Leakey Foundation scientist: orangutan bones tell us that biological sex is a spectrum, not a binary

June 19, 2023 • 11:00 am

Any YouTube video with the title “Orangutan skeltons bust the sex binary” is guaranteed to draw me in, and the very title makes me wary. How can skeletons bust the sex binary of any mammal, given that the definition of sex in mammals involves features (reproductive systems evolved to produce two very different kinds of gametes) that can’t be seen in a skeleton? Sure, you can often identify skeletons (especially of sexually dimorphic species like humans) by various bone traits, like hip-and-leg configuration, but such dimorphism is not 100% diagnostic, nor, more important, the definition of the sex binary.

What Alexandra Kralick has done in the video below, released three days ago,  is show that in orangutans, which have two morphs of males in the wild, give a more or less continuous distribution of bone-size measurements, since one morph of males is intermediate in size between males and females. But a continuous distribution of bone sizes does NOT “bust the sex binary.” What it does do is “break the bone binary”, but that says absolutely nothing about whether sex itself is binary. It’s like showing that the distribution of human heights is not binary and therefore human sexes are not binary!

The video is not only misguided, but is also an embarrassment to the Leakey Foundation, named after Louis Leakey, which sponsors research on human origins and evolution. The Foundation is, in fact, putting its imprimatur on work that purports to show that, in primates (and presumably in humans, since Kralick generalizes her results beyond orangs), sex is not binary. I doubt the Foundation would really agree with that, unless they’re terminally woke (and anti-science).

At any rate, here’s the YouTube introduction to the 48-minute video, which includes Q&A at the end.

Meet Leakey Foundation scientist Alexandra Kralick and learn how orangutan skeletons bust the sex binary in this rebroadcast episode.

Kralick declares that the theme of her work is “casting light on problematic assumptions that permeate scientific narratives of biological sex,” and adds that she’s bringing “a feminist and queer approach to this work to show how biological sex is more complicated than either ‘male’ or ‘female’ but in fact sits on a spectrum.”

One might sense that there’s an ideological motivation behind this work and its conclusion, a motivation that leads to misleading conclusions. One would be right.

Click to listen:

I’ve taken a few screenshots of the slides. The one below argues that “biological sex is not dichotomous” (i.e. “binary”), but she leaves out the one trait that defines sex and shows that it is binary: the reproductive system and the gametes that it makes. To repeat myself, males have a reproductive system evolved to produce small, mobile gametes (sperm), while females have a system evolved to produce large, immobile gametes (eggs). There is no third type of reproductive system, and no other type of gamete. This is the definition of biological sex, not the traits listed on the slide. (Note that the slide depicts humans, not orangutans, showing that Kralick is making a general statement, not one limited to orangs (which of course also fit the sex binary).

In the wild (though Kralick says “not in zoos”), the male orangs have two morphs, “flanged”, with big cheek pads, big vocal sacs, and large body size; and “unflanged”, males who are smaller, though not as small as females, and also lack vocal sacs and don’t have big cheek pads.  Flanged males are behaviorally and sexually dominant over unflanged males.  The two classes of males, of course, are still both male, for both can produce sperm and mate with egg-producing females (the unflanged males in nature may be “female mimics” that deceive females to get mates, but we don’t know).  There are three species of orangs (genus Pongo) that live in different places, and all three have the male size/feature dimorphism. This is what Wikipedia says about them:

Males become sexually mature at around age 15. They may exhibit arrested development by not developing the distinctive cheek pads, pronounced throat pouches, long fur, or long calls until a resident dominant male is absent. The transformation from unflanged to flanged can occur quickly. Flanged males attract females in oestrous with their characteristic long calls, which may also suppress development in younger males.

Unflanged males wander widely in search of oestrous females and upon finding one, will force copulation on her, the occurrence of which is unusually high among mammals. Females prefer to mate with the fitter flanged males, forming pairs with them and benefiting from their protection. Non-ovulating females do not usually resist copulation with unflanged males, as the chance of conception is low.Homosexual behaviour has been recorded in the context of both affiliative and aggressive interactions.

