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David Puts 4:59
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have inputs. I'm a professor of anthropology at Penn State, and I study sex, I guess sex differences, their evolution and development, especially hormonal causes, and especially focus on behavior in psychology. Okay, and do you focus mainly on humans? I do. And some work on nonhuman primates as well, but mostly, most of people. So, in terms of how the term is used for non human animals, at least, what is biological sex?
Yeah, biological sex refers to, we could be talking about few, a couple of different things. One is sexual reproduction, which involves producing gametes, and, you know, fusion of gametes and DNA from two different parents to form a new individual. But then if we're talking about sexes, like males and females, and that just has to do with gamete size. And so almost every time you know, most species that reproduce sexually have evolved sexes, there are a lot of species that reproduce sexually that don't have sexes at all. That is, like all the gametes are the same size. And roughly, there's sort of one mode in the distribution. But when sexes have evolved, they've they've evolved into two different morphs two different sizes, there's a small ones and the big ones. And males are defined as the sex that produces smaller gametes. And females produce bigger gametes. And there's a ton of variation across species and the sorts of characteristics that males and females have, it doesn't have anything to do with external anatomy, or body size, or anything like that. But if you if you're looking across species and want to know, do they have sexes? Well, then the question is, is there one sort of normal distribution of gamete? Size? Are there two different sizes of gametes? And then, if there are then which sexes are males, which you know, which are males and which are females, and that's just determined by which produces bigger gametes, and which produces smaller ones? And
Nick Jikomes 7:05
so what exactly are gametes?
David Puts 7:09
Sorry, sex cells. So in an animal's sperm and OVA, and in plants, pollen, Nova,
Nick Jikomes 7:17
I see. So so. So in humans, right males are associated with the sperm, and females with eggs are much smaller than eggs. They are less expensive in terms of their metabolic cost to produce. And that's an important thing here, how does that sort of tie into it the big one versus the small one? And how that relates to the sexes and how they differ in other ways.
David Puts 7:43
Yeah, on an evolutionary scale, that probably the we don't know this, but we, you know, we can guess that initial greater investment in gametes can lead to a parent that invests more in offspring overall. And you tend to see that across animal species that females defined as the sex producing the bigger gametes tend to invest more in offspring, not just in, in producing sex cells. But in you know, things like in mammals, internal gestation, and lactation, or in birds. If one sex spends more time caring for eggs and offspring, then it's usually females, although there's by parental care and a lot of bird species, but that initially larger investment. And gamete seems to have been related to increased investment and, and, you know, sort of post zygotic investment in offspring as well.
Nick Jikomes 8:41
So I see. And when we think about investment size, how should we think about that? Should we think about that? Or can we think about that in terms of something like calories, like the the sex of the beginner is literally devoting more calories from their body to the offspring or something
David Puts 8:55
like that? You can. It all has to do with trade offs, and the way that parental investment was defined by evolutionary biologist, Robert rivers back in the 70s. And I think this is a good definition. And the one that most people use is that it's investment in current offspring, whether that's calories time, whatever, that limits the parents ability to produce more offspring. And so that's the trade off that, you know, you only have so much, you know, sort of finite resources and finite time to spend reproducing, you've been able to sequester certain resources from your environment that you could use to produce offspring and care for offspring and you have to sort of make trade offs there and then any anything that goes into caring for and producing one offspring then can't be used for others, or you know, yeah, so that's that's what was meant by parental
Nick Jikomes 9:49
investment. And in sexually reproducing species, whether we're talking about humans, other mammals other animals, are they're always to gametes, or is there ever a Third gamete or intermediate gamete? Or is it always two,
David Puts 10:02
it's always two. And we don't know why. But probably the you know, there have been mathematical models that have shown this. And probably the reason is what's called disruptive selection that basically, it's again, you know about trade offs, that there's an advantage to producing smaller gametes, which is that you can produce more of them. And there's an advantage to producing large gametes, which is that they're higher in quality, they have more cellular resources that can increase the, you know, the survival and success of the, the fertilized thing the zygote at but you can't do both again, you know, finite resources. And so, you know, sort of intermediate sized gametes would be sort of jack of all trades Master of None, that it would actually in a sort of competitive situation lose out to the producers of smaller gametes are specialized in quantity. And larger gametes are specialized in quality. And so as a result, it's, you know, two sexes, like, the sexes, what's called an eye Sagami has evolved multiple times independently. And it's always two years sometimes in, you know, media pieces about oh, you know, there's 10s of 1000s of sexes in this fungus or something. But that's actually different. Those are mating types, and they don't have more than two different size gametes.
Nick Jikomes 11:24
I see. So there's always there's always two gametes. And yeah, I think what one thing you said is important there sexes actually vote multiple times. So there's, there's separate branches of the tree of life, and sex has evolved, you know, de novo at different points in the branches of that tree of life. But in each case, you know, despite any differences in the details, there are always two gametes. One, one is the big expensive one. One is the small, cheap one. That's right. And so, I want to get into talking about things like sexual selection, and how mate preference mate preferences and stuff and mating strategies relate, relate to some of this stuff. But why don't we just start off by having you talk about what is the difference between natural selection and sexual selection?
David Puts 12:06
Big question. So natural selection is usually now maybe I'll start with sexual selection, because that's a little bit easier. Sexual selection is defined as I would call it a kind of natural selection that favors traits that when mating opportunities, and so in species that, that have sexes and compete for mates, there are multiple ways that you could win mates by attracting them or by fighting off or threatening your same sex competitors are scrambled competition is another way of competing for mates where you just like locating mates. And there's competition to be quicker to get to those mates. But sexual selection favors those sorts of traits, ones that contribute to mating opportunities. And then natural selection, which is, you know, it's a little bit confusing, because, you know, is it different? Or is sexual selection, a kind of natural selection, sometimes people refer to the other kinds of selection is ecological selection. And that favors traits that enhance survival. It could also be like offspring survival, or care for offspring or anything like that. But basically, it's everything that's not sexual selection. And Darwin sort of noticed sexual selection, because he was thinking about, you know, he had this grand theory about how organisms evolved the traits that they have. And he thought, you know, well, you know, organisms seem really well designed for surviving and reproducing given their conditions of life that, you know, saguaro cacti, or plants and plants and notoriously need water. And yet these plants are adapted to living in a desert with low water, he sort of noticed like all these, you know, finches that seem to be adapted to the kinds of food they're eating, and so on. And so he's thinking that way, and then he started thinking about characteristics of organisms, like say peacocks tail feathers, and thought that he said a peacock made him sick to his stomach, which is, you know, sort of an odd reaction to seems like an attractive bird to me. But anyway, and the reason why is because it seemed like, major counter examples to what he had been saying, you know, how does natural selection favor a trait that seems really costly to produce and maintain? peacocks are prey for tigers, it's got to be make them easier to detect by by tigers and harder to to get away from tigers. And so we thought, How do I understand a trait like that? How does natural selection, you know, explain a characteristic like that I thought, you know, sometimes, selection can favor a trait, even if it's costly to survival if it compensates by increasing mating opportunities. And so maybe it male peafowl have these traits because even if it's costly to survival, it more than compensates by increasing their access to mate so increases their reproduction overall. And so, a lot of times we noticed sexual selection when it produces a trait that actually doesn't make sense under ordinary natural selection or ecological selection. But it's not necessary that that those two Types of selection are working in opposite directions. They could be kinds of traits. Yeah,
Nick Jikomes 15:03
yeah. And it's, you know, it's obviously convenient to talk about natural or ecological selection versus sexual selection, especially in cases where you have these weird phenotypes that seem to defy a survival based explanation. But ultimately, these two things are sort of like inseparable, because you have to be good enough at surviving, to get a meeting opportunity, and you have to be good enough at winning the attention of potential sexual partners. And you can only do that if you can survive long enough to get in front of them. So at the end of the day, they kind of converge in a sense. Yeah, right.
David Puts 15:36
And, you know, in the final analysis, what, what selection favors are traits that result in more offspring. And so, you know, there are multiple ways of accomplishing that. And, you know, lots of species have very short lifespans, but are successful reproducers. And so it just depends on the species in their conditions of life, whether selection tends to favor, you know, longer survival and low, low probability of mortality, or, or short lives a high probability mortality, just what in that species in that in their environment would lead to more offspring and the evolutionary past?
Nick Jikomes 16:10
So we've talked about gametes, in all sexually reproducing species that have evolved, there's always two gametes, there's always a big one and a small one, one that's more metabolically expensive, and one that's more metabolically cheap. But oftentimes, when you talk about biological sex, you hear people talking about things like sex chromosomes, or gonads, and other biological factors like hormones. How do all of these things sort of map map together? And by default, I guess let's just take primates as our sort of default animal group that we'll think about. So how does sex relate to things like gametes, and chromosomes and gonads? And is one of these more fundamental than the others? That's
David Puts 16:51
a really good question. First, I just want to clarify that. I think you said something like in every, every time that sexual reproduction exists, there are different sexes, but there are species that reproduce sexually that don't have sexes that have, you know, they're they don't have different size gametes. But when sexes have evolved, in sexually reproducing species, there have been two, but
Nick Jikomes 17:16
there's species that sexually reproduce.
