Daniel Bolnick wrote a thread on Bluesky about the biology of sex that I thought would make a great blog post. So I saved him the trouble and compiled it.
This has been lightly edited from the original thread.
Well, no surprise that (Jerry) Coyne used his blog (whyevolutionistrue) to argue with the SSE/SSB/ASN letter about definitions of sex. I’m not going to engage deeply with the letter here. Like any committee-drafted text it reflects compromises; I agree with some phrasing but some could have been clearer.
Instead, I want to respond specifically about his comments regarding my work on stickleback. He points out, quite correctly, that I have personally used the terms male and female, and sex, extensively in my own research on stickleback. True.
But it misses the point.
Yes, stickleback can generally be categorized as male and female. And they have a XX female / XY male sex determination system (in some species, not in others). And yes, when I run stats I often check for effects of sex, and often find them. I’ve had whole papers devoted to sexual dimorphism. But Jerry might want to talk to me (or another sticklebacker) before making claims about stickleback sex differences. Because its not as clear cut as he likes.
Yes, the large majority of stickleback I’ve caught are phenotypically male or female, in the sense that they have ovaries or testes. But I have caught quite a few stickleback (hundreds at least) with neither identifiable ovaries nor testes. I have twice seen stickleback with both (yeah, we dissect a lot of fish). These are rare, but they exist. In statistical analyses, these fish get coded as NA, and excluded.
So when I publish on sexual dimorphism, or use male/female as a factor in my stats (which I do a lot), I am simplifying, setting aside a small minority to focus on overall tendencies of the majority.
That’s okay for stickleback, but its not ethical when applied to humans.
I could readily have published my papers instead with three levels of “sex”: male, female, and “unknown,” with the third group being comparatively small and under-powered for statistical purposes given my sample sizes.
Then there’s the issue of trait differences between the sexes, which are clear-cut for most individuals in some environments, but less so in other environments. Stickleback are famous for males having bright red chins and blue eyes as a sexual signal... except when they don’t. There are plenty of lakes where I work in BC, in which males are dull brownish-green, and indistinguishable visually from females. Indeed, in some lakes we have a hard time generating in vitro crosses because we’ll grab a fish we think is male, only to find after we euthanize and dissect it to obtain sperm, that it had ovaries.
For many traits that we have measured (diet, isotopes, morphology, size, color, immune traits, gene expression, metabolomics), males and females are on average different. But for most of these traits the sexes distributions overlap extensively. The magnitude of that dimorphism varies. In some lakes it can be easy to distinguish the sexes (for most individuals), other lakes it can be very hard, even in multivariate trait space. We can measure this dimorphism using things like linear discriminant function analysis (LDA). This LDA can score individuals as to how distinctively male or female they are and it varies numerically in ways we can measure precisely. Some populations are more dimorphic, others less. These differences persist in the lab, meaning there is genetic variation in between-sex differences.
To conclude: yes, I use the terms male and female in my work, reflecting real features of the fish I study. But I also set aside individuals who don’t fit neatly into the categories; again, that's a fair simplification when working with fish, but not morally defensible with people.
I also emphasize that even after 25 years studying stickleback, I can mis-identify an individual’s sex, even when the gonads are unambiguous. In these cases the gametic sex is a real distinction, but its relation to other traits is complex.
I'm not personally a deep expert on the biology of sex, that's not my specialty. But, if Coyne wants to have a conversation about sex and sexual traits in stickleback, I'd be happy to point out the subtleties. Most of all, the subtlety that seems to elude him is we can recognize male/female differences that characterize many individuals, and use this distinction, yet still acknowledge that these differences do not apply completely to fully 100% of a population.
Anisogamy (Differences in size of gametes - ZF) will surely come up in comments. I don’t have a problem with this criterion as a broad rule, but again there exist exceptions that make it less than universal. Individuals exist that produce no gametes: genetic sterility, environmentally induced sterility, parasitic castration. For instance, the parasite I study, Schistocephalus, can completely sterilize its host in some cases: we find males with no functional testes, females with non-functional ovaries. Do they cease to be males/females because they don’t have any gametes (whether large or small)?
