chimpanzee males do better in terms of attracting females for mating if their mothers are around.


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