Estrogen – Part 2

How estrogen shapes various female characteristics.

This essay continues our exploration of the role of estrogen in the development of female sex traits through Cat Bohannon’s book Eve: How the Female Body Drove 200 Million Years of Human Evolution.

As Bohannon explains, estrogen has various effects on the hearing mechanism of female humans (which may, in some small way, explain communication issues between the two sexes):

We’re primates, too, which means we evolved from creatures that adapted to live in trees—most especially the terminal branches, where Purgi [the name Bohannon gives to the early human ancestor of female primates] and her kind needed a gift for acrobatics to be able to eat, along with a sensory array that could handle this new environment. We needed eyes that could see when fruits were ripening and distinguish when leaves were young and nutritious and tender. We needed ears that could hear our children in a loud, leafy landscape high above the ground. And while we wouldn’t use them nearly as much to find food as our foremothers did—a sweet fruit’s scent doesn’t always travel far—we needed noses that could handle a sex life in the canopy. Adapting to those needs changed our sensory array. But was it different for males and females? And if so, is it still different for humans today? … Primates are able to hear much lower frequencies than many other mammals. And the best theory going for why we can is our move into the forest canopy. It’s actually a physics problem: when you’re at ground level, you can bounce your sound waves off the earth, doubling your signal strength. When you’re in the trees, the ground is too far away to amplify your vocalizations. But that’s not the only problem that resulted from our Eves’ relocation to the trees. If I were to yell at you from across an empty room, you wouldn’t have any problem hearing me. But if the room were full of junk, you’d have a harder time. That’s because not only has the path between my mouth and your ear been obscured, but the stuff between us also absorbs some of the energy of my sound waves. Now add dozens of others yelling just as loud as me. That, dear friends, is the forest canopy: leaves, fruits, branches, moss, trunks, and many other screaming bodies between you and the ear you’re trying to reach. Animals generally adapt to a soundscape in one of two ways: they tweak their pitch range, or they boost their volume. Primates did both: they evolved to both hear and produce lower pitches, and they found ways to get louder. By lowering the pitch, they automatically gave themselves more distance, since the lower the pitch of a sound, the longer the sound wave, and the longer the wave, the farther it travels [note: this is because lower wavelengths transmit less energy to the surrounding environment, and tend to bend more easily as they travel] … [A]s their lifestyles changed, our tree-borne Eves needed those lower frequencies to cut through the sonic clutter. Up in the canopy, the ears of Purgi and her fellow Eves became specially tuned to the pitches that traveled best over the crowded, leafy distances that mattered. Modern human ears inherited those changes—many primates living today have them, in fact. We’re able to produce and hear sounds at greater decibels and lower pitches than is typical for animals of the same size … Among primates, females and males have slightly different hearing. That might be because the males don’t need to hear everything the females need to. It’s not that they have different ears—like a hi-def stereo, the equipment is largely the same. Rather, the tuning is a bit different, and that’s still true for men and women today. From what physiology labs have been able to determine, men’s and women’s ears respond differently to different pitches. Female-typical ears seem to be specially tuned to the range of frequencies that correspond to baby cries. Both men and women can hear and differentiate between noises in a certain range of pitches. Most can hear both bass notes and the high end of a violin. But generally speaking, men’s ears seem to be better tuned to lower pitches, while women’s ears are more sensitive to higher pitches—usually those above 2 kHz. That just so happens to correspond to the standard pitch of a baby’s cries. Now, if you’re a female primate, there are obvious evolutionary advantages to being able to hear your baby well. So, while the entire primate line might have shifted the bottom end of their hearing downward—presumably to correspond to long-distance, low-band communication through the forest canopy—being the primary caretakers, females would particularly need to retain their ability to hear their higher-pitched offspring … [F]emale-typical hearing became tuned to these higher pitches. Most women can hear them better than men even in noisy places. And while typical masculine ears tend to lose their higher range as they age, women’s ears are better at hanging on to those pitches. Importantly, our better ability to hear the very upper end of the human register is also tied to hardwired emotional response: baby cries alarm women more than men. It’s not that men can’t hear the kid crying, but that for many adult men, their ears snip off the upper end. Middle-aged and older men also have more trouble following a conversation in a crowded soundscape, especially if it involves a lot of higher-pitched sibilants. That also means they have difficulty hearing women’s voices, with their characteristic higher pitches, but retain the ability to hear men’s voices and other low, rumbly things.

