Origins in the Species
By Andrew Dolby
The remote Colorado ranching community where I grew up in the ’60s and ’70s only had a house or two in any several-square-mile area, but people knew their neighbors—no doubt out of necessity, if errant cattle needed rounding up or someone broke a bone. I have clear memories of a tough but charismatic and funny rancher down the road who would ruffle your hair and give you a good ribbing. Always smelled like whisky and ponderosa pine smoke and was never seen without a well-worn cowboy hat, an obnoxious belt buckle, and cowboy boots with a little horseshit on them. Clearly had command of all things four-legged on the ranch, including Bash (short for “Bashful”), the enormous Saint Bernard who was the first to greet you at the gate.
Yep, Margaret was a real character. To this day, she remains a symbol in my mind of the working-class Rocky Mountain West before high tech mountain bikes and wellness retreats—a cowboy to the core. Or cowgirl? Cow person? Margaret’s (and her equally rugged partner, Clara’s) gender or sexual orientation never entered my mind back then. These humans were far too fun and interesting for such questions to occupy a single brain cell. There were horses to ride, fences to repair, and witty one-liners to laugh over. Of course, I was just a kid, and I have no doubt that they had challenges I will never know. Fast forward to today, though, and society seems to be in a wave of disproportionate handwringing over gender identity and expression and even the very definition of sex. As a biologist, I have a few thoughts on the matter.
Let’s start with the fundamental question: how does biology define sex? A commonly held assertion in public discourse is that sex is among the most clear-cut biological concepts to define. Males have an X and a Y chromosome, and females have a pair of X chromosomes. Case closed.
But while simple black-and-white answers may feel satisfying, biology always finds ways to defy them. First, to place us in evolutionary context, we are part of a diverse animal kingdom, and chromosomes aren’t the only determinants of biological sex in animals. Sex is determined by temperature in some. For example, if sea turtles’ eggs are incubated at a temperature above about 82o F, the embryos become female, but if below this temperature, they become male. Some crocodilians show a more complicated version of temperature-dependent sex determination. Either high or low egg incubation temperatures produce females, and intermediate temperatures produce males.
Fishes’ sex determination is all over the map. In some species, sex chromosomes determine sex, but in others, environmental factors decide whether individuals develop testes or ovaries. Many fish species can change from one sex to the other (multiple times in some!), depending on their age, size or social status. In these species, we have to define sex by gamete (sex cell) size. Eggs are larger than sperm and contain all the ingredients and energy for growth of the fry. Sexual reproductive systems are so different from each other that we can’t even apply consistent criteria to all species in defining biological sex.
Human sex determination does indeed start with X and Y chromosomes. We all begin our development on a female trajectory until after about the fifth week. At that point, the SRY gene on the Y chromosome causes differentiation of our generic gonads into testes through activation of a complex gene network. Meanwhile, a different gene on a different chromosome puts the brakes on the female pathway. The testes then ramp up testosterone production, which guides the further development of male characteristics by selectively activating still other genes. Females continue the developmental course they started on, but estradiol kicks in as opposed to testosterone.
To be clear, both sexes produce both hormones, but at different concentrations and at different sensitivity levels. Sexual development is enormously intricate. Imagine multiplying the complexity of the most difficult British Bakeoff recipe by 10,000, then executing it flawlessly every single time, billions of times over, with never a shred of difference in the results. Not going to happen, you say? Well, of course it’s not going to happen! In my case, I can’t even get a grilled cheese sandwich to turn out the same way twice! The Human Sexual Development Bakeoff is no different, but sometimes the outcomes are people who can’t easily be placed neatly into a pink or blue box.
For example, hormones are useless without receptors. If a hormone molecule is a key, its receptor is the lock. Some people have XY chromosomes and churn out plenty of testosterone, but they don’t have the receptors for it. This condition is known as androgen insensitivity or testicular feminization syndrome. Their external anatomy presents as female, but internally, they have no uterus and a pair of undescended testes instead of ovaries. Most find this out when they fail to start their menstrual cycles at the expected age, or sometimes later when they can’t get pregnant. It may be hard for some to accept that a person with XY chromosomes is female. However, I challenge them to try telling the Brazilian volleyball great Erika Coimbra that she’s not a woman.
On the other side of the coin are XX males. During the formation of gametes, matched pairs of chromosomes exchange pieces of each other in a process called crossing-over. Crossing-over is a normal part of sex cell production and is one source of evolutionarily valuable variation among offspring. During the process of sperm production, parts of the Y chromosome can swap with the X. On occasion, the specific piece of the Y transferred to the X contains the SRY gene. If that sperm fertilizes an egg, the result is a person with XX chromosomes, but with male sex differentiation, a phenomenon known as de la Chapelle syndrome. Now what? We can dismiss such conditions as disorders, but they illustrate further that biology is a messy and moving target.
Additional sex differentiation is the product of even more layers of mind-boggling complexity and involves ongoing developmental forces that reach into adulthood. Humans on average show a modest degree of sexual dimorphism, which refers to collective differences between females and males in their secondary sex characteristics. Secondary sex characteristics are sex-differentiating traits not part of the reproductive system, such as differences in hair, muscle, and body fat distribution, conformation of the vocal tract, and shape of the pelvis. Their development is guided by sex hormones.
