Huberman Lab - Essentials: How Hormones Shape Sexual Development

Episode Date: February 13, 2025

In this Huberman Lab Essentials episode, I explain the crucial role hormones play in shaping the sexual development of both the brain and body. I discuss how biological masculinization and feminization depend on factors such as genetics, hormone ratios, and receptor availability. I also explore how hormones like testosterone, estrogen, and other steroid hormones influence sexual characteristics and brain development. Additionally, I examine the impact of environmental factors—such as herbicides like atrazine, cannabis, alcohol, and even cell phones—on hormone function and reproductive health. Huberman Lab Essentials are short episodes (approximately 30 minutes) focused on key science and protocol takeaways from past Huberman Lab episodes. Essentials are released every Thursday, while full-length episodes continue to be released every Monday. Read the full episode show notes, including referenced articles, resources, and people mentioned at hubermanlab.com. This Huberman Lab Essentials episode is from the full-length episode, available here: https://go.hubermanlab.com/PQYuc9i Thank you to our sponsors AG1: https://drinkag1.com/huberman Function: https://functionhealth.com/huberman Our Place: https://fromourplace.com/huberman BetterHelp: https://betterhelp.com/huberman Timestamps 00:00:00 Huberman Lab Essentials; Hormones, Sexual Development 00:01:25 Chromosomal Sex, Gonadal Sex, Hormonal Sex, Morphology 00:06:08 Sponsor: Function 00:07:56 Steroid Hormones, Sexual Characteristics 00:10:59 Primary & Secondary Sexual Characteristics, Dihydrotestosterone (DHT), Kisspeptin 00:15:12 Masculinization of Brain, Estrogen 00:16:29 Sponsors: Our Place, BetterHelp 00:19:15 Herbicides, Atrazine, Hormone Effects, Sperm Counts 00:25:04 Female Sexual Development, Androgen Insensitivity Syndrome 00:28:37 Sponsor: AG1 00:29:41 Cannabis & Aromatase Activity, Gynecomastia; Alcohol & Estrogen Activity 00:32:34 Cell Phones & Gonads 00:35:24 Beard & Hair Growth, DHT 00:38:42 Hyenas, Hormones, Androstenedione; Plants 00:43:44 Recap & Key Takeaways Disclaimer & Disclosures

Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. This podcast is separate from my teaching and research roles at Stanford.

Today, we're going to explore hormones, what they are, how they work, what leads to masculinization or feminization of the brain and body. What we're trying to do today is really get to the biology, the physiology, the endocrinology and the behavior. Hormones by definition are a substance, a chemical that's released in one area of the body,

typically from something we call a substance, a chemical that's released in one area of the body, typically from something we call a gland, although they can also be released from neurons, but they're released often from glands that travel and have effects both on that gland, but also on other organs and tissues in the body. And that differentiates hormones from things like neurotransmitters, which tend to act more locally.

Examples of tissues that produce hormones would be the thyroid, the testes, the ovaries, et cetera. And then of course, there are areas of the brain like the hypothalamus and the pituitary, which are closely related to one another and release hormones that cause the release of yet other hormones out in the body. So let's start with development.

Sperm meets egg. Everything that happens before that is a topic of the next episode. But sperm meets egg. This is mammalian reproduction. And that egg starts to duplicate. It starts to make more of itself. It makes more cells.

And eventually some of those cells become skin. Some of those cells become brain. Some of those cells become muscle. some of those cells become brain, some of those cells become muscle, some of those cells become fingers, all the stuff that makes up the brain and body plan. In addition, there are hormones that come both from the mother and from the developing baby, the developing fetus,

that impact whether or not the brain will be what they call organized masculine or organized feminine. And as I say this, I want you to try and discard with the cultural connotations or your psychological connotations of what masculinization and feminization are because we're only centering on the biology.

So typically people have either two X chromosomes and the traditional language around that is that person is female, right? Or an X chromosome and a Y chromosome and that person will become male. Now it's not always the case. There are cases where it's XXY where there are two X chromosomes plus a Y chromosome.

There are also cases where it's XYY where there are two X chromosomes plus a Y chromosome. There are also cases where it's XYY where there are two Y chromosomes. And these have important biological and psychological impacts. So the first thing we need to establish is that there is something called chromosomal sex. Whether or not there are two X chromosomes or an X and Y chromosome is what we call chromosomal sex.