Bohannon then explains how estrogen directs female noses to become chemical detectors of a particular sort:

Long before we could see, before we could hear, before we could feel anything at all, we could smell and taste. This is olfaction: our ability to sense chemical gradients. From the very dawn of life, single-celled animals needed to be able to distinguish chemicals in the water around them and sense their concentration. Are we getting nearer to food? Is that toxin getting farther away? The more mobile we became, the more important it was to be able to track the various chemicals in our environment. But our single-celled ancestors didn’t have sex to reproduce. We do. Once sex happened, male and female olfaction started to diverge, with each species’“nose” (or olfactory organ of whatever sort) tailored to the sex-specific needs of its carrier … Our hearing and sight sensors don’t require as much room in our heads as our olfactory system, which takes up a good third of the volume of our faces. Because olfaction involves molecules rather than waves of light or sound, and there are millions of different molecules in the air we breathe, being able to smell something requires a big, wet, warm surface area lined with sensors … [T]here are roughly four hundred known receptors in the human nasal tract, and roughly a thousand known genes for odor receptors in mammals, though the majority aren’t functional in the human body. Even setting aside the nonfunctional ones, these genes constitute as much as 2 percent of the mammalian genome—a truly massive number. So what do they build? Essentially, a bunch of receptors shaped a bit like catcher’s mitts … But each gene for an odor receptor builds one type of catcher’s mitt, and each mitt binds to only one molecule of the right size and shape … [M]en’s noses aren’t as good at that kind of granularity. Both women and men have those four hundred receptors, but women live in a more particular olfactory world … It’s one of those things everyone who works in human olfaction simply accepts: a woman’s sense of smell is more sensitive than a man’s. Women are better at detecting faint scents, telling the difference between different sorts of scents, and, once they catch a whiff, correctly identifying what it is. Though you can find some of these differences in newborn baby girls, it’s especially true of grown women around ovulation and pregnancy, and lessens in women after menopause … [J]ust as olfaction originally evolved to sniff out sex, food, and danger, most evolutionary theories for the feminine nose still fall under those three categories … Human puberty builds a bigger nose in boys in order to provide the oxygen they need to run their larger muscle mass. A typical teenage male will grow a nose about 10 percent bigger than a typical girl his size. The resulting adult male nostril sucks more air, and more odor molecules, into his nasal traps. And yet women are still better at detecting diluted scents—fewer molecules of the scent, in other words, in any given local quantity of air. Something is making a woman’s odor receptors function better … In 2014, one lab thought it might be a good idea to see just how many cells were in women’s olfactory bulbs versus men’s. Though the sample size was relatively small—only so many cadaver brains to go around—the results were clear: women’s olfactory bulbs have massively more neurons and glial cells [cells that protect neurons in the brain and spinal cord] than men’s do, even controlling for size. More than 50 percent more. Women’s are simply more dense. And given the way olfactory bulbs process signals, density might have a large effect on overall function. The density, and thereby strength, of any given signal is enhanced. The ripples spread faster over the pond. And given that women have the same number of odor receptors as men do, the primary site for how women’s olfactory system differs from men’s might be here in the bulbs.

Regarding estrogen’s role is shaping a unique female sense of taste, Bohannon writes:

[P]lant-eating mammals’ sense of taste has evolved … with females typically more sensitive to bitterness than males. After all, when it comes to passing down your genes, placental females are always eating for two. Because their bodies do so much of the heavy lifting when it comes to reproduction, the death of a female is always going to be far costlier for the species’ local fitness than the death of a male. So if having a nose that’s better at detecting threats and sex gives females an edge at the survival game, it benefits the species as a whole.

Regarding estrogen’s role is shaping a unique female sense of sight, Bohannon writes:

