Thirty-two years ago I wrote “The Five Sexes”, a critique of the medical treatments of children born with intersex conditions. Not at all accidentally, the essay also took a hard look at the idea that there are only two biological sexes. That essay has traveled far and in many languages. Many people loved it for opening their eyes to the beautiful complexity of human development.
Others were outraged. I think it was denounced by the Vatican, but that happened so long ago that my memory is a bit hazy. More recently, in heated exchanges on platforms such as formerly known as twitter, correspondents accused me of lying, of ruining lives, refusing to accept facts and of being ignorant of biology, PhD in developmental genetics notwithstanding. Many pointed to the world of non-human animals, especially other vertebrates (since few find the world of aphid sex to be as much fun as I do1), as proof that there are two—and only two—sexes.
And of course, now the President of the United States has decreed that there are only two sexes and that the entired nation must be organized on that principle. Non-binary existence: forbidden. MAGA gender ideology to the fore.
But what are we going to do about the Ruffs, a medium-sized shore bird that nests in the arctic tundra? As you might expect, there are female ruffs. They have the brown and white speckled camofluage frequently sported by ground-nesting shorebirds—coloration that is hard to spot in the sand or brown vegetation. So that’s one sex. Then there is a male, called an Independent (a.k.a. Indy). He is the largest ruff and comes in spectacular, intricately-patterned breeding plumage. During mating season he and other males gather, strut their stuff, and—figuratively speaking—butt heads while females watch and decide who to mate with. The Independent, which makes up from 80-95% of the males, is sex #2.
The Satellite males (a.k.a. Sat), who make up from 5 to 20% of the males, also sport the eponymous ruff, but they are a bit smaller, less colorful, and less feisty than the Independents. They also occupy the multimale playing field and show off their feathers, helping to attract females. Occasionally they get to mate with one. They are sex #3.
Finally, there are the rare (maybe 1%) Faeder (a.k.a. Faed) males—so-called after Ye Olde English word for father. They are sex #4. They look a lot like the females and sneak onto the mating field unnoticed by the larger, more aggressive males. They may not fight, but boy do they have balls! In fact their testes are twice as large as the Independents’. They are what field biologists informally call “sneaky fuckers”. They blend in while the Indy guy courts a female. When Indy’s efforts get her all fired up, the Faeder dashes in and does the deed before the Independent knows what hit him (so to speak).2 The sneaky fucker tactic, by the way, in which an unobtrusive male mates with a female on the Q.T., is reasonably common in the animal world, and often involves a different look and a different behavior pattern.
How can it be that one species has three males that look and behave so differently from one another? During the last decade+ scientists who study this bird have laid it all at the feet of a particular supergene. Each of the male ruffs have a different version of the supergene. A single gene variant involves a small change in a single base pair in the DNA. Changes of this sort end up leading to a white instead of a red fruit fly eye, or lactose in/tolerance in humans. But a supergene is a whole chunk of DNA that contains many of those simple genes and can dramatically influence physical and behavioral development. There are several “in the weeds” questions about how supergenes evolve and anybody with an interest can read up on the ruff supergene in the references in notes 2 and 3 of this essay.3
Here I want to tell you a bit more about the relationship between a few of the regular genes housed in the Indy, Sat and Faed supergenes and steroid hormones—mostly testosterone (T) and its precursor, androstenenedione (A4). Our three males parcel out testosterone and A4 to their testes and circulating blood. Physiologists assume that hormones circulating in the blood can reach the brain and affect behavior4, while testicular hormones affect sperm production. The big picture point, to which I also return at the end, is that there is a lot of room for physiological variation in the sexual development of vertebrates. And it is all perfectly normal.
Hormones and behaviors: it gets complicated
First a little more about where each kind ofmale hoards his testosterone and his A4. The testes depend on the right mix of hormones for overall growth and for healthy sperm cell development. Hormones in the circulating blood affect the rest of the body, including the brain (although see note 4 below), which influences behavior. The testes of Mr. Faed and Mr. Sat have 7-8 times more testosterone than Mr. Indy and 20 times more A4, and, as I already mentioned, bigger balls to show for it. But what about T and A4 circulating in the blood stream? Here the Independent excels with high levels of T, although he doesn’t have much in the way of A4. At the same time the Satellites and Faeders have almost no circulating T but loads of A4.5
So, to summarize: Indys have low T in the testes and high T in the blood but not much in the way of androstenenedione. Sats and Faeds have high T in the testes, low T in the blood and are replete with A4 in both tissues. To figure out how these hormone patterns might correlate with mating season behaviors, researchers rated aggression (chase, peck, forward, and fight) and courtship (full squat, mutual squat, circle, half-circle, and mount) in videos of the three kinds of male ruffs .