But the next stage of separating out the sexes is what we call gonadal sex. Typically, not always, but typically, if somebody has testes for their gonads, we think of them as male. And if somebody has ovaries, we think of them as female. Although that's not always the case either. But let's just explore the transition from chromosomal sex

to gonadal sex because it's a fascinating one that we all went through in some form or another. So this XY that we typically think of as promoting masculinization of the fetus, we say that because on the Y chromosome, there are genes and those genes have particular functions that suppress female reproductive organs. So on the Y chromosome, there's a gene

which encodes for something called Mullerian inhibiting hormone. So there's actually a hormone that's programmed by the Y chromosome that inhibits the formation of Mullerian ducts, which are an important part of the female reproductive apparatus. That's critical because already we're seeing the transition

between chromosome, Y chromosome and gonad. And other genes on the Y chromosome promote the formation of testes. So there are genes like the SRY gene and other genes that promote the formation of testes while they also inhibit the formation of the malaria ducts. So the transition from chromosomal sex to gonadal sex is a very important distinction.

It's kind of a fork in the road that happens very early in development while fetuses are still in the embryo. So we have to distinguish between chromosomal sex, gonadal sex, and then there's what we call hormonal sex, which is the effects of the steroid hormones, estrogen and testosterone and their derivatives, on what we call morphological sex

or the shape of the baby and the human and the genitalia and the jaw and all these other things. And so it actually is quite complicated. So, you know, it's a long distance from chromosomes to gender identity and gender identity has a lot of social influences and roles. This is an area that right now is very dynamic and in the discussion out there, as you know

but just getting from chromosomal sex to what we would call gonadal sex and hormonal sex and morphological sex involves a number of steps. So today we're going to talk about those steps. And there's some fascinating things that do indeed relate to tools, do indeed relate to some important behavioral choices, important choices about things to avoid while pregnant.

And for those of you that are not pregnant, things to avoid if you're thinking about eventually having children. And that is not to drive development in one direction or another, but there are examples where there are some deleterious things in our environment that can actually negatively impact

what we call sexual development overall, regardless of chromosomal background. So let's get started with that. I'd like to take a quick break and acknowledge one of our sponsors, Function. Last year, I became a Function member after searching for the most comprehensive approach to lab testing.

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to Huberman podcast listeners. Again, that's functionhealth.com slash Huberman to get early access to Function. Let's talk a little bit more about what hormones do. Hormones generally have two categories of effects. They can either be very fast or they can be very slow. There are hormones like cortisol and adrenaline, which act very fast.

And then there are hormones like testosterone and estrogen, which we refer to as the sex steroid hormones. These molecules, for those of you that are interested, are what are called lipophilic, which just means that they like fatty stuff. They can actually pass through fatty membranes. And because the outside of cells, as well as the what's called the nuclear envelope,

where all the DNA contents and stuff are stuffed inside, are made of a lipid of fat, these steroid hormones can actually travel into cells and then interact with the DNA of cells in order to control gene expression. So they can change the sorts of things that cells will become, and they can change the way that cells that cells will become and they can change the way

that cells function in a long-term way. And that's actually how the presence of these genes like SRY and Mullerian inhibiting hormone lead to reductions or elimination, I should say, of things like the Mullerian ducts and promote instead what's called in males, the Wolfian ducts or promote the development of testes rather than ovaries. So all you need to know is that hormones have short-term

and long-term effects, and the long-term effects are actually related to their effects on genes and how those genes are expressed or repressed, not in order to prevent them from having particular proteins made. So these hormones, these steroid hormones are exceedingly powerful. And if we're going to have a discussion

about masculinization or feminization, et cetera, you also need to think about the counterpart. It's not just about masculinizing the body or feminizing the body and brain. It's also about demasculinizing the brain in many cases as a normal biological function typically of XX females and defeminization, the suppression of certain pathways that are related to feminization of the body and brain.

So I've just thrown a lot of biology at you, but this is where it all starts to get incredibly surprising. You would think that it's straightforward, right? You have a Y chromosome, you suppress the female reproductive pathway like the malaria and ducks, you promote the development of testes and then testes make testosterone.