This suggests that the unflanged males are indeed either female mimics or males that are non-dominant or of lower fertility, and have to use force to get offspring. Regardless, they’re still called “males”: they are clearly not a third sex!

Here’s a picture of the two morphs of males taken from Wikipedia. First flanged, then unflanged; the differences in vocal sacs and cheek pads are clear. The unflanged male was at the San Diego Zoo, though Kralick says that unflanged males can’t be found in zoos (27:11).

Flanged male
Unflanged male

Below is a plot of the “long bone length” of adult (A) and juvenile (J) orangs: flanged male (FL), unflanged male (UFL), and female (F). Note that in each of the five long bones, the unflanged males are intermediate in length between big males and smaller females. Importantly, note that the orangs are classified as either MALE or FEMALE, which Kralick apparently got from the museum specimens that she studied. (The sample sizes were quite small: there were, for example, only three specimens of unflanged males.)

Here are data for the ulna (arm bone), again clearly showing the intermediacy of the unflanged MALES between flanged MALES and normal FEMALES.  But if sex is not a binary, how did they identify the specimens? No doubt the collectors used the presence of a penis or vagina (almost 100% correlated with biological sex in humans) as the identifer of males vs. females, though presence or size of penis is not the definition of “a male”.

You get the same kind of distribution from cross-sectional area of the long bones. Kralick says “These are the results that support the notion that biological sex is a spectrum.” (32:44).

Now where does all this come from? Whence the conflation of sexual dimorphism with the definition and dichotomy of sex itself? You might have already guessed based on the lecture and the fact that at the beginning it’s announced that the Leakey Foundation is collaborating on this with “the American Association for Biological Anthropologists LGBTQIAA group.”

Below is a slide from the talk asserting that these data “subvert the idea of the sex binary as natural and biological, thus altering the discourse that places value on biological causes for gendered social order.” Well, I get the part about subverting the sex binary, but to say that the sex binary is not “natural and biological” is simply wrong.  You know this from the immense irony that although Kralick finds a “spectrum” using bone dimensions, she sees this only by dividing the data by sex: males (two types) and females.  Here she recognizes that there are two classes, and not an intersex. As for its connection with gender, it’s opaque.

Note, though that in the talk Kralick does quote the outdated figure that “17 our of 1000 babies born are intersex” (1.7%), “the same proportion of individuals with red hair”. The intersex figure came from a flawed claim by Anne Fausto-Sterling in 2000, who used a very wonky way to decide who was “intersex.”  (Fausto-Sterling later retracted this claim, but you still see it everywhere).

In fact, developmental variants are very rare, constituting only about one in 5,600 people (0.018 percent), and also don’t represent “other sexes.” This is the same proportion of the time that a tossed nickel will land on its edge rather than on “heads” or “tails”, yet nobody thinks that the results of coin-tossing are “non-binary.” Further, just like coins that land on edge are neither heads nor tails, so an intersex individual, rare as they are, are not “other sexes”, but the results of development gone awry. After all, natural selection has created two endpoints for animal sexes, male and female, though the developmental paths to those endpoints can involve chromosomes, temperature (in turtles), the social environment (yes, clownfish) , or genes. Indeed, the paths are many but there are only two endpoints (males and females); and a deviation from those two goals represents a very rare straying from the path.

Towards the end, Kralick says explicitly that one of her aims is to deconstruct the narrative that unflanged males are “deviant,” which I guess a couple of anthropologists have said (but I bet few would say that today!). The language of “deconstructing”, “subverting”, “undermining”, and “busting” supposedly conventional science is straight out of postmodern discourse.

This slide, which conflates sex and gender again, supposedly gives us lessons about humans, the only species, Kralick asserts, that do have gender.