David Puts 17:20
Yeah, like, you know, bluger There's an a blue green algae called Chlamydomonas. That they reproduce sexually, they produce gametes. But they're all the same size. They don't have any sodomy. It's called ISO, same gammy gametes. And it's not same gametes. But so they don't yeah, they don't they have just one one size gamete. But when
Nick Jikomes 17:40
is the statement I made true for the animal kingdom?
David Puts 17:48
Probably mostly true teach this stuff, but I don't do research on it. So let me think. Hmm,
Nick Jikomes 17:55
I know, it's kind of a tough question. Because the whole tree of life and if there's one exception,
David Puts 18:00
yeah, that's right. Yeah, and that's kind of, you know, something I love about studying this stuff is the incredible diversity within species, like in humans, and then across species, it just makes it so so interesting to think about and study and, and I always feel like that whenever I tell my students anything, I'm like, yeah, there's probably an exception to this, you know, even if I don't know about it. But, okay, so, given sexes, what about these associations with sex chromosomes, gonads. Other characteristics? Yeah, there's no unnecessary Association. And I'll just give you some examples from other other animal species to, to illustrate the point, and then we can talk about people because they know that there's a lot of, you know, thinking and inks built in emotion about, you know, human sexes and, and so on. But there are plenty of animal species of vertebrate species in which the sexes are genetically identical. And so you've got species like, say, some turtles, where sex is temperature dependent, and it varies across species, whether, you know, the eggs closer to the middle of the clutch of eggs tend to be warmer. And so sometimes those developed into males, sometimes into females, but there are no genetic differences between the sexes. Sex is just determined by the environment by by temperature. And then there are other species like say, sequentially hermaphroditic fish, like the blue head, RAS is an example where they're there to tropical fish, females are yellow males are sort of blue and black and they have like a white stripe. And again, no genetic difference at all between the sexes, and they live in sort of they live in a reef and the male defends the reef and has multiple females in there. And you know, as a consequence has high mating success of a male's a territorial male because he has access to lots of females, if the male is removed, because he dies or is eaten by a predator, or an experimenter takes the male away, the biggest female changes into a male. And that's how, you know, within sort of minutes to hours, her behavior changes, she becomes territorial and starts to court females, and then over the next. And then her pattern of gene expression changes to male typical patterns of gene expression, her ovaries atrophy, and the testes grow. And her external anatomy changes. So by two weeks, she now no longer looks like a female, she looks fully like a male,
Nick Jikomes 20:40
she has her body now going to produce the other gamete. Yeah, produces
David Puts 20:44
produces sperm now. And again, there is no difference at all genetically between them. So, you know, the idea that like the essence of maleness is having a Y chromosome or something, isn't right. It really is just about gamete size. And, and you know, birds also I mean, they're they're also have genetic sex determination, but they're the other way around. So that females are the hetero comedic sex, there's Ew, whereas males have ZZ. So yeah, there's nothing about genes, or chromosomes or anything like that, that that is, you know, sort of fundamental to sex. It's really about gamete size. And, and same thing with like, you know, the traits that evolved to be associated with that. I mean, there are so called Sex Role reverse species, including some birds, where females are bigger and more aggressive with each other, more competitive for mates. So yeah, that varies.
Nick Jikomes 21:40
So so it sounds like another way of saying that is, you know, in some species, there is a strong genetic component as to what gamete is going to be produced. So look, your chromosomes, your sex chromosomes, and a primate will determine which gamete is going to be produced in other species, you gave the example of turtles, it's entirely environmental, the males and females are genetically identical. They don't have separate sex chromosomes, you know, things like temperature determining which gamete gets produced. But in any case, are the common denominator here is something ultimately is pointing to which of the two gametes you're going to produce, if there are two sexes and that species? Yeah, that's
David Puts 22:16
right. And so you know, sort of, if you think about the evolutionary past of any particular species, then there were, as long as there are separate selection pressures operating on males and females. In other words, the traits that enable males to leave more offspring are different to some degree, than the traits that enable females to lead more offspring, then you've got sexual selection pressures, sort of pulling the two sexes apart. And how do you accomplish that, especially given that males and females are the same species and have either identical DNA, like, you know, in blue head RASS, or in some turtle species or nearly identical DNA, like in humans, where, you know, males and females have all the same DNA? Except? Do you have a few genes, the genes that are expressed on the Y chromosome? Or do you have two copies of the x? And so how does selection pull the two sexes apart? In a way that, you know, when it creates species differences, it does that by favoring different genes in the two species, with sexes, it can't do that. So how does it do it? Some mechanism to get the process started. And then almost always, this involves producing gonadal hormones, you know, like testosterone or estrogen. And that causes different patterns of gene expression of the same genes. So the sexes have the same genes, but they're expressed differently, because of the hormones that they produce. And, and the, the sort of trigger that starts off development in a male typical, or female, typical pattern just varies across species, or this genetic or environmental.
Nick Jikomes 23:48
I see So so the differences in a sexually dimorphic species I want to talk about what that means is you've got, you've either got the small gamete, or the big gamete. And then you're going to have a certain good Natl identity, that is going to be tied to that. And that will lead to hormonal differences in development, that are going to change the expression of the same set of genes that both individuals of each sex have in common.
David Puts 24:14
Yeah, that's exactly right. And of course, there are exceptions. And they're well, you know, even in the same species, some traits are, you know, developed because of a different mechanism. But I'd say that's the general idea. And so, you know, what you have in say, humans is an undifferentiated gonad that then can go either way. And that's determined by in humans and other mammals by having an SR YG. And on the Y chromosome, I
Nick Jikomes 24:44
see. So if you have a Y chromosome, you have this particular gene that is going to switch this tissue to go down one developmental path and in this case, become the testes. And if that's not there, it goes down the other developmental path path by default. comes the ovaries. And so can we just explicitly define here? For people that don't know? What are gonads? Oh,
David Puts 25:06
yeah, the testes or ovaries? Okay. Yeah, I mean, I know when when I was a kid, I thought it was just testes, but it's the general general term, just like I believe phallus means clitoris or penis, but it's often used just for penis,
Nick Jikomes 25:21
I see little terminology. And does the analysis does. Is there an analogy here with the gametes? Are there? Are there two good Natl identities? Or are there more than two? Can there be intermediate?
David Puts 25:36
Well, let's see here. Impairment at least, yeah, primates? Yeah, I guess just to I mean, there could be all like, you just we just talked about how a gene on the Y chromosome sets development, one pathway or another, but there, there's all kinds of variation that can occur in. So are you still there? Yes, yes.
Nick Jikomes 26:01
Yeah,
David Puts 26:02
your image went away from it. It went away again. Alright. Is that? Yeah, no, you're good. Continue. Okay. Um, anyway, there's all kinds of variation that can occur, genetic variation, or other things that can cause the development of gonads to, you know, be perhaps be intermediate, but there's not sort of another mode in the distribution, like, you know, there's not a third gamma, third gonad type, or, you know, that produces a third type of gamete, or something like that, no. But there's all kinds of variation in any trait you can imagine. So including, like, you know, I mentioned the SR y gene, this gene on the on the Y chromosome that causes the undifferentiated fetal gonads to become testes. If that gene happens to be trans located over onto an X chromosome, you could have a person with two x's who develops a male a male typical phenotype. Or if it's deleted from a y, you could have a person that has x, y, but has a female typical traits phenotype. So
Nick Jikomes 27:13
the development of the testes in humans is it's really about that SROI gene, which happens to naturally be on the Y chromosome. But if you artificially move it, you can, you can get male typical development even in the absence of a typical sex chromosome configuration. Yeah,
David Puts 27:32
that's right. And, you know, that's sort of the genetic difference between males and females, it starts off a whole cascade of other differences. But like I said, there are lots of things that can vary along the way. Some people have a typical Y chromosome with an SR y gene on it that causes their gonads to become testes. Those testes produce really high androgen levels like, you know, normal male if not higher than male, typical levels, but they don't have Pepin have a functional receptor for testosterone. In other words, the way that sex hormones influence or ganando hormones influences influence development and gene expression is that they circulate around the body in the bloodstream. So it's a way of sort of integrating a whole bunch of developmental processes together, you know, producing hormones because they go everywhere in the body, and then any tissue in the body that has receptors for those hormones, it's a you know, like a protein that it would bind on to then that hormone receptor complex goes into the nucleus of a cell and it influences gene expression up regulates or down regulates various genes. But some people have a non functional androgen receptor. That means they can produce all the testosterone and other androgens, other male hormones, hormones produced more more by males and females, they can produce all that they want, but the message is never received. So the hormone doesn't affect gene expression. So it doesn't affect tissue development. And so there are people like that, who would have what's called complete androgen insensitivity syndrome, where they have testes undescended in the abdominal cavity, a Y chromosome and SR y g and high androgen levels. But they have externally you know, very female, typical appearance and psycho psychologically female, typical and so on. And usually, you know, their condition wouldn't be detected until they, they don't have ovaries, so they don't menstruate, so they don't go through, you know, female puberty and then then they would find out to go to a doctor and find out, you know, what their condition is. But so anyway, there are lots of different variations that can that can occur that don't involve the SROI gene. So
Nick Jikomes 29:37
as development proceeds, the go national identity that you have is going to determine sort of the the ratios and the amounts of the various different hormones and things that are going to be circulating depending on the pattern that one has, that's going to have a cascade of effects in terms of how different tissues and organs develop, and that will ultimately lead to systematically differences between the sexes, the the individuals in the population with the small gamete versus the large gammy? And if there's differences between them that are measurable is that's what we call sexual dimorphism.