And then it organizes the brain male and it wants to do male-like things. And then in females, you get estrogen and it wants to do a female-like And then in females you get estrogen and it wants to do a female like things and air quotes here for all of this. And it turns out that isn't how it works at all. Here's where it's interesting. We have to understand that there are effects

of these hormones, testosterone and estrogen on what are called primary sexual characteristics which are the ones that you're born with, secondary sexual characteristics, which are the ones that show up in puberty. And these are happening in the brain and body and spinal cord. And so I'm going to disentangle all this for you

by giving you some examples. First, let's talk about the development of primary sexual characteristics, the ones that show up at birth. And one of the more dramatic examples of this comes from the role of testosterone in creating the external genitalia. It turns out that it's not testosterone

that's responsible for the development of the penis in a baby that has an X chromosome and a Y chromosome. It's a different androgen. Androgen is just a category of hormones the penis in a baby that has an X chromosome and a Y chromosome. It's a different androgen. Androgen is just a category of hormones that includes testosterone, but testosterone is converted in the fetus to something called dihydrotestosterone. And that's accomplished through an enzyme

called five alpha reductase. Dihydrotestosterone is what we would call the dominant androgen in males. It's responsible for aggression. It's responsible for a lot of muscular strength. It's involved in beard growth and male pattern baldness. We're going to talk about all of that, but dihydrotestosterone has powerful effects

in determining the genitalia while the baby is still in the embryo. So there's testosterone that's made and that testosterone gets converted by this enzyme, five-alpha reductase, in a little structure called the tubercle. That tubercle will eventually become the penis. So you say, okay, straightforward,

this testosterone's converted to dihydrotestosterone. And then if there's dihydrotestosterone, it controls penis growth. And indeed that's the case. So that's a primary sexual characteristic. That baby will then grow up and later during puberty, there will be the release of a molecule. I talked about this last episode called kisspeptin,

K-I-S-S-P-E-P-T-I-N, Kispeptin, which will cause the release of some other hormones, conatropin releasing hormone, luteinizing hormone will stimulate the testes to make testosterone. So in puberty, testosterone leads to further growth and development of the penis, as well as the accumulation of or growth of pubic hair, deepening of the voice,

all the secondary sexual characteristics. There's a very interesting phenomenon that was published in the journal Science in the 1970s, for which now there's a wealth of scientific data. And this relates to a genetic mutation where 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone doesn't exist.

It's mutated. And this actually was first identified in the Dominican Republic. What happens is baby is born. If you were to look at that baby, it would look female. There would be very little or no external penis. And what was observed is that from time to time, that baby after being raised as a girl,

would around the age of 11 or 12 or 13, would start to sprout a penis. There's actually a name for this, it's called huevidosis, which the translation is more or less penis at 12. And as strange as this might sound, it makes sense if you understand the underlying mutation. What happens in these children, these juvedosis,

is that the child is born, it has testes which are not descended, so up in the body. They weren't able to convert testosterone to dihydrotestosterone because they lack this enzyme, five-alpha reductase. As a consequence, the primary sexual characteristic of external male genitalia, penis, doesn't develop. And then what happens is the baby grows up

and then testosterone starts getting secreted from the testes because kisspeptin in the brain signals through gonadotropin and the luteinizing hormone travels down from the testes because chispeptin in the brain signals through gonadotropin and the luteinizing hormone travels down to the testes. The testes start churning out testosterone and there's a secondary growth of the penis and all of a sudden there's a penis.

And the point here is that dihydrotestosterone, not testosterone is responsible for this primary growth of the penis and that testosterone later is involved in the secondary sexual characteristics, deepening of the voice, et cetera. Now this is where the information gets even more interesting and applies to essentially everybody. You might think that testosterone,

because it masculinizes the body in the secondary sexual characteristic way, and because dihydrotestosterone, another androgen, masculinizes the primary sexual characteristics, the growth of the penis early on, that testosterone must masculinize the brain. But the masculinization of the brain is not accomplished

by testosterone. It is accomplished by estrogen. Testosterone can be converted into estrogen by an enzyme called aromatase. There are neurons in the brain that make aromatase and convert testosterone into estrogen. In other words, it's estrogen that masculinizes the XY individual that masculinizes the brain.

And this has profound effects on all sorts of things on behavior, on outlook in the world, et cetera. But I think most people don't realize that it's estrogen that comes from testosterone that masculinizes the male brain, the XY brain, not testosterone nor dihydrotestosterone. I'd like to take a quick break and acknowledge one of our sponsors, Our Place.

