Born This Way? Gender-Based Toy Preferences in Primates

Last week, British parents who had hidden their child’s gender from the world finally revealed that their five year old, now ready to enter school, is a boy. While the parents had hoped to raise their son Sasha in a gender-neutral way (“Stereotypes seem fundamentally stupid. Why would you want to slot people into boxes?”), their approach raised eyebrows and controversy. Were they creating an environment where their child could find his own gender identity, free from crippling societal expectations, or were they conducting a bizarre and possibly harmful experiment on a family member?

Putting aside the issue of whether the parents acted appropriately, the story raises fascinating questions about gender-specific traits and preferences. To what degree are gender differences innate and biological, and to what extent do they arise out of societal modeling and environment?

Some (including Sasha’s parents) may see gender preferences as being primarily influenced by human social pressures, but there are indications of biological influences as well. For example, girls with a particular genetic condition that exposes them to high prenatal levels of androgen often show “masculine” toy preferences, even when their parents strongly encourage them to play with female-typical toys. Given the intertwining impacts of nature and nurture in human societies, can we learn anything from our animal relatives who grow up free from human societal norms?

In this post, I’d like to take a look at two recent studies that examine differing male and female toy preferences in primates.

Male Monkeys Prefer Trucks

First, in 2009 a research team led by Janice Hassett of the Yerkes National Primate Center at Emory University reported on experiments in which they the researchers to see whether rhesus monkeys (Macaca mulatta) would exhibit gender-specific toy preferences similar to those of human children.

In humans, studies have shown that boys gravitate strongly to stereotypically “masculine” toys such as trucks and other vehicles, while girls are less rigid, spending relatively equal amounts of time playing with boy-favored toys and with more traditionally “feminine” toys such as dolls. One hypothesis put forward to explain this difference has been that boys face greater societal discouragement when they play with “girl toys” than girls do in the reverse situation. The researchers figured that by looking at rhesus monkeys, who don’t face comparable social pressures to conform to gender roles, they might be able to illuminate biological influences on toy selection as well.

Of course I'm not playing; you gave me a Raggedy-Ann. Pass me that truck. Now. (photo credit: J.M.Garg, Wikipedia)

In their study, the researchers compared how 34 rhesus monkeys living in a single troop interacted with human toys categorized as either masculine or feminine. The “masculine” set consisted of wheeled toys preferred by human boys (e.g., a wagon, a truck, a car, and a construction vehicle); the “feminine” set was comprised of plush toys comparable to stuffed animals and dolls (e.g., a Raggedy-Ann™ doll, a koala bear hand puppet, an armadillo, a teddy bear, and a turtle). Individual monkeys were released into an outdoor area containing one wheeled toy and one plush toy, with the researchers taping all interactions using separate cameras for each toy, identifying all specific behaviors, and statistically analyzing the results.

The results closely paralleled those found in human children. As with human boys, male rhesus monkeys clearly preferred wheeled toys over plush toys, interacting significantly more frequently and for long durations with the wheeled toys. Also mirroring human behavior, female rhesus monkeys were less specialized, playing with both plush and wheeled toys and not exhibiting significant preferences for one type over the other. Here’s a chart illustrating the similar gender preferences of humans and rhesus monkeys (the information regarding human preferences comes from a 1992 study by Sheri Berenbaum and Melissa Hines):

The researchers noted that these similarities show that distinct male and female toy preferences can arise in the absence of socialization pressures and hypothesized that “there are hormonally organized preferences for specific activities that shape preference for toys that facilitate these activities.”

Barbie Really Is a Stick Figure

Next, in a brief paper published in 2010, Sonya Kahlenberg of Bates College and Richard Wrangham of Harvard University presented the first evidence of wild male and female primates, chimpanzees (Pan troglodytes) in the Kanyawara chimpanzee community of Kibale National Park, Uganda, interacting differently with play objects.

Over a 14 year period, Kahlenberg and Wrangham observed that juvenile Kanyawara chimpanzees tended to carry sticks in a manner suggestive of rudimentary doll play and that the behavior was more common in females than in males. Juvenile chimps, particularly females, would carry around small sticks for hours at time while they engaged in other daily activities such as eating, climbing, sleeping, resting and walking. While the same chimps used sticks as tools for specific purposes, the researchers were unable to discern any practical reason for the stick-carrying. The following chart shows the degree to which female chimps were more likely to engage the in stick carrying behavior:

Age and sex differences in the rate of stick-carrying in chimpanzees. Females: circles, solid line. Males: triangles, dashed line.