David Puts 30:13
Yeah, right. Some people, it's interesting, you know, people sort of vary and what they mean by that term. I think most people mean, just traits that exhibit a statistical sex difference. So it doesn't mean that the distributions don't overlap, but just that there's some, some statistical difference that's measurable. So you can think of like height in humans, you know, there's plenty of overlap. You know, we know lots of women taller than the average guy and guy is shorter than the average female, but there's a, you know, a sex difference that's easy to observe, and you don't need a big group of people to see you, the guys on average are taller than that, than females, with some people have, I think that's the typical definition. But some people have redefined the term recently, I think that's fair to say, redefine the term to mean, there is no overlap between the sexes. You know, I think, what, I don't really care too much, and I like to get bogged down on meanings, you know, just like, let's be clear about what we mean, and define the terms and then just try to understand the phenomenon we're trying to understand. But I don't think that's an especially useful meaning of the word. Because for one thing, how could you ever know that there's no overlap? Unless you've measured every individual in the population? And isn't? Isn't it unlikely that there's always going to be some because there's tons of variation in nature, and all kinds of environmental and genetic things that can cause differences? And so it just seems not not a very useful concept versus just sort of saying, Is there a sex difference? And if so, how much? How big is it? How much overlap is there some sex differences are bigger than others and sexual dimorphism? You know, I would use the word to refer to that. But sometimes I just say sex differences or sexually differentiated traits or something like that, to avoid any confusion.
Nick Jikomes 32:00
And when we think about sex differences and phenotype sexual dimorphism dimorphism, however, we want to talk about it. The the overall level of this does clearly vary between species. So if I think about a primate like a gorilla, it's a very high level of dimorphism. Right? The males are much, much bigger than the females. And the same is true, say, in humans, but to a lesser degree. What, what determines the level of overall sexual dimorphism? And how does that relate to things like population structure or mating strategies?
David Puts 32:34
Now? That's a great question. So I think in answering this question, we have to talk about causality at an on an evolutionary timescale. So, you know, previously, we've been talking about, you know, sex, chromosomes, genes, hormones, and so on. And, of course, that's those causes sex differences in gorilla anatomy in the same way that they do in humans. But if we're talking about what causes the species difference in that, you know, human males are about something like 35 or 40%, heavier in terms of fat free mass, and gorillas are almost like two to one, you know, like 100% larger male gorillas and females. What causes that difference? Almost always, I know of only one exception, and it's a fish. When males evolve larger body size, it's because that in that species, there's an evolutionary history of males using their size against each other, to win mates through what's called contest competition, either fighting or threatening one another, with the potential to fight. And you know, males evolve larger size grasses. Same thing. That's why the biggest female we've talked about blue header, so that's where the biggest female is the one that changes in the male because she's the one most likely to be successful at defending a territory from other males. But that's it I want to say this. The world record for the most sexually dimorphic mammal is Elephant Seals, where males are something like seven to 10 times the size of females in maths depends on whether we're talking about Northern Elephant Seals like come up to California, or southern which is like you know, Antarctica and Chile. That's pales in comparison to the world record for most female in most species, females are bigger than males. It's surprising because we're more familiar with mammals and birds. But if you look across insects where there are lots of insects and and fish and so on, females tend to be bigger than males. And the reason is because of selection for fecundity, they can produce more eggs, bigger eggs and so on. And the record for sexual dimorphism and body size is a deep sea angler fish called a devil fish. Females are 500,000 times the size of males, you wouldn't even recognize if a male is ever lucky. He's basically a swimming pair of gonads. And if he's ever lucky enough to find a female that he fuses to her, loses his internal organs, loses his eyes, and just becomes a pair of gonads on her body. And so you know, that's an extreme. But generally females are bigger, because there's always selection on females to be larger, for fecundity for producing more and bigger eggs and so on. But in some species, there's even stronger selection on males to be large, because successful males can reproduce at the rate of multiple females if they can, you know, acquire, defend whatever from other males, multiple females, like happens in grasses and gorillas and so on. I
Nick Jikomes 35:46
see so so it has to do with the nature of the male male competition for made access. Yep. Yeah, both
David Puts 35:52
things. Yeah, the the nature of a male male competition, whether it's whether males are competing, by attracting females, or by trying to locate females or by producing more sperm or more motile sperm, because you know, the female meets with multiple males at the same time, or whether they're competing through contest competition through the use of force, or threat of force against same sex competitors to win, mate. So number one, is that important to male? Was that important to male mating? Success? ancestrally. And number two, how important you know, like, what's the intensity of sexual selection? How, how big are the reproductive differences between males? You know, Are there lots of males that leave zero offspring? And then some males that you know, reproduce with dozens of females? And the more intense sexual selection? is, the bigger the sex different side of the as well? So
Nick Jikomes 36:41
how does the overall the overall amount of sexual dimorphism? When we look at cross primate species, we look at gorillas, chimpanzees, human beings, where do humans fall into the distribution? Are we relatively low in sexual dimorphism? High in sexual dimorphism? Somewhere in the middle? Yeah.
David Puts 37:03
I would say that we are relative, somewhere in the middle. And that, you know, if you look across primates, in general, in primates, there's a substantial degree of male mating competition. So if you if you said, Well, what's the average primate? Or is sexual selection, not very important, when I think about all the various, you know, saved monkeys that live in multi male, multi female groups, and so on. I mean, the average primate has pretty intense male competition for mate. So saying that we're like, the average primate suggests that there's pretty intense competition among our male ancestors for mating opportunities. But I will say that, you know, maybe especially anthropologists, but other social scientists, as well sort of disagree about the magnitude of the sex difference in body size. And I think one of the reason for that, it shouldn't be like an empirical question, though, where we've got data on millions of people. So why can't we agree on this? Right? I think the reason is that, in non human primates, when when you look or other animals, when you look at sex differences in say, body mass, normally, what you look at is just overall mass, you know, what's the weight of a male? What's the weight of a female, or the massive amount of massive female? And if you look at that, and humans, then you see that males are only maybe 15 to 20% heavier than females on average. And that's like, kind of in the middle between a monogamous species. And something with the species like say, chimps, where males are pretty aggressively fight each other for mates. However, other scientists studying this have said, Yeah, but that's not and it's kind of comparing apples and oranges because other primates don't have these huge sex differences in body composition, like humans do. human females have 40% More, more fat. And yeah, we don't know exactly why may have to do a sexual selection on females that kind of like that idea, but also, you know, providing resources for a big brain baby for a long time, you know, through gestation and lactation and sort of getting over the, the troughs and resource availability need to have it stored up on your body.
Nick Jikomes 39:21
I see. So this is this is interesting. So, you know, when we when we talk about sexual dimorphism, we can it's convenient, maybe to sometimes think about it and overall, and it's very easy to just look at the sort of most visually salient thing, which is just how big or massive the males are versus the females. But individual traits can go in either direction when you're comparing two different species. So So you just said, we are less sexually dimorphic than chimps and other apes in terms of mass, we just measure how many grams everyone weighs. But at the same time, we're more dimorphic in terms of body fat composition, like fat versus muscle composition. More
David Puts 40:00
More dimorphic by far the most of you know, we don't have great data on body composition on a lot of primate species. And I published a chapter with some colleagues recently, where we, you know, I went online and I found every primate that I could, where, where we there was information on male and female, fat free mass, fat, mass, and so on. And I only was able to locate maybe a dozen species. But of those we are far and away the most sexually dimorphic and fat mass. And so if you take that, and most primates are not highly sexualized, horrific and body fat, and so if you look at the mass that would be most relevant to our male ancestors competing for mate, which mates, which is really muscle mass, but let's just say we don't have that, but we do have fat free mass or lean body mass, then you look at that, then males are then humans are more sexually dimorphic than chimps. Oh, I guess I actually can only find data on the bonobos, sometimes called pygmy chimps. But they're probably about the same as as common chimps. So a little bit more section and morphic. But anyway, well outside the range of a monogamous species and in in with a species where males have, you know, established dominance hierarchies and pretty intense mating competition.
Nick Jikomes 41:25
So So, so if you look at overall mass, we are not as dimorphic a species that have pretty intense male male direct competition, and how you look at fat free mass, like muscle mass, essentially, we are that dimorphic we
David Puts 41:40
are and still considerably less than more fit than say gorillas, and, you know, orangutans, but hamadryas baboons. But yeah, more more slightly more than chimps. And that also accords with our skeletal, you know, if you sort of estimated how much bigger males would be than females for normal primate, based on our skeletons, then we look like a species where males are something like 45%, bigger than females as well. So
Nick Jikomes 42:11
what are some other human traits where we are? Especially sexually dimorphic?