The goal in this research is apparently to read into nature the spectrum of gender we see in human society, another example of what I call the “reverse naturalistic fallacy”—the idea that “what we consider good in humans must be observed in nature.” It’s a logically unsound way to valorize human identities. The double irony, though, is that Kralick not only affirms the sex binary, or at least accepts it, but also studies something in orangs that has nothing to do with gender.  Unflanged males are variants, apparently frequent, and may represent an evolved reproductive strategy.  Their existence raises a number of interesting and unanswered questions, but this study doesn’t go after any of them. Instead, it tells us what we already know from humans: males and females are of different but overlapping size, i.e., human height falls along a spectrum.

But human sex doesn’t.

h/t: Christopher

29 thoughts on “Leakey Foundation scientist: orangutan bones tell us that biological sex is a spectrum, not a binary

  1. Hey, nobody knows what it is like to be an Orangutan (or a bat – T. Nagel).

    Therefore, [ insert human-derived Orangutan gnosis here ]

  2. I have a cunning plan. Find a species that is not sexually dimorphic, so that males and female are the same size, then do a study of bone lengths showing that the means are the same for both sexes, and thus declare that sex doesn’t exist at all, and thus that biological sex (not just gender) must be a purely social construction!

    1. As cunning as a foxy fox that has just been appointed Professor of Cunning at Oxford University! /blackadder

      Anthropology was among the first of the soft sciences to be corrupted, and now it’s actually putrid.

  3. The goal — to subvert the idea that gender roles are biologically natural — doesn’t seem to mesh well with the idea that there are no discrete categories of male and female. On the one hand yes, if, scientifically speaking, there’s no such thing as a woman and nothing that distinguishes them from men then nobody can say that women who don’t have babies are bad. But on the other hand, we’re still talking about women and we have to anchor that concept in something . Using social expectations and innate feelings of womanhood instead of gametes seems like jumping into the fire.

    Isn’t it easier to subvert the Naturalistic Fallacy by simply pointing out that it’s a fallacy? This particular twisted attempt to find a spectrum in sex seems like a sublimated comparison of castrated human males with breast implants to unflanged orangutans on the assumption that both are “less male.”

  4. “… in the talk she does quote the outdated figure that “17 our of 1000 babies born are intersex” (1.7%), …”

    It seems clear to me this is supposed to illustrate the marginalization and exclusion of the intersex by cis-het domination, specifically with regard to knowledge.

    So by transforming society, especially in the realm of scientific knowledge (i.e. the source of the power to dominate), society will eventually become completely intersex, with bimodal sex the marginalized and in need of “transitioning”.

    IOW in Utopia.

  5. showing that the means are the same for both sexes

    Please, if you’re going to be comparing distributions by their easily-computed descriptive statistics, you need to compare at least mean, variance, skewness and kurtosis. Or go for a full probability distribution comparison.
    This might be the internet, but that’s no reason to lower your statistical standards.

    1. Huh? The conventional t-test or ANOVA of course uses variances in testing for differences in means.

      1. That was a reply to Coel, talking about comparing means. Using the other descriptive statistics of a distribution reduces the chance of comparing (say) a Poisson distribution with a hypergeometric.
        ANOVA was designed about normal (Gaussian) distributions. So if your actual distribution isn’t normal, you need to do something to convert your raw data into a metric which you can justify asserting that it is more-or-less normally distributed. Such as grouping your raw data into interval averages.

  6. Umm, isn’t variation kinda like the underpinning of evolutionary theory? Evolution isn’t going to occur if all individuals are going to be exactly the same for all time. Individual variation is how we get dimorphism in species and why the degree of dimorphism varies in species. I’ve been arguing all along that the reason we have all these so-called “genders” among humans is simply an expected result of a diverse population that isn’t always partnering according to strict stereotypes, and that our mating habits have changed over time from our presumably more polygamous ancestors.

  7. Why is she a “Leakey Foundation” scientist?

    Seems more accurate to call her a University of Penn PhD candidate.

    How have things gotten this far gone?

  8. The Foundation is, in fact, putting its imprimatur on work that purports to show that, in primates […] sex is not binary.

