Sexual Dimorphism is Due to Estrogen

Not a history of dominance and competition

Many animals, including primates, have varying levels of sexual dimorphism, physical characteristics that are different between males and females that are not related to reproduction. One of the most noticeable tends to be size. For example, gorilla males are roughly twice as large as females — although female stingrays are typically a little bit larger up to three times as large as males. Human males tend to be bigger and have greater upper body strength than females, with a rate of dimorphism closer to 15%. However, around 10% of females are taller than the average male, and there are some females who are stronger than the average man as well.

A lot of things play into height, including genes and consistent access to good nutrition. “The average young adult today is around 8 or 9 cm, or about 5%, taller than their ancestors 100 years ago. Some countries saw very different changes for men and women. In South Korea, for example, mean height for women increased by 14% versus 9% for men. In the Philippines the opposite was true: male height increased by around 5% versus only 1% for women.” (source)

It’s been hypothesized that in general men tend to be taller than women because that serves an evolutionary purpose, particularly as relates to mate competition. Charles Darwin certainly thought that was the case.

“There can be little doubt that the greater size and strength of man, in comparison with woman, together with his broader shoulders, more developed muscles, rugged outline of body, his greater courage and pugnacity … have been preserved or even augmented during the long ages of man’s savagery, by the success of the strongest and boldest men, both in the general struggle for life and in their contests for wives,” he wrote in The Descent of Man. (source)

This dovetails nicely with Darwin’s other supposition that across species, males are naturally randy and promiscuous and that females are naturally coy, sexually reticent, and choosy. It’s a plausible story if you are assuming the entire natural world operates under the same sexual mores as Victorian England, but neither supposition stands up to actual scientific scrutiny. Primate females, in particular, are far from sexually reticent or choosy.

In fact, female primates couldn’t be further from reluctant breeders or seekers of “intimacy” with a single “best” mate, or dead set on doing it with “the alpha.” Indeed, (primatologist Meredith) Small suggests that it is difficult for us humans to wrap our minds around “just how little importance nonhuman female primates attach to knowing a male before they mate with him.”

Martin, Wednesday. Untrue (pp. 164–165). Little, Brown and Company. Kindle Edition.

In both animals and humans, sexual promiscuity can offer an evolutionary advantage — for both males and females. “A female who mates with several different males will have more genetically diverse offspring, boosting the chances that at least some of them will thrive.”(source) Additionally, there are times when it is evolutionarily advantageous for a male to be sexually choosy. (I’ve written several stories that delve into these myths; one is linked at the bottom of the page.)

In any case, it’s reductive to say that all human mating follows certain rules since mating strategies are highly dependent on circumstances. As one of the many groups of researchers who debunked the “Batemen Paradigm” that supposedly proved Darwin’s theory about the binary sexual nature of males and females noted, “We argue that human mating strategies are unlikely to conform to a single universal pattern.”

Additionally, humans evolved as multi-maters, not by locking horns and beating each other up for mate access like elks or gorillas. The coronal ridge on the human penis is shaped that way in order to scoop out semen left there from other genetic competitors. It wouldn’t be necessary if that competition had already taken place prior to coitus. And because you don’t want to accidentally scoop out your own semen, it’s typical for men to become flaccid after ejaculation and to need that refractory period before they can go again.

The human cervix works in much the same way that it does in some of our primate cousins, to sort and assess sperm quality. “For the human female cervix, like that of a promiscuous macaque who may breed with ten males or more in rapid succession, actually serves not so much to block sperm, as was previously believed, as to busily filter and assess it, ideally several different types of it from several different males, simultaneously. It evolved not as a simple barrier but to sort the weak and bad and incompatible sperm from the good, suggesting by its very presence that there was a need to do such a thing — i.e., that females were mating multiply.” (Martin, Wednesday. Untrue (p. 145). Little, Brown and Company. Kindle Edition.)

Besides the ways that our biology reflects that we were multiple maters, early humans wouldn’t have routinely physically fought each other for mate access — firstly because there was no need to. If everyone shares food and looks out for each other, and there are few personal possessions to give to heirs, who cares who the father of a child is? As primatologist Frans de Waal has noted, “When group stability is important for individual advantage, selection will favor active peacemaking and cooperation in our closest relatives and ourselves.” (source)

And secondly, constant fighting would have been counter-productive to the survival of the band as a whole. Life was precarious enough in the Paleolithic era, without members of your tribe or your neighbors who all depend on each other for survival risking injury or death in this way. It simply doesn’t add up.