The researchers hypothesized 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 (i.e. carrying sticks like mother chimpanzees carrying infants).” In support of this proposition, they pointed to several factors. First, they never observed stick carrying by any female who had already given birth; that is, stick-carrying ceased with motherhood. Also, the chimps regularly carried sticks into day nests where they “were sometimes seen to play casually with the stick in a manner that evoked maternal play.” Finally, nurturing behavior towards objects like sticks had previously been reported in captive chimps and documented on a couple of occasions in the wild.

Also, the researchers suggested a social rather than biological basis for the behavior. Because regular stick-carrying hasn’t been reported in other wild chimpanzee communities, they proposed that that young Kanyawara chimpanzees may be learning the behavior from each other as a way of practicing for adult roles – a form of social tradition passed between juveniles previously described only in humans. Kahlenberg and Wrangham conclude by noting that:

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.

So there you have it. One rhesus monkey study positing a biological and hormonal basis for gender-specific play, and another chimpanzee study emphasizing social learning… At least for now, the threads of nature and nurture impacting gender roles seem difficult to disentangle for non-humans, just as they are for us.

_____

ResearchBlogging.orgHassett, J., Siebert, E., & Wallen, K. (2008). Sex differences in rhesus monkey toy preferences parallel those of children Hormones and Behavior, 54 (3), 359-364 DOI: 10.1016/j.yhbeh.2008.03.008.

Berenbaum, S., & Hines, M. (1992). EARLY ANDROGENS ARE RELATED TO CHILDHOOD SEX-TYPED TOY PREFERENCES Psychological Science, 3 (3), 203-206 DOI: 10.1111/j.1467-9280.1992.tb00028.x.

Kahlenberg, S., & Wrangham, R. (2010). Sex differences in chimpanzees’ use of sticks as play objects resemble those of children Current Biology, 20 (24) DOI: 10.1016/j.cub.2010.11.024.

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18 Comments

  1. “One rhesus monkey study positing a biological and hormonal basis for gender-specific play, and another chimpanzee study emphasizing social learning…”

    I don’t really see it that way, because the first study is still introducing arbitrary and artificial human meaning into objects, and placing the monkeys in a context filled with human meaning. The Wrangham study is far more agnostic; it assesses the behavior in a chimpanzee context.

    To illustrate what I mean a little more clearly, there was recently a tumblr post (http://r0bertbrowniejr.tumblr.com/post/16423006783/yesterday-my-mom-posted-a-picture-on-facebook-of) about a boy who picked out pink zebra-patterned ballet flats to wear to his first day of school. The mother’s friends excoriated her for “warping” the child, etc., so she firmly reassured the boy that it was perfectly fine for him to wear pink if he liked it. The boy replied that he liked the shoes because they were zebra-patterned; he didn’t care what color they were, and his admiring peers at school didn’t care either.

    I cite that story to illustrate that the adults were concerned with what they prioritized in their social context: the color pink. The children were effectively color-blind in this case. The rhesus monkey study may be a similar case where the human researchers are dividing toys based on wheels vs. plush, when the monkeys do not consider these to be distinctions worthy of consideration.

    Reply
    • Hi Jennifer,

      Thanks for the thoughtful comment – you raise very good points.

      You’re absolutely correct, of course, that the Wrangham study is an observational study of behavior in the wild, so there’s no danger of scientist-introduced experimental biases, etc. Also, regarding Hassett study, I too wondered about whether the scientists choices about designating certain classes of toys as either masculine or feminine (Who “decides” that boys/male monkeys like wheeled toys better? What about balls? What about little tea sets? What about toy weapons? Etc.)

      I can’t speak for the scientists and have to take some of their assertions at face value, but did want to point out that they appear to have have taken into consideration at least some of the issues you bring up. For example, as to the “masculine” label for wheeled toys, the scientists were building on prior research that had empirically shown that boys spend more time with wheeled toys (this is reflected in that first chart in my post above). So, while this doesn’t get at the mental state of how the boys perceived these toys, it at least means that the scientists weren’t selecting the “wheeled toys” category in a completely arbitrary manner. That is, they knew that human boys display a marked preference for wheeled toys and now they wanted to see if male rhesus monkeys would have a similar preference. Since (the researchers assert in the paper) there is no evidence of social stigma or pressures surrounding toy choices in rhesus monkeys, finding out whether male monkeys and boys display similar toy preferences would at least inform discussions and theories regarding the biological vs. social underpinnings of gender preferences.