David Puts 42:17
Good question. Well, you're illustrating one with the, the beard. So it, you know, that's the kind of a I think we kind of can take some of these traits for granted sometimes, like, what is what is up with growing hair, from our faces all the time. And, and so that's one and voice, you know, my, my lab has used large sex differences and voice pitch. So there's no there's not. I just give you one example, when I was a postdoc in neuroscience at Michigan State and a big sort of psychology study, and recruited over 600 people. And the average speaking fundamental frequency, like the average speaking voice pitch, for every male was higher than every female, out so lower than every female there. In other words, there's no overlap in voice pitch between males and females and over 600 young adults. So that's a really big sex difference that emerges at sexual maturity, you know, pre pubertal boys and girls don't differ in voice pitch, it just happens that when you know, the testes start producing lots of testosterone, then all their various target tissues are affected. And that includes the vocal folds, it makes the vocal folds grow much longer and thicker. You know, males are about seven to 8% taller than females in humans. And so you'd expect vocal folds in males, if it just scales with body size that males vocal folds should be like seven or 8% longer, but they're like 60% longer almost, you know, 10 times isn't as big of a sex differences you'd predict based on the difference in body size. And so as a consequence of some of some of these facts, and you know, traits that are favored by sexual selection, like say, deer's antlers, and peacocks tail feathers, they tend to emerge at sexual maturity, when they can start paying for their costs by helping when mating opportunities. And so I thought about voices, you know, it seems like a trait that could well be related to male mating success and favored by sexual selection and so that we've used that it's also eminently quantifiable, you know, you can just record people's voices and you know, voice pitch is just like one, one number, it's, you know, what's the rate of vibrations per second, we're opening openings and closings of the vocal folds per second, or Hertz. And so it's been a really useful model trait for us to study sexual selection in humans, and that's highly sexual dimorphic. You know, muscle mass as we talked about, there are lots of behavioral sex differences as well. In psychological one's sexual orientation. You know, there's very little there's overlap, but by far most males are attracted to females. And vice versa, you know, that's one of the most sort of psychologically largest sex differences in humans.
Nick Jikomes 45:05
I see. And so in terms of speech is interesting, or vocalization is interesting for a number of reasons. Because we have speech, we can talk and use language in ways that other primates can't. But there's all these non linguistic elements like like pitch, and just sort of the pattern of auditory stimulus that you're omitting what, you know, what are how do lat non linguistic elements of speech in the human voice like pitch? How do they play into how individuals perceive the opposite sex? So what are the specific features of the male voice that females tend to evaluate when they're doing mate selection? And vice versa?
David Puts 45:45
Yeah, that's a good question. The the work that we've done is mostly focused on the sexually differentiated or sexually dimorphic parts of the voice. So voice pitch, and also timbre, which has to do with the resonant frequencies of the vocal tract. So you can think of sort of voice production as being well, we start to produce the sound. By pushing air from our lungs past the vocal folds are sometimes called the vocal cords in the lyrics, and that causes them to vibrate. And that's the source of the sound vibrating vocal folds. And so longer, thicker. vocal folds vibrate more slowly. And if you've ever looked in a piano, you know, the longer thicker strings are the ones with that are associated with lower notes, right. And so that's the source of the sound. And then the vocal tract above that determines what are called formant frequencies, that the resonant frequencies in males also have a vocal tract that's about 15% longer than females. So again, about twice as big of a difference, as you predicted, based on the differences in body size. And so that makes a voice sound sort of richer and fuller. And, and that affects how people perceive the sort of dominant status. Threat potential. I think that, you know, those sex differences probably evolved, you know, 10s of millions of years ago and in primates is a way of intimidating other males that males sort of exaggerate their size, acoustically by producing low pitched and low timbre. vocalizations. And we've done some cross species research in primates to suggest that females prefer lower pitch, but not too low. And they don't care as much, it doesn't affect female preferences as much as it does. Males impressions of the sort of threat potential or status of another male. And then experiments, males prefer higher pitched, and higher timbre female voices, although timber, that's when you manipulate things, but I'm a little worried about studies like that sometimes, because I think when you've manipulated an acoustic parameter, and nothing else has changed, then listeners kind of just focus on that. And then you can get an effect that's actually much bigger than it would be in nature, when all these other things vary among, you know, across voices. Because when we've looked at a large sample of female voices that weren't manipulated at all, and we just had males rate them on how attractive they are. And then we said how important is how much does pitch or, you know, influence ratings of attractiveness? It was didn't it didn't matter. Statistically timbre did you guys prefer to sort of smaller sounding or shorting shorter sounding vocal tract, but pitch actually had no, no offense?
Nick Jikomes 48:40
You said you said there are there are things like female preferences for lower pitches, but up to a point, but you said it sounded like you said there's a bigger effect in terms of how males perceive other males based on the how low their voice is basically, to what extent is that vocalization, like how low your voice is the pitch of your voice, a reliable indicator of something like how threatening you are or how dangerous you are or where you fall in a social status hierarchy. Another way of asking that question would be, if males, you know, whether we're talking about humans or other primates, if a male moves positions in a status hierarchy, Will his voice reliably change in one direction?
David Puts 49:18
Ooh, that's great. Yeah. So there are two. Two answers to your initial question, which was sort of how accurate are assessments made from these acoustic parameters? And then the other one is does voice change as a function of status? What's the I'll ask the I'll answer the second one first, because it's maybe a little bit quicker. There is some evidence that that does happen across contexts and the same individual. Say, with some colleagues of mine, I probably I was a little author on this. published a paper a few years ago, showing that people tended to lower their pitch when they were sort of an authority. And, you know, they were knowledgeable about something. And when they weren't, that was it was a higher, higher pitch. And I think that that does happen across social context that people raise their pitch, when they are when they feel in control or in authority, and they sorry, lower it when they feel in control or authority, and they raise it to sort of single deference. And I don't think that's probably almost never a conscious thing. I think that when you're nervous, that produces tension on the vocal folds. And that's something that was adaptive in our ancestors, that it doesn't make sense to, you know, it can be costly to signal dominance in a situation where you're not dominant, you know, better to signal deference. And of course, we can sort of volitionally Learn to control those things. But then there's another sort of question in there about between individual differences. I think maybe we also asked you about that that like, say, are males with lower pitch? Are they? Do they have characteristics that would make them more dominant or something? Or were you asking about that?
Nick Jikomes 51:07
Yeah, I think that's related to the question of, you know, if you if you're a guy with a really low voice, is that actually statistically going to be related to the probability that he is, you know, relatively high in the Status hierarchy? Yeah, yeah. That
David Puts 51:19
that's something that we and others have been working on quite a bit lately. Because we recognize it's an important question like, why would you defer to a male with a deep voice or a low pitched voice? And give up all and have all these costs of differing like, because, you know, the other studies have shown like males with low pitch, have more mating opportunities, some studies find more offspring, more resources, you know, like higher incomes, and so on? Like, why would you give up all those things to male in deference to males with deep voices unless there's actually some kernel of truth and a low, deep voice? And the answer is that probably the biggest thing that a deep voice signals his sexual maturity and males, that it's you've gone from pre pubertal, to going through puberty. And so in that sense, it's strongly associated with physical from inability, because there's a huge difference between an adult male and everybody else, in terms of, you know, size, strength, aggression, and so on. And so, in that regard, if you sort of look across all humans, then a low voice pitch is a very good indicator of the threat potential of that individual. But then if you look, among adult males, the relationships are weaker, there is an association with body size, taller males have lower pitched voices, but it's not a strong association. There are associations with very weak one with strength, that with testosterone is a bit stronger, that lower testosterone is associated with higher voice and vice versa. But, but this relationships are certainly weaker, at least in the studies that have been done. I don't know how, you know, you can imagine that in the evolution. Yeah, sorry, we're gonna follow that up. But okay, well, I just thinking, most of the studies that have looked at this, though, are in societies with good access to resources, and healthcare, and so on. And if voice pitch in adult males is associated with things like testosterone, and higher testosterone, lower pitch, or cortisol, which is a stress hormone, you can imagine that in our ancestors and populations where healthcare, you know, great modern healthcare wasn't available, people were much more at the mercy of their own immune systems and staying healthy resources were less reliable, that there might have been sort of bigger differences in some of these sort of health related characteristics that would affect hormone levels that would affect pitch. So it's possible that ancestrally there was a bit stronger relationship between pitch and somebody's sort of overall health, vigor and ferment ability than you would see in a in, say, the modern US where everybody's, you know, relatively high in health.
Nick Jikomes 54:12
You know, one of the things that we've mentioned, or that's been implicit in some of the stuff we've been discussing is how sort of mate access and mate choice relates to individuals perceived position and a social status hierarchy or dominance hierarchy. So what exactly are social status hierarchies and social primates? And what are some of the main factors that determine their structure? And, you know, I think it could be helpful to talk about maybe species differences here.