    This could get the Leakey Foundation into significant trouble, if any East African politician latches onto their support for a not-exclusively straight model of humanity.
    East Africa is not a safe place to be anything other than “straight”, and the Leakey name has a long history of political difficulties already in the area. Nothing to do with their science, but getting site access, protecting sites (and wildlife) frequently steps on established toes in the region. Promoting ideas which are locally considered illegal and immoral is a good way to get your operations shut down.
    Oh well, it’s their bed ; they’ve made it, and someone is almost certain to use this against them. Talk about giving gifts to your political opponents.

  9. Wasn’t there a story on here (or somewhere else ; I’ve been busy elsewhere) recently about palaeontologists re-classifying specimens from a single, normally distributed “morphological species” into two (or potentially more, as this study describes) morphospecies which may (or may not) reflect sexual dimorphism (trimorphism) in a single biological population?
    People have been having this argument over dinosaur (saurischian, ornithischian, not so often avian) bones since the 1980s. Some people lump a set of specimens into one species with bimodal measurements ; other people split the same specimens and the same measurements into two species. Personally, I tend towards the lumpers (with a likelihood therefore that we’ve a number of examples of sexually dimorphic dinosaur species). But absent identification of medullary bone (a bone texture associated with rapid calcium release to form mineralised shells ; has been done for T.rex, I’m not sure of any others), assigning one of those morphospecies to male (or female) is a lot harder.
    If there were a taxonomist’s heaven, would St Peter let everybody in (lumper) or send the splitters to eternal [glory or damnation] and the lumpers to eternal [damnation or glory] ?

    1. But it’s usually disputed whether different morphotypes are different species (genera) or different ontogenetic stages (not sexes). For example, Stygimoloch and Dracorex are juvenile forms of Pachycephalosaurus, and Torosaurus may be a mature form of Triceratops.

      If you mean the study by Paul et al. published last year, the authors identified three morphotypes in T. rex that they called distinct species. Other paleontologists, including specialists in tyrannosaurids, criticized their work, saying that they presented weak evidence (see Carr et al., 2022). But there was nothing about sexual dimorphism. Morphotypes associated with SD were previously identified in T. rex, but later studies (with more careful statistical analysis) didn’t find it.

      Yes, it’s generally difficult to detect sexual dimorphism in non-avian dinosaurs. But in the paper published last week (Pintore et al.), paleontologists detected SD in a herd of ornithomimosaurs (ostrich-like dinosaurs), based on the structure of the femora, which also differs in males and females of modern archosaurs (birds and crocodilians). But it remains to be determined which were females and which were males. You mentioned medullary bone—yes, it can help with this, although it’s not a panacea, because it can only detect females that are in the egg-laying cycle, and it’s very difficult to distinguish it from pathological bone in fossils.

  10. “Unflanged males wander widely in search of oestrous females and upon finding one, will force copulation on her, the occurrence of which is unusually high among mammals.” That is what we would call rape in common language (forced copulation) as Galdikis pointed out, basically a reproductive strategy in this case.
    Since we, well she, are generalising -and extrapolating to humans (a different species, to be clear)-, and rape of biological females by ‘trans-women’ is not exactly unheard of, may we presume that Kralick contends that being a ‘trans-woman’ is a strategy by omega males to get access to biological females? On the Orangutan’s pattern? Or do I misunderstand her?
    Just to make sure: /s

    1. I wondered whether anyone would mention the “rape prone” orangutans. A quick google search pulled up the below ten-year-old gem on NPR. The lesson I draw is that when the animal kingdom does things that you do not want to see replicated in humans, then you deny that such behaviors are “natural”, or you rename them so as not to draw human parallels.

      On the other hand, if trans apes or promiscuous female birds are what you like, then “natural” it is . . . and wholesome and good or something.

      Remember, “truth” is what gets you whatever it is that you want. And if you want something different tomorrow? That’s okay; truth is fluid that way.


    2. Oh, wait – oh wait – I do remember learning about this:

      “Several studies have found that small male fish will look and behave like the female of their species in order to gain access to female territory and copulate with them.” https://en.wikipedia.org/wiki/Sexual_mimicry

      This factoid should be the “gotcha” any time someone trots out the “clownfish” argument…

    3. When I read the part with the unflanged males, I thought “ah, so, the primate version of ‘male feminists'”… yeah, I’m getting cynical.