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how could tools possibly come up at this stage of the conversation where we're talking about sexual development and we're talking about the differentiation of tissues in the body. Well, this is true both for children and parents and adults. I want to emphasize that there are things

that are environmental and there are things that people use that actually can impact hormone levels and can impact sexual development in fairly profound ways. And I want to be very clear, this is not me pulling from some rare journal of never heard of it. This is pulling from textbooks in particular, today I'm guiding a lot of the conversation

on work that on behavioral endocrinology. This is a book by Randy Nelson and Lance Crickfield, true experts in the field. I'm going to talk about some of the work from Tyrone Hayes from UC Berkeley about environmental toxins and their impacts on some of these things like testosterone and estrogen. I'm going to touch into them.

I'm going to give some anecdotal evidence that's grounded in studies, which we will provide in the caption or that I'll reference here. You know, again, I'm just going to highlight when one starts talking about environmental factors and how they're poisoning us or disrupting growth or fertility rates, it can start to sound a little bit crazy

except when you start to actually look at some of the real data, data from quality research labs funded by federal government, funded not from companies or other sources that are really aimed at understanding what the underlying biology is. And for that, I really, we should all be grateful to Tyrone Hayes at UC Berkeley. I remember way back when I was a graduate student

in the late 90s, goodness, at UC Berkeley. And I remember him, he was studying frogs. He was talking about developmental defects in these frogs that live in different waters around, it was California, but also elsewhere. And he identified a substance which is present in a lot of waterways throughout this country and other countries, so US and beyond,

certainly not just restricted to California, which is atrazine. This is A-T-R-A-Z-I-N-E. Again, this is the stuff of textbooks and it causes severe testicular malformations. So again, atrazine exposure is serious. And what's interesting is if you look at the data, what you find is that at sites in Western

and Midwestern sections of the United States, 10 to 92% of male frogs, these were frogs, mind you, had testicular abnormalities. And the most severe testicular malformations were in the testes rather than in the sperm. So it's actually the organ itself, the gonad itself. Now, it's very well known now that atrazine

is in many herbicides. And so, you know, whereas I would say in the 80s and 90s, the discussion around, you know, herbicides and their negative effects was considered kind of like hippie dippy stuff or the stuff you hear about it, you know, your local community markets and these kind of new agey communities. Now there's very solid data from federally funded labs

at major universities that have been peer reviewed and published in excellent journals, showing that indeed many of these herbicides can have negative effects, primarily by impacting the ratios of these hormones in either the mothers or in the testes, altering the testes of the fathers or direct effects on developing young animals

and potentially humans. And so you ask, well, what about humans? Frogs are wonderful, but what about humans? So here are the data on what's happening. And this isn't all going to be scary stuff. We're also going to talk about tools to ameliorate and offset some of these effects, depending on your needs. But across human populations, sperm counts

are indeed declining, okay? So in 1940, the average, the average density of human sperm was 113 million per milliliter of semen. That's how it's measured. How many sperm per milliliter of semen? That's how it's measured. How many sperm per milliliter of semen. In 1990, this figure has dropped to 66. It went from 113 million per milliliter to 66 million per milliliter

in the United States and Western Europe. So it's not just a US thing. Researchers also estimated that the volume of semen produced by men has dropped 20% in that time, reduced sperm count per ejaculation even further. So between 1981 and 1991, the ratio of normal spermatogenesis has decreased from 56.4% to 26.9%.

So there's a lot that's happening primarily because of these herbicides that are in widespread use to reduce sperm counts. And these are going to have profound effects, not just on sperm counts, but on development, sexual development at the level of the gonads and the brain because you need testosterone to get you to dihydrotestosterone

for primary sexual characteristics. You need estrogen that's come from testosterone to masculinize the brain. And of course, we're not just focusing on sperm and testosterone. You of course also know that many of these herbicides are disrupting estrogens in a similar way, which might explain why puberty is happening

so much earlier in young girls these days. So there are a lot of things that are happening. Now, does this mean that you have to run around and neurotically avoid anything that includes things like atrazine? And should you be avoiding all kinds of herbicides? I don't know, that's up to you, but it does seem that these have pretty marked effects

in both the animal studies and in the human studies. So let's talk about female sexual development. And as always, what we'll do is we'll talk about the normal biology. Then we'll talk a little bit about a kind of extraordinary or unusual set of cases, but we'll talk about them because they illustrate an important principle about how things work

under typical circumstances. So there is a mutation called androgen insensitivity syndrome. And understanding how androgen insensitivity syndrome works can help you really understand how hormones impact sexual development. So here's how it works. There are individuals who are XY,

so they have a Y chromosome, that are born that make testosterone. They have testes and they don't have malaria in ducks because on the Y chromosome is this malaria inhibiting hormone. However, these individuals look completely female. And in general, they report feeling like girls when they're young, women when they're older.