      Also, there’s one part of the Hassett paper that may touch upon the issue raised by your insightful observation that the nature of the toy can be in the eye of the beholder, and where a parent may see something pink the child may see something that’s got cool zebra stripes. Although I didn’t mention it in my post, one of the “masculine” wheeled toys was actually a shopping cart, a toy that might – at least to some – carry some “feminine” connotations. Here’s a relevant excerpt from the study:

      Because we chose toys based on object properties and not on previously established sex-typed categorizations, our wheeled and plush toys are not entirely analogous to the more stereotypical categories used in the human studies or to toys typically marketed as for boys and girls. Our findings suggest that sex differences in toy preferences in humans and nonhuman primates rely to some extent on physical object properties, but that social characteristics likely also influence preference, and some of these may be unique to humans. For example, a toy such as a plastic shopping cart, one of our wheeled toys, might appeal to boys or rhesus monkey males for its physical properties, but the same shopping cart also has symbolic properties related to imaginative play, and in humans may be socially stigmatized for boys. Because the shopping cart relates to a specific human activity, the toy facilitates different activities for humans than for rhesus monkeys. However, our finding that male monkeys show a preference of comparable magnitude to those seen in boys makes a cultural devaluation explanation unlikely.

      All that said, I think the concerns you raise remain quite valid, and probably also serve to underscore how difficult it must be to design this type of experiment. Thanks again for commenting!

      Paul

      Reply
      • Jen

         /  January 26, 2012

        Hi Paul,

        That’s really interesting! I’m glad they allowed for that consideration in their study. You’re absolutely right–it’s do hard to control for observer and researcher bias. Do you think they could have had a double blind setup, where non specialist observers could assess a rubric of “preference” without being told which juvenile monkeys were male or female? I don’t have access to the paper right now, so I don’t know their specific protocol. I’m still wary of placing animals in human context or values (the same way I’m wary of attributing human ideas of success to, say, nondominant orangs who actually value sexual fitness over social dominance because of their unique strategy), but I do see what they were trying to get at here, and I appreciate that you pointed out the study trying to isolate factors by using this analogy.

        Reply
        • Hi Jen,

          If you’d like, there’s actually a free version of the Hassett paper here, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2583786/, and a free version of Wrangham here, http://www.cell.com/current-biology/fulltext/S0960-9822(10)01449-1. (I register my posts with a site called “Research Blogging,” which automatically generates the footnotes, but invariably links to pay walls!)

          They did have two observers reach consensus, but it wasn’t a double blind setup. The relevant sentence reads: “The identity of every animal interacting with the toys and specific behaviors (Table 2) directed towards the toys were coded from the videotapes by two observers working together to achieve consensus on both identity and behaviors.” Agreed that it would have good if it were double blind, although, I have to confess, I don’t know whether it’s obvious from looking at them whether a rhesus is a male or a female… :) Also, at least to my eyes, the interactions that they were recording seem relatively concrete and non-subjective in nature, so maybe bias observer bias wasn’t too big a problem.

          I’ll have to check out the orang fitness strategy you mention – sounds interesting!

          Paul

        • Oh great! Thanks so much for linking to the free ones for me! I’ll definitely give them a look. I guess I’m just biased against humans! It’s the processualist in me.

          I can’t remember the paper I’m thinking of that refers to the strategy. There’s been some debate about how social hierarchy functions in comparatively solitary orangutans, but there’s really interesting sexual dimorphism among the males particularly. Dominant orangs have cheek flanges, and unflanged males are often prevented by flanged males from mating with females. Some speculate the actual presence of the flanged orangutans will keep the other males from developing flanges, but it’s true that flanged orangs are more hostile towards other flanged males than unflanged. Naturally, it’s hard to test a lot of the theories surrounding this. Flanged males have it pretty easy–they just call out and the females come. Unflanged males, if they’re not impeded by a flanged male and if they encounter a female in heat, will force copulation. Rape isn’t unusual for primates–chimps do it all the time–but unflanged males do it in the absence of flanged males if they can.

          This is as opposed to chimps, where the non-dominant male sometimes goes off with a female in heat, hiding from the rest of the troop, and mating exclusively with that female. It’s a little weird, but the female in estrus gets to not be beaten up and raped constantly by other males who are trying to be the one with the most secure sperm plug. I can’t recall if she gets raped by him exclusively or if she chooses the times instead. (I was always a little confused why the female would fight back so viciously, given that the successful male would be supposedly the strongest, etc. though mating season does sound fairly disruptive for the female, super-obvious estrus signs aside.) Her kids may not have the best fitness depending on what kind of genes that non-dominant chimp can pass on, though, so there’s a trade-off.