David Puts 54:39
Yeah, yeah, I think that those are really relevant because in your typical mammal, then, you know, position in a dominance hierarchy, pretty much just has to do with a couple of things. One, your history of success and fighting other same sex competitor errors, and possibly an often your ability to oppose and to pose an apparent threat. So, you know, there are species like say red deer, where both males would prefer not to fight because that's costly. And so they roared each other, they, you know, go side to side, and they show how big they are and they roar. And then that often results in one saying, okay, you've got a lot more energy than I do. I differ. And there's no fight that happens. But so in other mammals that it's sort of fighting or threat. And then in social primates, there's that but then if you look at say, our closest living relatives, chimpanzees, they're also coalition's and so you can be successful against a more physically formidable, bigger, stronger male, if you have allies. And so I think that something like that is probably characterized. It's hard to say but uh, probably all of all of hominid evolution for the past 7 million years but you know, the fossil record is sort of unclear one indicator would be how sexually dimorphic were our ancestors and body size. But in humans anyway, for sure, there's that you know, human males are not like elephant seals that you know, they're unabated Lee at one another's throats during the mating season, you know, there's low frequency in compared to chimps, there's much lower frequencies of male fighting and humans than in chimpanzees. There's a lot more sort of deference according to information you have about the other individual, there's coalition forming, and and then there's also a sort of another avenue to social status called prestige. It's freely conferred deference that, you know, it's not based on fighting ability. It's not based on the other males ability to hurt you physically, but rather, things that they can offer like leadership and knowledge and skills and that sort of thing, that we value those in our societies too. And so we afford status and give deference to some individuals based on prestige that doesn't have necessarily anything to do with their their fighting ability or the threat potential. Yeah.
Nick Jikomes 57:16
So what would be the common denominator status, you know, if you're going to integrate this across, elephant seals, gorillas, chimps and humans, so obviously, elephant seals, they don't care if you've got, they don't give each other degrees from prestigious institutions. It's all about size, and then fighting physical fighting ability, human beings, you know, we've got all of these other markers of status and prestige. But across species, what is that what is the high status exchanged for what's the, the ultimate thing here?
David Puts 57:48
resources relevant to reproduction. And so that could be mates in species where members of one sex compete for mates. But in most primate species, females, for example, might have status hierarchies, they may have dominance hierarchies, but their reproductive success is not limited by access to males, one male, they can't, a female that has 10 males cannot reproduce at a higher rate than a female with one male. So they're not competing so much for mates, but they are competing for resources for themselves and their offspring, females are using their bodies to turn resources into offspring. And so, you know, that's what females primates are competing over. But that said, it's, you know, access to reproductively relevant resources, whatever those happened to be.
Nick Jikomes 58:30
I see. And so I think you just touched on to an important difference between males and females, the small gamete, sex or morph and the big gametes extra morph, you know, if a female so you said a female with access to one male can't. But with 10 access to 10, males can't reproduce 10 times faster than then a female that just has access to one, because she's sort of the rate she's the rate limiting step in terms of reproductive output. So it would make sense they're not competing for the number of males, but they would be competing for the quality of the male in terms of the reproductively relevant resources he could provide.
David Puts 59:09
Yeah, absolutely. If, if what males are providing is something that there is competition over that means that the use of that characteristic by one female makes it at least partially unavailable to others. So if it's sperm, you know that like some males are higher in genetic quality than others than females might not be competing very much for those males because that male can inseminate all the females in the group and there's not a an issue with you know, many, the one female meeting with this male makes that male sperm unavailable, but others, but if males are doing something like providing parental care or resources or something like that, and males differ in their ability and willingness to provide those things then females could compete over, over those over man over mates. And it will say also that it's not always I just want to sort of make This little tweak, because I think it's interesting. It's not always the sex with the smaller gamete competing for mates. Because in some species like say, spotted sandpipers, females, reproductive success is more dependent upon their access to mates than males. And the reason why is because the female lace or the male makes the nest, the female lays her eggs in the male's nest. The male incubates the eggs and cares for the offspring, and the female then can go off and mate with another male. And so in spotted sandpipers, females, you know, a female with three mates leaves about three times as many offspring per season as a female with one mate. And so in spotted sandpipers, females are limited by their access to mates. And as a consequence, they're bigger than males and more competitive and for mates and more aggressive than male. So there are some species like that where the speed the sex producing the bigger gametes, females is actually their reproductive success is more strongly tied to mates than males. But mostly it's the other way around.
Nick Jikomes 1:01:03
Yes, and how does how do things like? So we can think across species here, but you could also just to think across present day and historical human cultures, you have different meeting systems or meeting cultures that emerge and stabilize, you know, sometimes some cultures are largely monogamous. Sometimes they're large, largely polygynous. So what does that have to do with this? What Why do different meeting? I don't know what you call that meeting systems or meeting architectures. Why do they emerge? And what does that have to do with things like meat availability, and and population structure?
David Puts 1:01:42
That's a really good question. I don't think anybody really agrees on why these different men, I guess, you'd really probably call them like you're meeting about marriage systems or something, because, you know, that's pretty much a human universal that humans engage in long term relationships that are sexual slash romantic, and involve, sort of, they're socially agreed upon. And they pretty much involve people agreeing that these people are allowed to have sex with each other, but other people are not if they're not in the marriage. And so whenever that's sort of marriage in a nutshell, I suppose, and that those things differ like some societies have. Most societies described by anthropologist allow polygynous marriage because most societies are small scale societies that were not made.
Nick Jikomes 1:02:42
And just to find that for people so polygyny would be one male with multiple wives,
David Puts 1:02:47
one male with multiple wives, and which means that given a sex ratio is about the same number of males as females, that that means adult males are also more likely to be unmarried. There are other males out there with no no mates at all. In you know, the levels of polygynous marriage across societies are Whoa, you know that. Even in a polygamous society with polygynous marriage, most marriages are monogamous. Other societies allow only monogamous marriage. A small number of societies also have polyandrous marriage when female married to multiple guys, they're often brothers, usually brothers. And so there are two issues here. One how those different marriage patterns develop. And that's not clear. You know, probably things like religion, and have have a lot to do with it. Another one is, how does that affect the intensity of meaning competition? And the answer to that one is probably not a whole lot. And the reason is that even in monogamous societies, societies that only allow monogamous marriage, the mating system is sort of effectively polygynous because males are more likely to divorce and remarry. When they remarry, it's usually a younger spouse than their previous one. And they're more likely than to have to reproduce again with a second spouse. And so it's really just sort of like polygyny, but expanded over time rather than, you know, single time, in
Nick Jikomes 1:04:17
terms of in terms of had never occurred to me in terms of sort of reproductive output, or structure here. serial monogamy is more like, Yeah,
David Puts 1:04:30
that's right. De facto or something. Yeah. And so in the studies that have looked at say, typically, when you have polygyny, then you have bigger variation and male reproductive success because some males are leaving lots of offspring and other males are leaving no offspring. And so there's more variation among males in their reproductive output. And when you compare that those variances across monogamous and polygynous societies, there really is not any systematic difference. You get big differences in May I was having big variation and reproductive success in monogamous societies as well. So
Nick Jikomes 1:05:08
interesting. And so what about like species differences here, so, so humans sort of have this extra layer of, of culture and you know, formal taboos on things like, like marriages, and you know how people relate to each other sexually. But even in non human primates, you have species that are more or less likely to form long term pair bonds. And so, in non non human mammals, why does something like pair bonding, in the long term evolve, what kind of population structures or whatever influence that
David Puts 1:05:51
there are, this is getting slightly outside of my expertise, but I offer some some informed speculation. So, there are some species like say Gibbons and Simon's where monogamy seems to be about defending a mate from competitors. And in those species, both males and females can defend their mate. And so both sexes call, like, call, they sing. And they sometimes do duets. And both sexes seem to be sort of advertising to their same sex competitors. I'm here, so you can stay away. And that defends me. So that results in monogamous mating because they're each individually scaring away the, you know, other members of their sex. In some South American monkeys that have either monogamous or even polyandrous meeting. Females are able to most primates, few females give birth to Singleton's they don't we're not like, you know, pigs where you can have, you know, whatever, I don't know how many offspring they have in a typical litter 810 12 Something a lot. We don't have litters, we have one. But in some South American monkeys, females give birth to twins. And that requires a lot of parental care. And so, you know, a male can do well, by tying himself to a single female and helping care for the offspring, because she can produce a lot of offspring. And so, you know, for males, it's like, Well, should I, you know, not invest in this mate and her offspring and reenter the mating pool and try to find another mate, or should I stick here and put my reproductive effort into caring for this mate, and their offspring will a female and when a female can produce kind of a lot of offspring, then maybe sticking around is a better strategy? And so the I think that that's probably the the two sorts of types of monogamy that evolved in primates, but yeah, I would defer to any one of my primatologist colleagues on that one.