[O]ver the eons up in the trees, our noses shrank, our eyes moved forward, and the visual centers of our brains exploded. If you line up fossilized skulls in chronological order, you can see the eye sockets move toward the front of the head. And as this happened, the size of the visual processing portions of the brain increased dramatically. If you want to optimize how you interact with your local environment, where you place a pair of sensors matters. Because lungs constantly suck in new air, the best way to orient an olfactory sensor is to place it in the path of that river of odor-laden air—it makes sense for our nostrils and their corresponding olfactory bulbs to be smack in the center of our faces. The ears, meanwhile, are best placed on either side of the head so they can hear sounds radiating from both sides of the body—better for triangulating how far away a sound might be and what direction it’s coming from. Eyes use similar strategies, but generally speaking, which ones they use depends on what sort of creature you are: predator or prey. In mammals, there are essentially two strategies for eye placement. Prey animals usually have their eyes on either side of the head. Think of deer, rabbits, small birds: by having eyes on the sides of their heads, they’re able to keep watch for predators over an incredibly wide field. What’s directly in front of them matters a lot less than spotting the lion in the grass. Meanwhile, predators—dogs, eagles, snakes, cats—generally have their eyes on the front of their heads. While this produces blind spots at the far left and right of their visual field, it greatly increases the amount each eye’s visual field overlaps. That overlap—the parallax—makes it a lot easier to see how far away something is from you in space. It’s also easier to make out fine-grained features of items in that overlap zone. Having a large parallax means we can see farther away, in greater detail, and are better able to judge the distance between ourselves and faraway objects … [P]rocessing a lot of 3-D visual data takes a lot of computational firepower. Indeed, when paleontologists measure primate fossils’ skulls, the more stereoscopic the eye placement, the bigger the brainpan. Binocular, stereoscopic vision is a convergent trait that has evolved a number of different times. Owls and bats, both predators, move through the air at night, and both have eyes on the fronts of their faces. Not all predatory birds have binocular vision, nor do all insectivorous mammals. The defining circumstance is hunting at night, when it’s that much harder to see things, so being able to utilize a parallax is important. In this line of thinking, maybe primates’ eyes slowly moved forward because it’s hard to catch insects in the treetops at night. So they twitched and skittered in the nighttime canopy for hundreds of thousands of years. The bugs got better at hiding. Our ancestors got better at finding them. Predator and prey body plans competed with each other in their slow, evolutionary dance. The more time passed, the more our ancestors started eating other things: leaves and fruits, particularly. So even if our binocular vision did initially evolve in service of following insects in a 3-D space, that predatory advantage rapidly fell to the wayside. The bigger brains, however, stayed … [F]inally our primate ancestors were fully diurnal: daytime dwellers who slept at night. The reason for that was, in all likelihood, fruits—that fantastic food supply in the canopies of the angiosperm forests that usefully advertises its readiness by color. Most mammals are color-blind—unable to differentiate between red and green. Their world is more blue-gray, or even sepia. This is how color vision works: special receptors on our retinas, called opsins, respond to different wavelengths of light; longer waves skew red, while shorter waves are bluish. The retina takes these different color wavelengths and “mixes” them in the underlying nervous system. One receptor activates for blue, and another for red, and the brain sees purple—so long as you have those two different receptors. If you don’t, you’ll just see variations of blue. Most placental mammals are dichromatic, meaning they have two primary types of color receptors: blue and green. If you don’t have a red opsin, you simply can’t differentiate between red and green very easily. Which doesn’t matter when you’re nocturnal—there’s not a lot of red and green going on. Birds can all see red. Most fish can see it, too. But not cats, not dogs, not cows or horses, not rodents, not hares, not elephants or bears. Their worlds are red-less … [B]ack in Africa, primates became increasingly frugivorous and foliage-friendly—away went the insect diet and in came the tender young leaves and ripe fruits. These primates became the Catarrhini: Old World primates, a select group of monkeys and the apes, some of whom would eventually evolve into humanity. To eat all those tender green and ripe red things in the daytime required a retina with a red opsin. The genes for creating that opsin, as luck would have it, are located on the X chromosome. If you have two X chromosomes, as most women do, it’s incredibly unlikely that you’ll end up being red-green color-blind, whereas roughly 10 percent of men are. If red-green color vision was obviously selected for in diurnal primates, why was it located on the X chromosome? It’s possible this type of color vision was more advantageous for the primate Eve than for her consorts and sons. Perhaps being more efficient at spotting more nutritive foodstuffs (extra-sweet berries, extra-tender young leaves) made a real difference in pregnancy and breast-feeding. If Purgi utilized the same sex-specific parenting strategies as many living primates do, foraging for herself and her infant offspring, then the survival of the young depended far more on the female than the male. In other words, there was more pressure to see red and green on the newly diurnal Purgi than there was on her male counterparts … Generally speaking, human eyes do two things: saccades and fixations. Saccades are the twitchy ways eyes move from one spot to another in a visual field, and when they linger on a spot, it’s called a fixation. There are known sex differences in these patterns when people look at human faces—adult women tend to have more saccades that move between different parts of a person’s face and eyes, whereas men tend to fixate a bit more around the nose. No one knows why. But this might be why women are famously better than men at learning new faces, and it might also be why women seem to be a bit better at accurately judging what emotion that face is conveying. We also tend to focus on the left eye region a titch more, which is likewise the side of the human face that tends to be more emotionally expressive.