But there's something unusual that's happening in these individuals because they have an XY chromosomal type and not XX. So what's happening? Well, what's happening is the testes are making testosterone but the receptor for testosterone is mutated. And therefore the testes never descend. They don't have ovaries, they have testes

but the testes are internal. And so typically these individuals find out that they are actually XY chromosomes so that their chromosomal sex is male, if you will, and their gonadal sex is male, but the gonads, the testes are inside the body, they don't actually develop a scrotum, they don't make ovaries,

and when they don't menstruate around the time of puberty, that's a sign that something is different. And so they never menstruate around puberty, and if they look into this deeply enough, what you find is that they are actually XY, they make testosterone, but their body can't make use of the testosterone because they don't have the receptors.

And the receptors are vitally important for most all of the secondary sexual characteristics that we talked about, body hair, penis growth during puberty, et cetera. So again, we're talking about this in order to illustrate the principle that in order to, body hair, penis growth during puberty, et cetera. So again, we're talking about this in order to illustrate the principle that in order to have its effects, a hormone doesn't just have to be present,

that hormone actually has to be able to bind its receptor and take action on the target cells. Perhaps the simplest way to understand how estrogen and testosterone impact masculinization or feminization of the brain and behavior is from a statement. It's actually the closing sentence of an abstract that my colleague, Nirao Shah

at Stanford School of Medicine published, which is that estrogen, again, it's estrogen that is aromatized from testosterone by aromatase sets up the masculine repertoire of sexual and in animals and in humans, territorial behaviors. So it sets up the circuitry in the brain. Estrogen does that.

Estrogen sets up the masculine circuitry in the brain. And testosterone is then what controls the display of those behaviors later in life. And I find that incredibly interesting. You would think it was just testosterone did one thing and estrogen did another, but it turns out that nature is far more interesting than that.

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to claim a special offer. Right now they're giving away five free travel packs plus a year supply of vitamin D3 K2. Again, that's drinkag1.com slash Huberman to claim that special offer. Okay, so what are some things that impact sexual development early in life and later in life? Let's talk about cannabis.

Let's talk about alcohol. First of all, cannabis, marijuana, THC. There are many studies that point to the fact that THC and other things in cannabis promote significant increases in aromatase activity. Now, pot smokers aren't going to like this, especially male pot smokers aren't going to like this, but it's the reality.

Here's the deal, that cannabis, and it's not clear if it's THC itself or other elements in the marijuana plant, promote aromatase activity. Now, this has been observed anecdotally where pot smokers have a higher incidence of developing something I mentioned before, gynecomastia, breast bud development

or full-blown breast development in males. Now, earlier I said that estrogen is what masculinizes the male brain. In utero, that's true, but the way that cannabis seems to work, at least from the studies I was able to identify, is that it promotes circulating estrogen in the body and therefore can counteract some of the masculinizing

effects of things like testosterone and dihydrotestosterone on primary and secondary sexual characteristics. So I mentioned this because, you know, I think nowadays marijuana use is far more widespread and certainly during puberty, it can have profound effects on these hormonal systems. And so we'll do another episode that goes really deep into this,

but yes, cannabis promotes estrogenic activity by increasing aromatase. Most everyone can appreciate that drinking during pregnancy is not good for the developing fetus. Fetal alcohol syndrome is a well-established negative outcome of pregnancy. And it's something that there are cognitive effects that are really bad.