Nick Jikomes 1:07:59
I see. But the level of parental care required and and how that's related to things like the litter size, the number of offspring there had at one time, that would that does seem like it's a factor.
David Puts 1:08:10
Yeah, it does. And also sort of the degree to which males can do something, you know. And so in birds 90% of birds are have at least pass around birds have social monogamy, where both sexes make the nest together, both sexes incubate the eggs, and when the eggs hatch, both sexes go off, and get, you know, invertebrates and feed them to the nestlings. But those are species where both sexes can do those activities, you know, that it's not like one sex would be better than the other. And so a male can increase his own reproductive success by increasing his mates reproduction. But in mammals, that's much harder, because, you know, in most mammals are have internal gestation, right. And that's something that males can't do. And then all mammals produce milk. And that's something that males can do. And so then everything is sort of biased against males being able to invest in offspring, in mammals, but there's some cases where you just need a parent to care for, you know, the carry the offspring around or something like in, you know, marmosets and tamarins, or in humans, and I think, you know, the not enough has been written about this, but certainly it's been discussed before, that there was a point in hominid evolution in which human males can invest and could really carry a big share of the load compared to other primates, and that is with hunting, which evolved somewhere around sort of one and a half to 2 million years ago and Homo erectus probably, that we went from a typical, great ape that ate a lot of plants and some meat to being a primate that ate a ton of meat. And now all of a sudden, you can get big packets of calories, protein fats. And that's something that a male could go off and acquire and provide to his made an offspring and increase her reproductive success and, and the survival of his offspring as a result. And I think that was a major turning point in human evolution where, you know, probably not only decreased male competition for mates, but greatly increased male investment and mates and offspring and had other ramifying consequences that I'm happy to talk about to like, concealed ovulation, I suspect was that Strassman. So I came up with this idea one time, I thought, this is brilliant. I'm going to write a paper on it. And then I did a little more reading and realize that Beverly Strassman, had published it 30 years earlier. And for some reason, the idea just wasn't, you know, favorite at the time. But this idea that females suppressed cues to ovulation and the human lineage as a way of obtaining male investment, enabling some males to invest in them.
Nick Jikomes 1:10:57
Yeah, I know. Let's talk about that. Because I did want to bring that up. Because one of the ways that we differ from a lot of primates, I believe that we, I believe most primates, right, like chimpanzees and lots of other species, when the females ovulate, not only do they basically, they are only interested in sex when they're ovulating. So they're really not receptive much at all, outside of that window of their cycle. And too many of them I think, literally, like visually display that they're ovulating. So there's this very, very clear signs that the males can literally see that, okay, this female is ovulating, and therefore, it's going to be worth my time to try and get her attention now. But humans don't do that. So can you unpack that for us? What is concealed ovulation and why? Why would that even be a thing? Yeah.
David Puts 1:11:47
So in group living primates, females often advertise estrus, that is the fertile time around ovulation with genital swellings. And I wish I had some pictures to show you right now. I mean, I do I could like pull them up or something. But I'll just, you know, wax poetic here. But the Yeah, the genitals swell up to, you know, amazing, conspicuous proportions. And that happens in chimps. It happens in lots of group living monkeys. And it's a signal. You're right, females are more sexually interested. They're more receptive, and they solicit copulations around ovulation. And they also produce these signals that then males can tell, roughly the time I mean, the genital swellings are usually start before the fertile point in the cycle and continue afterwards. But it's at least it's a window where males can tell this is around when the female is ovulating. And in some speed, it varies across species. And so like in gorillas, so chimps females have big general swellings and gorillas, not really. But apparently, you can tell even a human you know, is can tell when a female is near ovulation and, and gorillas, because there are some changes in coloration and appearance and that sort of thing. But in humans, it's really like we suppressed cused ovulation is not just that there wasn't strong selection to produce this big advertisement. It's like, there was selection to cover up any possible cue. And I think the reason why that happened in humans, when you look at chimps, and you see which males mate with fertile females, when females are in estrus, and there they have these big generals of the alpha male, that is the most dominant male in the group, monopolizes copulations. The whole fertile part of the cycle and even the day before and the day after. Okay, so for females, that's okay, because males are not providing anything other than sperm. And so if that you let the males duke it out, the most successful the most vigorous males, the males with the highest genetic quality we're going to be tend to be the ones that achieve highest status. And so sure, mate with that male and your offspring will have those genes can be healthier. But, you know, once you know, in humans, males can provide and did provide more than just DNA to offspring, but say, could provide resources through hunting and child care potentially in other things, then females have an interest in not allowing that alpha male to bully his weigh in when they're fertile. If you if you're if you have a male who would be seen as subordinate male, who would be willing to invest in you and your offspring, you have to facilitate that by not allowing the dominant male to fertilize you and your fertile because then why would the subordinate male invest? It's not his offspring anymore. And so you know, this is stress man's idea, but the idea is that concealing ovulation from everybody makes it possible for a long term mate to invest in his own offspring and that means that those males could profitably go out and collect resources and provide them to the main an offspring and be relatively assured that the offspring were going to be the males own offspring rather than some dominant male. So anyway, that's that's the idea. And it makes a whole lot of sense to me. So I would think that, you know, sometime, after the evolution of our own genus Homo, you know, probably an homage erectus that Aveo Tory Q has started becoming suppressed, you know, a million and a half or 2 million years ago. Yeah.
Nick Jikomes 1:15:28
So I would imagine that that is plausibly related to a number of things in our lineage, you know, one would be, you know, what you said around hunting. So, as soon as we developed the cognitive and the cognitive capacity, and the dexterity and so forth to do things, like, make hunting tools and go out and, and kill animals reliably, you know, we now had a much larger, much more reliable, high density calorie store that an individual who's good at that could build up. And so you know, you could provide that to a female in her offspring. But also, you know, as, you know, as our lineage evolved in certain ways, such as, you know, just becoming smarter and having a longer lifespan and having a longer childhood, that required a lot of input, it probably became, at some point impossible to do that, or very, very difficult to do that, with only one parent doing the lion's share of the child care early in life. And so as our lineage went down that way, would it make sense to me, that you, you need to have to parents investing a significant degree. And that seems like it would match up with the idea that you'd want to conceal ovulation so that you're not, you're not merely advertising fertility to the most aggressive and plausibly the most aggressive male who may or may not be willing to invest anything other than sperm?
David Puts 1:16:56
Yeah, I think you're right. And but there's quite a bit of variability in the extent to which males invest in their mates and kids across societies as well. So it's really hard to kind of, you know, I, I just relayed the story about what our ancestors were like, and what, you know, meeting structure, and, you know, a single male and a female, investing in their offspring together. And it's hard not to think that that was an important part of our evolution, but you also see a ton of variability across societies, in sort of males involvement, Kin in their offspring, you know. And so there's a lot of research on what's called Allo, mothering that, you know, other females sort of help each other in the group care for babies and, you know, hold your baby, while you need some time to do that something else. And so, I say,
Nick Jikomes 1:17:45
that is a level of male parental investment, does that correlate at all or map potente degree to things like, the climate as it relates to say, food availability. So for example, I can imagine that if you're living in a climate where you can, you can grow stuff very easily year round, it's very easy to take care of a child compared to say, someone living in a climate where you have to go out and, you know, go into the tundra, and hike for miles and hunt an animal. And that's, that's gonna make it very, very difficult to get enough food for to raise a child or something like that. Yeah,
David Puts 1:18:29
I'm sure it's related to not just to sort of the importance of having another parent caring for the offspring. And the extent to which, yeah, the extent to which males can actually contribute in meaningful ways to the offspring survival and reproduction, and for the females, you know, reproductive success. And on the other hand, males ability to increase their reproductive success by having more mates, you know, so if they're, if you're in a society where a male can have a lot more offspring by marrying polygynous, Lee, then, you know, that's going to tend to happen more often, and males will, consequently tend to invest less in each female and her offspring. And so both of those things play a role, no doubt.
Nick Jikomes 1:19:19
So in our lineage in humans, there are not there are very few visible signs of where a woman is at in her operatory cycle. It's not nearly as conspicuous as other primates. Are there zero signs? Or are there some physiological indicators that
David Puts 1:19:39
are reliable? Yeah. There are physiological unreliable, let's say physical, physiological indicators. As far as we know, and so it first there is no debate that there are huge changes in ovarian hormones over the cycle, you know that well like it the after menstruation at the beginning of the cycle, the follicular phase of the cycle. Everything all estrogens and progesterone progestogens, like progesterone are low. And then as ovulation nears, estrogen goes away up. And then ovulation happens and estrogen goes way down. And progesterone goes way up into in the luteal phase of the cycle. And then in the middle of that, estrogens go up anyway, big changes over the cycle and hormones and have, you know, it's not surprising that those should influence aspects of the body that might be observable. And so we've observed, we've seen this in, you know, carefully designed studies with a good sized sample. Looking at the same women at different times in the cycle, we found that voices were more attractive when estrogen was higher relative to progesterone, suggesting the fertile part of the cycle faces were also more attractive at that time. And we're interested in looking we haven't found like what acoustic parameters are causing these changes. And I'm not sure about what changes in the face either. I mean, some things that change over the cycle are things like skin redness, acne, oiliness, and things like that, that might that might play a role that other people have found. But you have to do some and maybe some behavioral changes as well. So there do seem to be some changes. But you know, there's current debate about I don't look like signals to me that it looks like leakage of information, as Steve Gaines did. Randy Thornhill wrote in a 2008 paper that it's, you know, sort of, there are changes over the cycle on hormones necessarily that involve, you know, ovulation and growing the endometrium of the uterus and things like that. And those big hormonal changes have some effects that are visible.