Bohannon then explains the role estrogen plays in reducing female humans’ ability to run:

“Running” is a key word here: we’re the only living apes that do it … Ardipithecus ramidus walked the earth—the Eve of human bipedalism—about 3–4 million years after the last common ancestor of chimps and humans … Scientists found Ardi’s skeleton near Aramis, Ethiopia, in the mid-1990s, but it took the better part of a decade to analyze the fossils and realize what they’d found: the earliest bipedal ape, the Eve of women’s legs, hips, spine, and shoulders. Ardi is the best evidence we have for the root of the sex differences in men’s and women’s musculoskeletal system. She is the reason there are men’s and women’s divisions of competitive sports. She is the reason women have crappy lower backs and knees. When you look at a modern woman’s skeleton, you’ll still see a lot of Ardi. For example, modern women’s feet and knees kind of suck. Because our leg and foot joints naturally absorb a lot of the pressure of our body weight when we move, you’d think their failures would simply depend on how heavy that body is. But though women tend to weigh less than men do, we’re still more prone to trouble in our feet and knees than men are. Some of that has to do with modern footwear, but not all. Even when we wear the most supportive orthopedist-recommended shoes, women’s feet and knees still falter. Becoming upright was in some ways harder on Ardi and her granddaughters than it was on the males … Modern humans inherited the problems that come with any sort of bad design. Our feet are, in many ways, the biological equivalent of duct-taping your car’s bumper back on when you don’t have the money to send it to the body shop. But it’s worse for women. Stiffening the upper- and mid-foot bones so we can walk means a lot of force is transferred from our ankles to our forefoot. All that force on the forefoot, especially the big toe joint, weakens it over time. Combine that with a female body that tends to “sway” in motion (wider hips, funky knees, more butt fat), and eventually something’s gotta give. It’s probably going to be the big toe joint—both the most flexible part of the foot and the one that receives the most pressure. That’s what bunions are: the physical reminder of how hard it is to turn a grasping hand into a foot. It’s just physics: force has to go somewhere. Our foot distributes pressure down toward the forefoot as we walk. The rest radiates back up through our leg bones, knees, hips, and spine. Unlike men’s, women’s femurs come into the knee joint at an angle. This was true of Ardi, too, but it’s much more pronounced in modern women. Because our hips are wider than men’s, our knees are somewhat closer together to help balance that differing center of gravity. That sexual dimorphism lines the pockets of orthopedic surgeons, who regularly perform significantly more knee replacements on women than men. Consider that every pound of body weight normally puts an extra pound and a half of pressure on the knee joint when we walk around barefoot. It goes up to four times the pressure when we jump. Our bodies have evolved to mostly handle that. But modern, gendered footwear can pull the rug out from under us: in high heels, our center of gravity is tipped forward, meaning that instead of the buttocks and hamstrings, the quadriceps at the front of the thighs have to do the lion’s share of the work, yanking the top of the knee upward, further compromising the joint. Over time, that can damage the ligaments in the knee, wear away at cartilage, and generally wreak havoc. It’s bad for our toe joints, too: walking in heels eliminates the “roll” of normal walking and instead can mean, depending on the heel’s height, a repeated slamming of all your body weight and momentum onto the forefoot. The heel of a high heel is mostly there for balance, which is precisely why stilettos work at all—we’re just tiptoeing our way down urban streets like bewildered ballerinas … In high school physics, you probably learned that the length of a lever had a lot to do with how much potential force that lever could wield. Shorter arm, less force. Longer arm, more force. That’s why the arm of a car jack needs to be long to let you apply enough force to lift the car so you can change your tire. Right. Now think of the bones of your legs. Your femur is one arm of the lever that folds at your knee. How strong your leg can be, therefore, has a lot to do with how long your bones are. The same is true for any other joint in your body: your muscles are there to support, stabilize, and pull on your skeleton. There are ligaments and fascia to connect muscles to bones, muscles to muscles, and cartilage plays a role, too. But fundamentally, a musculoskeletal system is a set of levers. Lots and lots of levers—things that pinch closed and widen, depending on the task at hand … [M]ost modern human women don’t tend to have as much upper body muscle mass as men. Somewhere in puberty, men’s and women’s average body plans diverge, with men’s shoulders and chests broadening and bulking up, while women’s hips widen and their breasts develop … [I]t may be better to consider men’s muscular upper bodies—along with their ability to do all those chin-ups, push-ups, and burpees—as something closer to our tree-dwelling ancestors’. Though boys and girls are relatively similar as children, adult men distribute muscle mass over their upper bodies much more than adult women. While many women are great explosive athletes, they rarely approach the same speed as men over short distances. In feats of strength, we likewise don’t generate as much force on average. Being bigger animals, men also have bigger lungs and hearts, which helps to get that extra oxygen to working muscles.

(The above graphic is from the image archives of the Washington Post.)

That concludes this essay series on estrogen.  In the next essay series, we’ll explore the role of testosterone in human male development.