It also stands to reason that if men evolved to be bigger and stronger than women as part of a pattern of domination and mate competition no woman would be taller or stronger than the average man. In fact, no short men would probably even exist any longer, right? But this isn’t the case so, if males didn’t primarily evolve to be generally (but not universally) larger than females for these so-called evolutionary reasons, why does it take place at all?

One word: Estrogen

Ovaries matter because they produce a lot more estrogen than testes do, and estrogen helps direct bone development. “In all human skeletons, a lot of estrogen stimulates long bone growth,” Dunsworth explained. Before puberty, people with ovaries and people with testes grow at roughly the same rate. Then those with ovaries ramp up estrogen production, which stimulates the growth plates in their bones and causes the long bones in particular to lengthen. That’s why, during early adolescence, girls are generally taller than boys.

The spike in growth isn’t long-lived, however, because high levels of the hormone make the growth plates fuse, Dunsworth explained. That is why height differs between the sexes: People with ovaries experience the growth-stopping peak in estrogen soon after puberty, “right after their ovaries start to kick in and regularly contribute to monthly cycling,” Dunsworth said. Meanwhile, the bones of people with testes continue to grow for several years until their estrogen peaks, so they end up taller. (source)

This theory about estrogen isn’t new, but it’s previously been overshadowed by the somewhat blind focus on sexual selection. Even though social anthropologists tend to assume that inter-male competition was relatively low in the Pleistocene (source), a persistent belief in pervasive male competition fits in well with relatively new to humans dominance hierarchy beliefs. There’s a vast quantity of evidence to indicate that cooperation and not competition was the key to early human evolution, but the patriarchy does love to try to justify itself with stories that paint it as timeless and inevitable.

In addition, until quite recently male bodies were considered “the default” as far as scientific study goes, and there are many, many aspects of female physiology that have been under-studied or discounted entirely. This has further compounded the belief that sexual selection and not estrogen is the cause of size dimorphism in humans. We tend to see what we want to see and to believe stories that confirm our pre-existing biases.

Not only have doctors, scientists and researchers mostly been men, but most of the cells, animals and humans studied in medical science have also been male: most of the advances we have seen in medicine have come from the study of male biology. Dr Janine Austin Clayton, an associate director for women’s health research at the United States National Institutes of Health (NIH), told the New York Times that the result is: “We literally know less about every aspect of female biology compared to male biology.” (source)

But this is starting to change. Things related to physiology, endocrinology, and bone growth are all evolutionary factors as well, not just social conditions. When we look at why humans experience puberty when they do, it gives us a deeper look into not only how but why estrogen has impacted average height differences.

In essentially all other species including the great apes, puberty occurs as growth is tailing off. While there may be some weight gain especially in males, there is no significant change in skeletal size in other primates at puberty (Bogin, 1999). It has been suggested that the pubertal growth spurt is a form of catch-up growth which is delayed until after brain growth is largely completed: this would serve to protect the brain in the competition for nutrients and growth to optimal size. (source)

In other words, because humans continue growing, even after the onset of puberty in a way that is not typical of other primates, the large increases in estrogen in the bodies of those with ovaries leads first to a growth spurt and then to a fusing of the growth plates that males do not experience due to significantly lower levels of estrogen. It’s a byproduct of other evolutionary mechanisms, not an indication of aggressive sexual selection processes — something that seems to have been assumed without actually consulting any social anthropologists who specialize in pre-history simply because it makes sense in the context of modern social dynamics. And because until fairly recently, science rarely considered women in anything but a passing way.

“We need much more evidence than just the existence of body size differences in and of themselves,” said Louise Barrett, an evolutionary anthropologist at the University of Lethbridge who co-authored a 2016 review paper on the evolution of human height variation with Gert Stulp of the London School of Hygiene and Tropical Medicine. The sexual selection hypothesis certainly has an intuitive appeal — scientists reasoned that “men are bigger than women, and men like to fight a lot, so those two things must be connected,” said Barrett — and it lined up well with popular cultural conceptions of masculinity and femininity. “The thing is, the studies aren’t particularly good.”

“We need better or rigorous tests of evolutionary hypotheses generally, and particularly when you’re dealing with humans,” said Barrett. “The picture we have of ourselves influences how we behave and act. It’s too important to have an inaccurate picture.” (source)

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