There's actually physical malformation, et cetera. So drinking during pregnancy, not good. Probably drinking during puberty, not good either because alcohol, in particular, certain things like beer, but other grain alcohols can increase estrogenic activity. Now, this isn't just about protecting young boys from estrogenic activity. It's also protecting girls from excessive

or even hypoestrogenic effects of alcohol in puberty. Now, many teenagers drink, college students drink, and it's important to point out that puberty doesn't start on one day and end on another day. Puberty has a beginning, a middle, college students drink, and it's important to point out that puberty doesn't start on one day and end on another day. Puberty has a beginning, a middle, and an end, but development is really our entire lifespan. Okay, so we talked about cannabis,

we talked about alcohol, let's talk about cell phones. First of all, I use a cell phone, I use it very often, and I do not think they are evil devices. I think that they require some discipline in order to make sure that it does not become a negative force in one's life. So I personally restrict the number of hours that I'm on the phone and in particular on social media.

But what about the cell phone itself? When I was a junior professor, I was a pre-tenure early professor, I taught this class on neural circuits and health and disease. And one of the students asked me, you know, are cell phones safe for the brain? And you know, all the data point to the fact that they were,

or at least there were no data showing that it wasn't. I still don't have the answer on that, frankly. I'm not personally aware of any evidence in quality peer-reviewed studies showing that cell phones are bad for the brain or that holding the phone to the ear is bad or that Bluetooth is bad or any of that. I'm just not aware of any quality studies.

However, I was very interested in a particular study that was published back in 2013 on rats, it was basically took a cell phone and put it under a cage of rats and looked at basically testicular and ovarian development in rats and saw minor but still statistically significant defects in ovarian and testicular development.

Since then, and now returning to the literature, I've seen a absolute explosion of studies, some of which are in quality journals, some of which are in what I would call not blue ribbon journals, identifying defects in testicular and or ovarian development by mere exposure to cell phone emitted waves. Let's just call that.

We don't know what they are. And this sounds almost crazy, right? Anytime somebody starts talking about EMFs and things like that, you kind of worry, like, is this person okay? But look, the literature are pointing in a direction where chronic exposure of the gonads to cell phones could be creating serious issues in terms of the health at the cellular level

and in terms of the output. So the output for the testes would be sperm production, swimming speed in sperm is an important feature of sperm health. In the ovaries, it would be estrogenic output, how regular the cycles are. I think that it's fair to say based on the literature that there are effects of cell phone emitted waves

on gonadal development. The question is, what is the proximity of the cell phone to the gonads? So you have to take these sorts of studies with a grain of salt. There's some interesting effects of hormones that actually you can observe on the outside of people that tell you something about not just their level

of hormones, but also about their underlying genetics. And these relate to beard growth and baldness. And it's fascinating. The molecule, the hormone dihydrotestosterone made from testosterone, is the hormone primarily responsible for facial hair, for beard growth. As well, it's the molecule, the hormone,

primarily responsible for lack of hair on the head, for hair loss. Not incidentally, the drugs that are designed to prevent hair loss are five alpha reductase inhibitors. So remember five alpha reductase from the Hueva dosis? Well, the people that discovered the Hueva dosis went on to do a lot of research on the underlying biochemistry

of this really interesting molecule dihydrotestosterone. They identified five alpha reductase and five alpha reductase inhibitors are the basis of most of the anti-hair loss treatments that are out there. And so there are some interesting things here. First of all, the side effect profiles of those treatments for hair loss are quite severe in many individuals.

Remember, DHT is the primary androgen for libido, for strength and connective tissue repair, for aggression, even if that aggression, of course, is held in check, but just sort of ambition and aggression is related to dopamine, but within the testosterone pathway, less so to pure testosterone, although pure testosterone has its effects,

but DHT is, at least in primate species, including humans, is the dominant androgen for most of those sorts of effects. And if you look at somebody, everyone can predict whether or not they're going to go bald based on looking at their, we're always taught our mother's father. So if your mother's father was bald, there's a higher probability that you're going to go bald.

The pattern of DHT receptors on the scalp will dictate whether or not you're going to go bald everywhere or just in the front or so-called crown type baldness. And the density of the beard tells you about the density of DHT receptors. Now this varies by background, by genetic background. There are areas of the world where all the men seem to be,

have the same pattern of baldness, like a strip of baldness down the center with hair still on the sides and full beards. That's because these patterns of DHT receptors are genetically determined. Elsewhere, testosterone levels can still be very high. DHT levels in the blood can be very high, and yet people will have very light beards or no beards, and that's because

they don't have a lot of DHT receptors in the face. There are a lot of effects of DHT that you can just see in male phenotypes. And it's interesting that these hair loss drugs that are, or to prevent hair loss drugs, are directly aimed at preventing the conversion of testosterone into dihydrotestosterone. And that's why they, to some extent, prevent hair loss, but also to some extent have a bunch of side effects that are associated with low DHT. I want to tell you a story about hyenas

and clitorises the size of penises. So when I was a graduate student at UC Berkeley, we had a professor in our department, phenomenal scientist named Steve Glickman. Steve Glickman had a colony of hyenas, spotted hyenas that lived within caged enclosures, of course, in Tilden Park behind the UC Berkeley campus. The hyenas are no longer there.