Nick Jikomes 1:21:50
Yeah, but there's no, there's no, there's no drive to make these things conspicuous.
David Puts 1:21:54
It seems like not I mean, when you compare, it's always useful to have this sort of cross species perspective, because, you know, I feel like some social scientists don't have much of a broad knowledge about other species. And so they might conduct studies like this and say, Look, see if there are changes, that's a signal. But if you compare us to non human primates, it just looks like these are very subtle cues that are very subtle. Yeah, that probably reflect you suppression of cues rather than, you know,
Nick Jikomes 1:22:24
signals. I could be wrong, but that's what it looks like. And does human female sexual interest generally, or interest in particular, traits in males change across the the avatar ambulatory cycle
David Puts 1:22:40
for sexual interest? Yes. And for especially interest in uncommitted sex. Now, two studies have shown, one from our lab and one from another lab have shown that, that peaks around mid cycle around ovulation, they're not huge differences. You know, again, like it's not something that you just easily notice looking around. It's not like a female, chimp who all of a sudden, she's really interested in, you know, receptive and soliciting copulations. And stuff, it's a subtle change, but detectable. It's hard to imagine that these changes in sexual interest, don't also lead to or correlate with changes in the kinds of characteristics that females are interested in. If you're more sexually interested at midcycle, than wouldn't you be more interested in traits that are sexually attractive, versus other traits outside of that fertile window. But I will say that the literature on that on sort of mate preferences changing over the cycle is quite murky at this point. And a lot of the you know, if you asked me 10 years ago, I would have said, and I did say, you know, in public talks, you know, this the, you know, women's preferences change over the cycle so that they prefer more masculine males, at you know, near ovulation, because at the time, the literature really including stuff from from, you know, our own lab and stuff that I published as a grad student, part of my dissertation, really struck, you know, suggested that it was many convergent lines of evidence, but since then, better design bigger studies, including ones that we've done and been collaborated with other labs are not seeing those changes. So now I kind of, you know, throw my hands up a bit and say, well, it's, there are some studies that suggest that it's easy to imagine that there could be those changes, but the evidence isn't great.
Nick Jikomes 1:24:31
Yeah. So okay, so there's not a lot of clear cut stuff here. Yep. Not
David Puts 1:24:35
not unmeet preferences, but I think the data are, are substantially better on sort of sexual interest changing over the cycle. And again, though, they're not huge changes. And,
Nick Jikomes 1:24:45
you know, sort of implicit in most of our discussion is, is that we're talking about heterosexual mating. But obviously, especially in humans, you have a variety of sexual orientation. So you've males prefer females, obviously. But you also have males for males. And same same thing on the female side. So where does sexual orientation even come from? What determines it? Is a lot of this stuff determined genetically, prenatally perinatally, very early in development, and what are the factors at play here? What do we know about the biological basis for sexual orientation in humans?
David Puts 1:25:25
That's a great question. So first, I'd want to sort of define what we're talking about by sexual orientation. Because, you know, you could mean people's identity, you know, you say you're straight or gay, lesbian, bi, whatever. Or the talking about behavior. And what I think probably most people studying sexual orientation, usually made, at least what I'm going to refer to is attractions fantasies, that sort of thing. Who do you want to have sex with? So you do you want to have sex only with males only with females, both more equally? Up? So in terms of that, are you more Andrew philic attracted to males are gynae philic attracted to females? Most almost all of the human variation, and that probably comes from early, going out on hormones, and mainly testosterone, that there's just too many converging lines of evidence, my my co authors, and I just published a review paper, you know, like a month or two ago on this, and we just looked at sort of all the different lines of evidence from various endocrine conditions and so on biomarkers of early hormone exposure. And it just seems strongly that the reason why the average male is attracted to females, and the average female is attracted to males is because of early testosterone exposure, that that organizes that regulates patterns of gene expression in the developing brain to make a brain that is more attracted to one sex versus the other. That's the sex difference. But then there's within sex variation, like Yeah, but what are the causes of the differences between gay and straight males or between, you know, straight women and lesbians, and the evidence there is a lot murkier. I don't think that overall differences in sex hormone action going on a hormone action account for much of the variation among males and sexual orientation. There are other factors, potentially, that we can talk about. And for females, I also think that not a lot of the variation among them, is due to differences in gonadal hormones. But but more it's the evidence is a little bit better within females that that, you know, sort of exposure to elevated androgen action like testosterone action, early in development prenatally, let's say that that results in more gyno philia more attraction to females, the evidence is a little bit better in females. But yeah, I think that's most of the variation is sex variation, whether you're a male or female that determines, you know, that's strongly related to whether you're attracted to males or females. And that difference is driven by androgens probably, mostly, maybe estrogens can talk about that, too. But then the within sex variation has less to do with with hormones. Okay,
Nick Jikomes 1:28:13
what do we know what it has to do with? Well,
David Puts 1:28:18
which sex to me to start with? Let's start with females about there. There are various sort of biomarker studies that, say, looking at anatomical characteristics that suggest that lesbians on average had higher androgen action prenatally. So that suggests that some of the variation within females is due to early sex hormone exposure. For males, maybe there's some research that's been published recently that suggests that it's not helpful to treat all say, gay males as monolithic, that they're different. You say you could separate them on the role that they like to play and sex, whether they're more of a player receptive role or bottom or, or penetrating role or top, and that there are differences among them, and that you could actually explain some of the differences by early sex hormone exposure. And so that past attempts to look have had been looking at it wrong because they were treating, you know, all males that were say attracted to males as the same group. But there's other evidence that some of the variation has to do with like, maternal immune system response to, to male male proteins within moms carry us on. There are proteins that are produced because males and females have some different DNA. Males have a Y chromosome. There are some genes on the Y chromosome Produced proteins, that the mom's body, if she's exposed to them, would she, they, her body would recognize that as a foreign protein. And her immune system would melt the response to it say that's not mine, I'm going to produce antibodies to it so that I can not that immune the typical immune response that all of our bodies have to foreign proteins, and that if moms carry more sons, then they'll be exposed more often or, or to higher levels of male proteins, and they could have an immune response that could then affect the neural development of subsequent sons. And there's some evidence that that's the case, then moms who have who have given birth to more sons are more likely to have leader born sons are more likely to be gay. And even some evidence that moms who have who have gay sons that have had more previous males that they have a higher levels of a, of a protein that's made on the on the Y chromosome, so that, you know, that explains some of the variation may be about a quarter of the variation in male sexual orientation,
Nick Jikomes 1:31:01
quarter the army, so that would be fairly substantial amount of variation explained.
David Puts 1:31:06
Yeah, actually, it would be a substantial I guess, I think I misspoke, because it's a quarter of the cases of gay males. Is that I guess that's not quite the same thing anyway. So if you if you think like one of the predictors of a male being attracted to females or male of the males attracted other males about maybe a fifth to a quarter can be attributed to the so called birth order effect fraternal birth order. Interesting.
Nick Jikomes 1:31:36
So so there's a correlation between how many sons a woman has had, and the probability of a son being homosexual later in life? Yeah, so more have more sons as positively correlated that. Yeah,
David Puts 1:31:51
and the least the last estimate, I knew of each subsequent sign increased, each, each son increases the subsequent son's chance of being gay by about a third of the base rate, can so if you think of a male has about male random in the population has about a 3% chance of being gay, then if he has one older brother, it's a third of that. So he has a 4% chance and then two other brothers a 5%. So you need like, you know, whatever it is 48 older brothers to probably be gay. It's not like, right, right. Right. It's a lot of older brother doesn't happen. Yeah.
Nick Jikomes 1:32:27
Interesting. And so what like, what do we know about? You know, are there any is is the prenatal or perinatal environment, the hormonal environment that that a woman has in her uterus while she is pregnant? Has that been changing over time in any systematic way? Like, there's been a lot of chatter recently about, like things in our diet in the environment that are having, say, estrogenic versus androgenic. Effects is, is our other environmental factors that we know are making hormonal environments in utero, say, more estrogenic or androgenic, or anything like that.
David Puts 1:33:10
I didn't see where you're going with that. I get it. Yeah, no doubt. And I mean, that's something that's not unique to humans, like there are others a bat species where males, sometimes lactate. And it's not that that's because the fruit they eat has high estrogens. And so I mean, it's, you know, a probably across species, their environmental hormones that have similar effects in the body once they get in there. And so that seems to be the case and people too, but I don't really know, you know, the extent of it. And I don't think that the data are very good on sort of how how big these effects are, and what what consequences they're having. But I also don't know that literature very well. So I don't know how expertly I can, I can answer that question.