          I’d like to return the gesture and link to these papers, but most of this is from memory. Wrangham’s done a lot with dominant-vs-nondominant mating strategies across primate species, and he put out an academic trade book that was unfortunately though excitingly titled Demonic Males. It’s a great, well-written book, though it’s obviously his perspective on the literature at the time. [Also I feel like I should disclose that I took quite a few classes with Wrangham as the professor, so I’m definitely biased here as far as the quality of his work!] :p

        • I should also amend my last comment because I’m using a fuzzy version of the word “rape”. To clarify, male chimps will intimidate, harass, or strike female chimps so they can copulate. Female chimps resist whenever they want, and how it plays out depends on their relative strengths. It’s a little harder and unclear when you have a group of males together, and they’re often aggressive towards both members of a copulating pair, and it all gets a little confusing. So I misused rape in the human sense, which is lazy (and insensitive of me in human societal terms) of me, I know. The pair going off and mating exclusively definitely happens (females sometimes just leave the group to go mate with other males from outside groups). Whether the pair copulate forcibly or agreeably, I don’t recall.

          Orangs definitely force copulation a lot though. Male dolphins sometimes will pair off to corral a female and mate with her for weeks, which is an even more interesting strategy when you think about that kind of cooperation.

        • All very fascinating! I knew a bit about some of this, but not at this level of detail. There are lots of cool mating strategies out there (sneaky non-dominant squid, spiders bringing offerings so they can mate rather than being eaten, etc.) – maybe a future post!

        • Looking forward to that one! I feel really bad for the male anglerfish though…and the female octopus.

          Male anglerfish are tiny and mostly testicles. They latch onto the female, injects most of himself when they detect ovulation, and then die. Scientists at first thought they were parasites and didn’t realize the extreme sexual dimorphism until later!

          Female octopodes are in the other direction. The female guards the eggs, never venturing out of her shelter, and continuously blows water over the eggs to ensure sufficient oxygen circulation. By the time the eggs hatch, the female has starved enough to be too weak to do anything else, including defending herself against predators, which is how the mothers often die.

        • I know – poor mother octopus – so smart, and such a short life!

  2. Sharon

     /  January 26, 2012

    You’re trying too hard to discredit the study, @jzgongen, because it doesn’t confirm your ideological view of the world (taken from feminist blogs).

    Reply
    • I didn’t take the comment that way at all.

      Reply
    • Hi Sharon,

      I’m not trying to discredit the study. I think there can be improvements made on this study that’s trying to suss out a very complicated question. Discrediting any study is really only necessary when there are questionable ethics involved (data forgery, data theft, etc.).

      I’m not sure why my original comment inspired you to assume that I have a feminist ideological view of the world, let alone one taken from feminist blogs, particularly because I am very wary of Third Wave feminists and “grrls” because of the uneven quality of their argumentation and the battles that they do choose to pick.

      Reply
  3. Sharon

     /  January 26, 2012

    “Do you think they could have had a double blind setup, where non specialist observers could assess a rubric of “preference” without being told which juvenile monkeys were male or female?”

    Bringing in lay people to administer this scientific experiment on animal behavior? Seriously?

    Reply
    • Non-specialist does not necessarily mean lay people, or nonscientific. I specified “non-specialist” because a researcher who spends all day looking at rhesus monkeys might easily sex juvenile monkeys at a glance. I know I certainly could do so with other species. I don’t think I could do the same for rhesus monkeys. Surely colleagues or other scientists could participate in a standard rubric evaluation?

      Non-specialist observers are observers, and not administering the experiment. Hopefully you don’t allow your test subjects or participants to administer your experiments! ;)

      Reply
  4. I think the thing that jumped out at me about the results (graphs) of the first study is just how similar we are to our animal counterparts. Think we could all do to remember that :)

    Reply
  5. One question came to mind on reading this – and al the comments. This presumes the existence of only 2 separate and identifiable genders, when was this absolutism in chromosomal variation categorised? Did I miss the memo?

    Male or female is really not synonymous with masculine or feminine.
    And what of all the stops in between?
    That one section of a breed of animal carries sticks like babies surely cannot be extrapolated outwards to support the narrowing of criteria defined behavioural practices in an altogether separate species.

    Interesting but written to support one view from limited evidence, in my opinion, which is a different view from yours I feel. Interesting nonetheless.

    Reply

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