  11. Has no one explained why the two male forms differ? Is it inherited, or environmental? Do unflanged males develop the cheek pouches egc in certain conditions?

    Can anyone suggest a good Orang book, preferably recent?

    1. No idea. I had not heard about this thing in Orangs. But in other species ranging from arthropods –> birds there are commonly different kinds of males (and sometimes different kinds of females), where contrasting mating strategies are maintained by natural selection in their own way, and these all have a genetic basis as far as I’ve heard. For example, there is a marine crustacean that lives in sponges. Some males are large and aggressive, and they hang on to up to several sponges with a “harem” of females that they oversee and protect. That is a successful strategy held up by selection. But that results in an opening for selection of alternative genes that produce small “sneaker” males that resemble either females or juveniles. These forms don’t stir up aggression in the big males, and that allows the sneaker males slip in, copulate with some females, and skedaddle. So they too are maintained by natural selection.

  12. The expression on her face says enough. I mean no disrespect, but when someone is using a look like that when describing something I just don’t feel right.

  13. If ““a feminist and queer approach to this work” means replacing good science with ideology, then perhaps people who take that approach are unsuited to science.
    It is as if she is making an argument against diversity.

    However, the two types of males, with very different physical features, is really interesting.
    The human analog that it made me think of is a certain type of male feminist who give off sort of rapey vibes to other men, which women seem to not notice. I guess it could be considered a mating strategy. But not a different sex.

  14. So male orangutans have a couple of different strategies for getting their sperm hooked up with a females egg in super-straight sex, and therefore a spectrum of something or other?

  15. Jerry says, “One might sense that there’s an ideological motivation behind this work and its conclusion, a motivation that leads to misleading conclusions. One would be right.”

    If there’s any doubt, this from her ResearchGate bio:

    As an anthropologist trained in a four-field approach, Alexandra Kralick pushes disciplinary boundaries by engaging with queer theory to showcase how great ape skeletons defy normative expectations of biological sex and gender. She has been integrating contextual reflexivity and decolonial theory to foster conversations regarding ethics and equity in great ape skeletal research.

    In her 2019 Youtube video, she claims that XY women* with androgen insensitivity disorder (or syndrome) also refute the sex binary. This is nonsense. For one thing it is an exceedingly rare genetic disorder and cannot be argued that it represents normal variability in sex traits. It occurs in 2 – 5 per 100,000 live births (0.003%), one-sixth as common as a flipped nickel landing on its edge. For the second, affected persons produce either spermatozoa or (more usually) no gametes and lack the associated organs to conduct them to the outside world. They never produce some imagined spectral or third (non-egg, non-sperm) gamete. What she’s trying with these red-herring arguments is to conflate the phenotypic appearances displayed by sexually binary individuals (in their normal and defective development) with the definition and determination of sex itself.

    She’s messing with our minds, just as queer theory says.

    (Comments on the YouTube video are disabled.)
    * They are entitled to be called women because they look phenotypically indistinguishable through childhood from typical XX girls/women. Making the diagnosis (eventually) does not change the way they see themselves (except to discover they will be infertile) or how society sees them. Most strangers need never know. If they are elite athletes they can, and do, compete as women despite “failing” a genetic test of woman-hood. No androgen receptor -> no male puberty.

  16. It’s so sad. Ideology is invading deeper and deeper into biology, and there is no end in sight…

  17. “the American Association for Biological Anthropologists LGBTQIAA group.”

    is that satire?

    Regardless, LGBTQIAA is a good start for an adequate password. You have to add a number and a lowercase letter, however.

    Oh. Also, you have to bust a queer character into a valid password these days.


    1. There was a Gary Larson Far Side cartoon showing the police with dogs and SWAT gear handcuffing the suspects whose hideout they had just raided. One of the suspects says to the ringleader, “See? I told you ‘Shave and a haircut’. was a stupid idea for a secret knock!”

      Strong passwords indeed.

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