To wrap up at last: my impression as an empiricist specializing on a particular fish species: stickleback sex is clearly delineated most of the time (hence, I use the term a lot), but not in every single instance.
The latter is where Jerry misses the point: Once we pivot into the world of humans, the biology of sex in stickleback (and how I simplify it slightly in my work) is irrelevant to the ethical standards by which we treat each other.
Update, 21 February 2025: Nathan Lents provides another great thread on the biology of sex. (Lightly edited.)
Biological sex and sex differences are real, but poorly captured by a simple binary.
No one is saying there is no such thing as “male” and “female.” (Of course there is.)
No one is saying there aren’t difference between “male” and “female.” (Of course there are.)
No one is saying that “male” and “female” aren’t real categories. (Of course they are.)
We’re saying that these categories are continuous. Continuous categories are still categories. There are many examples of continuous categories in biology, but the simplest is developmental age. Child and adult are real categories, with real and important differences. A child and an adult are different and must be treated as such. But there is no hard line we can draw between those two categories. It’s a spectrum, and a gradual transition. They are continuous categories. But that doesn’t mean the categories aren’t real and important.
There are borderline cases where it’s not simple to say which category someone belongs in – child or adult – but that doesn’t mean the categories are meaningless. All of adolescence could be seen as fuzzy gray area. Child and adult are still different.
Dawkins and Coyne’s “gotcha” moments of finding authors using the term “male” and “female” in their publications isn't the flex you think it is. They (and you!) probably use other continuous categories like adult, juvenile, adolescent. Continuous categories are still categories.
The same is true for speciation, of course. There wasn’t a Homo heidelbergensis woman that gave birth to a H. sapiens baby. (See? I used “woman” and no one will take issue!) The transition between species is gradual and continuous, a spectrum, but species are still different.
And this is what we’re saying with biological sex. The categories are not as easy to define as we thought because almost all sex differences are bimodal, not binary. Except for the gametes (I’ll get to them in a sec), pretty much all sex differences are continuous.
Examples:
Height, muscle mass, hormone levels & ratios, red blood cell count, white blood cell count, basal metabolic rate, bone density, thickness of the corpus collosum, ratio of white matter and gray matter, size of pre-optic area, breast size, facial hair, waist-hip ratio, libido… These are all features of sexed bodies that show continuous bimodal sex differences. Don’t all of those matter a whole lot more to health, daily life, relationships, than what gamete you make? Yes, gametes are binary, but they also matter very little most of the time.
There are important health differences. Women are nine times more likely to get lupus. Men two and a half times more likely to get Parkinson’s. Women suffer depression more. Men die by suicide more. Women more often anemic. Men more heart attacks. These are important, but have nothing to do with gametes. There are two liver diseases that are, bizarrely, reciprocally bimodal: Women are nine times more likely to get primary biliary cirrhosis cases, while men are nine times more likely to develop primary sclerosing cholangitis.
Recognizing that biological sex is complex doesn’t deny sex differences, it affirms them. I understand why some want to center the gametes – they are simple and binary. But that ignores all kinds of important sex differences all over the body that actually matter. In my view, the fact that people are retreating to gametes to define sex shows that they know that all of the other sex differences are continuous / bimodal.
Men and women are real categories with real differences, but the categories are continuous and differences are bimodal.
And I haven’t even gotten to intersex people, who are real people that exist. They are almost always healthy, usually fertile, and in any case deserving of dignity and respect. They, too, cannot be ignored or discarded, simply out of preference for nice neat categories.
Biology is messy, adaptable, and endlessly creative. It rarely traffics in binaries and loves wondrous variety. Life will always frustrate the human desire for neat categories with simple, universal definitions. Diversity is a universal feature of life. Let's acknowledge celebrate that!
Photo by Jason Ching/University of Washington on Flickr. Used under a Creative Commons license.