Distributed around any average, though, is a range of variation, especially if you measure a biological feature. As is true of any characteristic, or phenotype, the point where each individual person lands on the physical masculinity-femininity scale is the product of a complex, inextricable mix of both genes and environment. Try replicating that same British Bakeoff recipe, each time with different kitchen equipment, room temperature and humidity levels, pastry chefs, and ingredients from different sources! Genes play a role in everyone’s neural development, hormone production, and brain physiology. Everything a person eats, stressors they encounter, activities they engage in, amount of sleep they get, pollutants they consume in their tap water (especially endocrine-disrupting industrial chemicals), etc., affect exactly how their genes are expressed.
Many social and psychological scientists draw a sharp distinction between biological sex and gender. Sex is defined by reproductive anatomy and physiology, and gender, especially gender roles, is strictly a social and cultural construct. Certain factions within our society believe that gender roles, such as who is the provider and who is the nurturer in relationships, must strictly comport with biological sex. I’ve even heard the argument that according to science, male dominance is the natural pattern, so we should accept male dominance in our own cultures.
Female spotted hyenas would take exception to that assertion, though, and would back it up with one of the most powerful bite forces in the entire animal kingdom. In this species, females dominate males and sport “packages” that would be indistinguishable from any males’ in a hypothetical hyena locker room. Their elongated clitoris, known as a pseudo penis, is fully erectile, and they mate and give birth through it, the painfulness of which should be relatable to anyone, regardless of sex. The dominant matriarch’s offspring occupy privileged positions in the clan. Other species, such as orcas, African elephants, and bonobos, our closest ape relatives, also exhibit strongly matriarchal social systems.
Gender identity is yet a different matter. Some people, from a very young age, are resolute that they identify with a gender not consistent with their reproductive anatomy. Others feel deeply that they are neither male nor female. The brain is an organ, and its structure and function are not exempt from the influences of genes. Scientists are studying the physiology underlying sex change in anemone fish. Anemone fish are protandrous, meaning that they start life as males and transition to female when they reach larger body sizes. These scientists have found that the first signs of sex transition appear as differences in gene activity in the brain. We have only scratched the surface in understanding sex and gender identity, but is it possible that brain chemistry plays at least some role in gender identity? I find it difficult to believe that a biological girl would begin confidently professing their male identity at the age of 3 as a product of an intentional, deliberative decision-making process. (As an aside, I don’t remember having to make a conscious decision as a teenage boy about whether to hang Leif Garrett or Cheryl Ladd posters in my bedroom.) We can label transgender identity as “gender dysphoria,” but maybe it is just another example of biological variation.
Ultimately, all of it comes down to the fact that humans are just driven to categorize things. Some people seem to believe that sex, masculinity, and femininity are transcendent, immutable binary “truths,” and anyone who does not conform to traditional ideals is considered defective or “sinful” and needs to change. But the only truth is that biological sex and everything we associate with gender are categories generated by human thought processes. They are what we decide they are. Scientists do their best to apply objective criteria when creating categories, of any sort, with the aims of being able to make predictions and understand natural processes. We are satisfied when we devise taxonomic schemes that work consistently, but again, biology often doesn’t like to play. (Organic evolution’s penchant for blurring boundaries, by the way, is far from confined to sex and gender.) It may be no more scientifically valid to force certain people into a sex or gender binary than it is to accept that they are neither sex, or both. The only consistency in biology is variation, and sex-based variation is completely normal and natural in the context of the full tree of life.
This is not to say that sex or gender should be ignored. Just like race, which is also a social construct, they have real impacts on people’s lives. My point is that society should also protect the life, health, and civil rights of people who cannot be neatly pigeonholed, and we should generally question culturally imposed gender roles and norms. Moreover, maybe we can focus more on our shared humanity and what truly matters in our lives. How about we respect each other’s natural differences, while appreciating what makes us interesting, which, far more often than not, has nothing to do with gender or sex? Instead of defining transgender identity as a disorder that needs a cure, how about we define hostility toward transgender people as a disorder that needs a cure? I also find it rich that people who adamantly proclaim that the science is on their side when it comes to traditional ideals of sex and gender are often the same people who work fervently to undermine or outright dismiss science when it delivers answers that don’t agree with their predetermined ideologies or agendas.
Of course, there was much more to Margaret and Clara than their physical can-do toughness and adventurous sense of fun. They were schoolteachers by day, intellectuals who purposefully chose their rugged lifestyle on the Colorado range when they were not in the classroom. They even built and ran a small ski area across the road from their ranch, which is where I first learned to ski. My dad would drop me off for the day with $5—enough for a rope tow ticket and a sloppy joe.
I had the pleasure of meeting Margaret and Clara again when I was in my 20s. Only then could I fully appreciate how sophisticated they were and understand their incisive commentary on the world. They are no longer with us, but if there is an afterlife, I hope they are sharing a laugh and surrounded by horses and dogs and good books. I hope they can hear the whisper of ponderosa pines and smell the sagebrush after it rains…stuff that truly matters.
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Andrew Dolby is a biology professor at the University of Mary Washington, where he has been a faculty member since 2000. A self-professed wildlife nut, a young Andrew was studying encyclopedia entries for “okapi” and “cassowary” while the other boys were out playing pickup baseball. He now teaches courses in animal behavior, ornithology, ecology, and evolution, and leads student field trips to Central America and the Galapagos Islands. In his free time, he enjoys running the Fredericksburg trails, drinking locally brewed beer, and plotting his next opportunity to add some new bird, monkey, or carnivorous mammal to his sightings list.