The results are pretty interesting, and also leave us with some standing puzzles. Only the Independents showed aggressive behaviors, and they did so at the rate of 0.6 behaviors per minute. In addition to aggressing, the Indys courted at the rate of 0.2 behaviors each minute. But the Satellites courted even more, logging in at a rate of about 0.4 behaviors per minute. In contrast to both the Indys and Sats, the Faeders neither courted nor aggressed. Clearly, high T/ low A4 is associated with aggressive bird actions. Courtship, however, is more complicated. High T/low A4 (the Indys) correlates with some courtship behaviors while Low T/High AF is matched with high courtship behaviors for the Satellites but none at all for the Faeders. I don’t have an explanation for this difference in courtship between Sats and Faeds. Nor it seems, at least for now, do the researchers in question. (Researchers have their own frustratingly common behavior. If something doesn’t make sense, they simply don’t mention it, which left me spending hours trying to figure out what I might be missing. Grrr.) In an earlier paper they did worry about it and suggested some future lines of research that might solve the problem.
A little biochemistry can go a long way
But in the 2025 paper they focused more on a related question: how do the different males manage to set different levels of T and A4 in different tissues? To address this problem we need to return to the supergenes and ask how they regulate hormone synthesis.
I start with some basics: how do ruffs (and humans) make testosterone (and A4). Neither are direct gene products. There is no gene for testosterone. Rather, these two steroid molecules are assembled, step by step from a starter molecule familiar to most of us—cholesterol. We get cholesterol from our diet, and enzymes (proteins that facilitate a chemical reaction) help transform cholesterol into androstenendione and hence to testosterone. The image below is a simplified map of steps in this transformation. Here’s where the supergene comes in. Genes code for proteins, many of which are enzymes. And the supergene variants associated with the different ruff males code for a passel of enzymes each facilitating a step that turns cholesterol into testosterone.
The above (simplified) diagram names 7 of these (e.g. CYP17A1 and HSD17B3), coded for by 7 simple genes that are located within the supergenes. These 7 enzymes govern most of the steps taking cholesterol to testosterone.
The higher T in the Satellite and Faeder testis probably results from higher concentrations of three enzymes in the early part of the T-synthetic pathway, the steps that start the transformation of cholesterol to testosterone. But one enzyme—HSD17B2 (B2 for short)— at the very end of the pathway from cholesterol to T is the most likely culprit for the higher levels of circulating T in the Indy blood stream. B2 takes testosterone and turns it back into T’s precursor—A4. Because of the structure of the Indy supergene, B2 gene expression is negligible, so once they make testosterone they get to keep it. In contrast the structure of the Stat and Faed supergenes, the B2 gene is “highly expressed”. This means that even though their blood cells make T, the B2 enzyme converts it back to androstenenedione. The result: low concentrations of circulating T in Sats and Faeds.
Some Ruff life lessons
Living life—successfully accomplishing daily tasks such as resting, eating, relating to others and longer-term ones such as reproducing—is complicated. Problems arise—where shall I find food, or a comfortable spot to sleep, or can I (or do I want to) find a life companion? How can I raise myself from a sitting position, or lower myself to the floor from a standing one? For each problem there are many possible solutions. In this substack I focused on one big issue for ruff males—how do they manage to woo and mate with a female?
As a species , the ruff deploys three different approaches to mating. Each of the three approaches takes advantage of the huge flexibility inherent in biological organisms. To regulate hormones, each kind of male uses biochemical approaches to regulating hormone levels in different tissues. As a result, during mating season, each kind of male express different behaviors. Some males are found more commonly, others less. But each represents normal variation within the ruff population.
I don’t think humans are any different in principle. In fact we are way more complex than ruffs, and we have open to us a far wider range of possible behaviors, self presentations, likes and dislikes. Given what we know—what the ruffs tell us— about biological complexity and diversity within species even down to the level of individual genes, the idea that our best self-description is that there are two and only two sexes, types of desire, or “normal” physical appearances is—at the very least—improbable.
Brain cells also make hormones on site. Researchers are still figuring out the direct effect of gene variants acting directly in the brain on Ruff male behavior.
For graphs of these results see Figure 4A for Testosterone and A4 in the testes and Figure 1B for circulating T and A4 in the blood stream
A public intellectual in the best, most relatable sense!
Food for thought and weapons against mis-information.
Both informative and fun to read. Thank you!