Nick Jikomes 1:34:00
Got it. So what, what other types of things have you published on recently? Or is your lab working on? What are some of the big questions that you've been thinking about?
David Puts 1:34:09
Well, yeah, that's thanks for asking that. The research in our labs sort of is two pronged and it's all focused on sort of understanding sex differences, sexual variation. And so the one is the sort of evolutionary questions and so we tend to focus on sexual selection and in understanding that we're using voice but we're looking at lots of characteristics, faces and so on. And then the other prong is more of the sort of proximate developmental stuff like sex hormones. And so one of the one of our lines of research recently is looking at people with an endocrine condition called IHH, idiopathic hypokinetic tropic hypogonadism, which is a mouthful, so IHH words and people with this condition can be both sexes. Lose very low levels of going out on hormones before they're born. And then none after they're born until they would never do that until they get on hormone replacement therapy sometime, usually after the normal time of puberty because they don't go start going through puberty. The reason why has to do with their hypothalamus part of the brain, just never, it doesn't have the cells or their non functional to start off the pathway to tell the gonads to produce go down to hormones. So the hypothalamus doesn't tell the pituitary to tell the gonads, to produce going out of hormones. And so we can study people with this condition and say, what are the effects on males or on females of having been exposed to very low levels of androgens like testosterone or estrogen is like Esther dial on psychology and behavior. And so that's been really interesting to us because that condition IHH was almost unstudied, there was one paper published in 1981, I think looking at psychology, and people with this condition. And it provides a really useful source of information, I mean, not just to help people with IHH, and make decisions about hormone replacement therapy, and so on and understand themselves their psychology a little bit better. And clinicians helping people with IHH, but also helping us understand sort of the processes of the sexual differentiation of the brain psychology and behavior in humans. And one of the really useful from a scientific standpoint, aspects of this condition is that people with IHH rarely know they have this condition until they don't go through puberty. So it's not like they're treated differently as a result of their condition by parents, physicians or whatever. Because if you don't test for it in the first few months of life, then you can't test for it. Until, until the normal time of puberty. So it's been really useful for that, and we published a few papers, but we're, we're just realizing that, and I hope, you know, I'm very grateful to the, to all of our research participants and I, and I want to start sort of working more with, you know, communicating our findings with them, and the broader medical community to see how it can, you know, can help people understand how going out of hormones, you know, play important roles in our, in the development of our brains and behavior.
Nick Jikomes 1:37:24
Any other topics that you think are worth discussing that have been top of mind for you, just in terms of sex differences in your research?
David Puts 1:37:32
Let me think there's so many, so many papers, right now. Some of these things I don't want to talk about, because I'm going to publish them. And I want them to be more interesting. If I if I already said what I'm gonna say, you know, when I mentioned that this big review paper that we published this year, as well on? What's the name of it? I don't remember, but it's about it's something like, you know, contest competition and the evolution of human males. I think maybe that was the the title of it. But it was really reviewing data on how strong sexual selection was in our male ancestors. What sorts of traits that produced when these traits evolved? You know, are these really common to all the great apes? Are they really specific to us than our closest relatives, chimps? Are they specific to hominins, or maybe even our own genus Homo, and sort of tracing the evolution as best we can of when these various sexual dimorphism ZZ that evolved in males and why and, you know, traits that seem to be involved in male male contest competition, like bigger body size, and aggression, and, and, you know, greater muscle mass and things like that. And I tried to I publish something like this in 2010, that you I'm really glad I wrote, I'm really glad I published it, because it felt like it kind of needed to be said, because the the research on in the literature on sexual selection, and human males at the time, mostly seemed to be saying our traits evolved to attract females. Beards, why do we have those because females find it sexy, deep voices? Why do we have that because females find it sexy. And what I was seeing was that these traits have much bigger effects on appearance of fighting ability and from inability among males, or they directly help win fights, and they're maybe not attractive at all, like physical aggression, same sex aggression. And so I said, I said those things first, in a review paper that's, you know, been cited a lot in 2010. But I think I was mostly talking to psychologists. And so in this review paper, I collaborated with a biologist and an anthropologist. And I really wanted to talk more to my fellow anthropologists who I don't think that that message from 2010 ever sunk and for most of them, and so I tried to talk much more about rather than the behavioral things in some sort of basis. anatomical things a lot more about hominid evolution in the fossil record, non human primates, the genetic evidence and so on population genetic evidence, the kind of data that anthropologist Biological anthropologists tend to focus on more, because I wanted to speak to my fellow anthropologists a little bit a little bit more, so I feel good about that. And I, it only came out recently, it's been cited four times. And for all I know, I was the one who cited it four times. I don't remember. But I do hope that, that that contribution, gained some traction, because, you know, I feel like anthropologists in general, sort of look at look at human sexual selection and sexual dimorphism is in Yeah, the wrong way? I don't know. Under emphasize it.
Nick Jikomes 1:40:46
I see. And so like, I mean, kind of circling back to stuff we talked about earlier. The, in your view. So when you look at all of the sex differences in humans than you consider them in phylogenetic context, comparing them to other species. Humans are quite sexually dimorphic. Probably more so than you would expect, if you just only looked at Raw mass. And so just can you just clarify again, like what do you think that means in terms of the level of male male competition for mates in our lineage?
David Puts 1:41:20
I think that it suggests that the level of male male competition for mates and our ancestors over the last several 100,000 years, let's say, was maybe pretty average for a primate, which is pretty high for a mammal. And, you know, we're not an extreme. We're not like, you know, gorillas, orangutans, but I think it was pretty high. And I also think that something that gets maybe lost sometimes when you're looking within a society is that a lot of that male competition for mates is not just males within a society, asserting dominance over each other and getting a resource because others differ, because if you don't, I'm going to kick your butt. But there's also coalitional aggression, that that's something that's happens in chimps. And it's happened in every human society. We're seeing it today, sadly. And that's something that's been important in our ancestors as well, that groups of males will attack other groups of males, kill those males, some of those males, and you know, abduct females mate with the females in that group, there's genetic evidence that that's been happening for 1000s of years in our lineage. But for all we know, millions of years, given that really similar patterns occur in chimpanzees. And we share a common ancestor with them, you know, 7 million years ago or something. So anyway, I think that's an important form of male competition for mates, that it's not just
Nick Jikomes 1:42:47
individuals against individuals, but its ability to form competing
David Puts 1:42:51
groups. Yeah, and, and, you know, plenty of anthropologist and other social scientists, they focus on that, you know, in coalition aggression, how important that is. But other ones who are just sort of trying to estimate how strong was sexual selection and our male ancestors? Maybe missed that, because they're looking only at within group variation?
Nick Jikomes 1:43:14
Um, well, David, we've covered a lot. Is there anything that you want to reiterate or emphasize, based on everything that we said before we sign off?
David Puts 1:43:21
Um, let me think here. I guess there isn't. Maybe one thing? Yeah, I have two messages, I guess what one is something that I liked it. I realized several years ago that I wanted to tell my students when I when I first introduced them to the idea of sexual selection in humans. And I got to a point in the lecture where I thought, I am worried that I gave you the wrong impression that I've depicted a picture of humanity where males and females are so different, we might as well be different species, and we can understand each other. And, you know, I show this, the cover of men are from Mars, and women are from Venus. And I say, I'm worried I gave you this impression. When truly and I show a nice picture of our beautiful blue planet, you know, we're from Earth. And we're the same species, males and females have almost the same DNA. And we're similar on many, many dimensions. However, there are somewhat dimensions where we differ a lot. And the interesting thing is that we can use evolutionary theory and in particular sexual selection theory to make predictions about where those sex differences were will why and you know, the direction of those sex differences and so on. And that's interesting because it allows us to use the tools of evolutionary biology to understand ourselves and our differences. But I also don't want it just feel like it's a false impression to say, to give them the only emphasize the differences and not emphasize not only how similar we are in many dimensions, but also the degree of overlap and even the traits that show sex differences. So that's one thing and the other is is sometimes worry that I give this impression that then we're fated to be the way our ancestors were that you know, because it was in our evolutionary past, human males are just going to be aggressive. And they're, you know, with each other and with females and that sort of thing. And, and that's not right, because you know, we're incredibly intelligent and social, and we respond to social learning and social conditioning. And so there may be factors like going out all hormone production, that predisposes a brain to like engaging in rough and tumble play, and play fighting and to be more tend to respond to social situations more with physical aggression than with other things. But that doesn't mean that we're fated to do that, because we are highly skilled social learners that, you know, we can we can design social programs that, you know, minimize or reduce these, or maybe obliterate the sex differences. So, yeah, I think those are maybe two of those sort of takeaway messages that I like to have. Because I don't want to leave people with a bleak view. I just think it's important for us to understand, you know, who we are, what, what causes those things, and maybe sort of what we're up against as well.
Nick Jikomes 1:46:17
All right. Well, this has been a fascinating, fascinating conversation. Dr. David puts thank you for your time. Thanks a lot.