Hyenas exhibit an incredible feature to their body, their hormones, and their social structure. Hyenas, unlike many species, have a situation with their genitalia where the male penis is actually smaller than the female clitoris. And I should say that the male penis itself, having seen a fair number of hyena penises,

is not particularly small, which means that the hyena clitorises are extremely large. This was well known for some time. It turns out that in the spotted hyenas, the females are dominant. So after a kill, the females will eat, then their young will eat, and then the male hyenas will eat.

As well, when the female hyena gives birth, she gives birth not through the vaginal canal that we're accustomed to seeing, but through a very enlarged clitoris-like phallus, although it's not a phallus, it's a clitoris, and it literally splits open. So the many fetuses die during the course of hyena development and birth. The baby hyena actually comes through the tissue

and it's a very traumatic birth. It was a mystery as to how the female hyenas have this, we'll call it masculinization, but it's really a androgenization of the periphery of the genitalia. And it turns out through a lot of careful research done by Steve Glickman, Christine Dre, and others, that it's androstenedione,

what is essentially a prohormone to testosterone, it's androstenedione at very high levels that's produced in female hyenas that creates this enlargement of their genitalia. So if you want to read up on androstenedione, androstenedione is made into testosterone through this enzyme 17-beta hydroxy steroid dehydrogenase. It's a complicated pathway to pronounce.

It's a fairly straightforward pathway biochemically. You may recall during the 90s and 2000s, there were a lot of performance enhancing drug scandals in particular in major league baseball. And it was purported, although I don't know that it was ever verified, but it was purported that the major performance enhancing drug of abuse at that time

in particular players whose names we won't mention, but you can Google it if you want to find out, was Androstenedione. And the last little anecdote about this, which is also published in the scientific literature, which is weird, but I do find interesting. Hormones are so fascinating. They're just incredible to me,

is going back to the marijuana plant. The marijuana plant has these estrogenic properties. And I asked a plant biologist whether or not this was unusual. But this plant biologist told me, oh yeah, there are plants that make what is essentially the equivalent of testosterone, like pine pollen, as it looks a lot like testosterone.

And there are other plants that make what is essentially estrogen. And I said, well, why would they do that? He said that one of the reasons why some plants have evolved this capacity to increase estrogen levels in animals that smoke, not smoke it, but then animals that consume them, I'm guessing that animals aren't smoking marijuana,

although I don't know, send me the paper if you've heard of this, is that plants have figured out ways, they've adapted ways to push back on populations of rodents and other species of animals that eat them. So plants are engaged in a kind of plant to animal warfare where they increase the estrogen of the males in that population to lower the sperm counts

to keep those populations clamped at certain levels so that those plants can continue to flourish. And I find this just fascinating. And hormones, therefore, aren't just impacting tissue growth and development within the individual and between the mother, remember the placentas and endocrine organ and the offspring, but plants and animals are in this communication. So it's a fascinating area of biology. And as you've noticed today, none of this deals with the current controversies

around gender and how many genders and sex, et cetera. That's a separate conversation that is by definition grounded in the kind of concepts we've been talking about today and needs to take place taking into consideration all of the aspects of sex and the effects of hormones, both on the body, on the brain. We didn't talk a lot about spinal cord,

but we will in the next episode, but we can just say on the brain and the periphery, early effects, late effects, acute effects, meaning effects that are very fast of levels of hormones going up or down, something that absolutely happens during and across the menstrual cycle, as well as long-term effects, like the effects of these hormones on gene expression.

So today, as always, we weren't able to cover all things related to sex and hormones and sexual differentiation or development. There's no way we could, but we have covered a lot of material. So once again, I want to thank you for embarking on this journey through neuroscience and today, neuroendocrinology with me.

And as always, thank you for your interest in science. ["Science on a Planet"]