The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships

by Nathan H. Lents

An Immense World meets Sex at Dawn in this fascinating exploration of sexual behavior throughout the animal kingdom, as evolutionary biologist Nathan H. Lents argues persuasively that many of our supposedly modern ideas about gender and human sexuality are, in fact, deeply rooted in our animal ancestors. 

Evolutionary biologist Nathan H. Lents knows what makes humans unique—and it’s most definitely not our sexual diversity. A professor at John Jay College, Lents has spent his career studying what makes us, well, us, and contrary to what the culture warriors want people to believe—diverse sexual behavior is not a new development, or even a human one. It didn’t just emerge from a progressive culture; it’s the product of billions of years of evolutionary experimentation throughout the animal kingdom. It’s not a modern story, a Florida story, or even a human story. It’s a biological story.

In The Sexual Evolution, Lents takes readers on a journey through the animal world, from insects to apes, revealing what the incredible array of sexual diversity can teach us about our own diverse beauty. Nature, it turns out, has made a lot of space for diverse genders and sexual behaviors. And why? Because when it comes to evolution—diversity wins. This is not just a political or social message, instead it’s rooted in science and cultivated from understanding the full breadth of sexuality that exists throughout the world.

With shades of both Frans de Waal and Esther Perel, Lents’s storytelling is as fascinating as it is topical, offering eye-opening stories about the diversity of animal life, while relating it to our own sexual journey as a species. At once a forceful rebuttal to bigotry and a captivating dive into the secret sex lives of animals, The Sexual Evolution is the rare book of pop science that leans into the controversy. Sex, the reactionaries say, should only be for procreation between a man and a woman, anything else goes against nature. Well, nature would like a word with them.

Genres: Science, Nonfiction, History, Biology, Evolution, Sociology, Psychology

326 pages, Kindle Edition

Published February 4, 2025

About the author

Nathan H. Lents is professor of biology and director of the Honors College at John Jay College of the City University of New York.

His research has been published in a dozen leading science journals, including the Journal of Biological Chemistry, Molecular Cell, the Journal of Forensic Sciences, and the American Journal of Physiology, as well as science education journals such as the Journal of College Science Teaching and The Chronicle of Higher Education.

He also maintains The Human Evolution Blog and blogs for Psychology Today under the tagline, "Beastly Behavior: How Evolution Shaped our Minds and Bodies." His articles occasionally appear in magazines such as Skeptic.

Reviews

[Graeme Newell · Rating 5 out of 5]

What an absolutely delightful read. For me, the mark of a great nonfiction book is this: does it seriously shake up something I thought I understood. This book checks all the boxes and really got me thinking in so many new ways.

Reading this book was like having someone gently open a door in your brain that you didn’t even realize was closed. I went in expecting a dry, maybe mildly interesting take on biology and left completely rethinking what sex, reproduction, and gender mean - not just for humans, but for the entire animal kingdom. This was such a fantastic book because it really got me thinking about the evolutionary development of sex, something I had never seriously considered before.

We’re so often taught a very rigid, oversimplified story: male meets female, they reproduce, the end. But Lents rips that wide open and shows how unbelievably creative nature can be when it comes to procreation. He makes the point - backed by some truly wild examples - that survival is all about adaptability. The species that make it through the hard times - whether that’s a climate shift, scarce food, or new predators - aren’t necessarily the strongest or fastest, but the most diverse. And that goes double for sexual strategies.

I found myself absolutely captivated by the incredible array of reproductive tactics Lents shares from across the animal kingdom. Fish that change sex depending on the environment? Check. Insects that pass on sperm packages like love notes? Yep. Birds and reptiles with multiple categories of “male” that serve entirely different purposes in reproduction? It’s all here, and it’s jaw-dropping. The creativity on display in the natural world is honestly more impressive than anything you’d find on a streaming service.

One of the things that struck me the most was how Lents reframed something we all take for granted: that there are just “males” and “females.” But nature is far more nuanced than that. In many species, you’ve got dominant males, satellite males, sneaker males - each with distinct roles and reproductive paths. And the same goes for females. It’s like a choreographed dance, with all kinds of roles, all designed to maximize the chances of survival for the next generation.

Then Lents turns the lens on us, and that’s where things got really interesting. Compared to the rest of nature, humans seem, well… pretty darned unimaginative. Our cultural narratives around gender and sexuality are astonishingly narrow when stacked up against what other animals are doing. In most of the animal kingdom, same-sex pairings, fluid gender roles, and sex that isn’t strictly for reproduction are not just common - they’re often central to social life and group cohesion. In fact, one of the most eye-opening ideas Lents presents is how sex, in many species, is less about reproduction and more about bonding, trust, and group dynamics. In some cases, it’s the glue that holds the tribe together. Refusing to participate might make you an outcast. Nature, it turns out, is far less prudish than we are.

There’s also a deeper philosophical shift that this book encourages - at least it did for me. Lents challenges the binary, buttoned-up version of human sexuality that most of us are raised with, and shows how the natural world exists on a continuum. Reading this made me realize that sexual expression is less of a rigid system and more of a bell curve, with endless permutations that all serve a purpose. It was a beautiful reminder that variety isn’t just okay - it’s essential.

If I had one critique, it’s that the section on the crazy search for a human “gay gene” felt like it dragged a bit. It wasn’t uninteresting, but it went too deep into the weeds for my taste and lost the snappy pace that the rest of the book had. That said, it didn’t derail the overall experience.

What I loved most, though, was how this book managed to feel both expansive and intimate. You’re taken on this global tour of animal behavior, but it always circles back to the personal, to the way we see ourselves and each other. Lents writes with clarity and compassion, and a refreshing dose of humor, and who doesn’t love a scientist with a funny bone? He’s not trying to be edgy or provocative - he’s just showing us what’s already there, hiding in plain sight, waiting to be understood.

In the end, “The Sexual Evolution” left me feeling more curious, more open-minded, and truly hopeful. There’s something really beautiful about the idea that sexuality isn’t a rulebook - it’s a toolkit. A wildly inventive, evolution-approved way to make sure life goes on. And nature, as it turns out, has written far more chapters than we ever imagined.

[Cher 'N Books · Rating 5 out of 5]

5 stars = Utterly incredible. One of the best books I've read this year.

“Concentrated power and control brought an end to the era where everyone was free to do as they pleased.”

In almost 15 years of journaling my reading history on Goodreads, I believe this is the first time I have ever read two 5 star books back to back. December has been very kind to me! A lot of ground is covered in this nonfiction selection, but the main takeaway is that the most notable feature of the human sexual experience is diversity. It clearly demonstrates that nature loves diversity and we should too.

“If you could choose what species in which to be reincarnated, you’d be crazy not to choose bonobos.”

This was an incredibly riveting read that had me nerding out, going down research rabbit holes and telling others about tidbits that I learned. It covers gender and sexual behaviors of all kinds of animals, how this has evolved over time, and gives social philosophical takeaways. The writer uses a casual narrative that is engaging and easy to digest. This is an excellent choice if you enjoy books filled with strange and fascinating facts about animals, including us humans. It is overflowing with examples of truth being stranger than fiction.

“The common factor, at least in these two primate species, is the empowerment of females. When males are in charge, as in a harem, life can be pretty rough - especially for the males! But when females are in charge, harmony and equity seem to be the natural results. It’s something to ponder.”

I appreciated that Lents comes across as a champion of women and the LGBTQ+ community without being preachy. As a childfree person, I cheered his demolishment of the argument that sexual activity is narrowly purposed towards procreation. He clarifies that if you feel a monogamous heterosexual relationship is what is right for you, that choice is validated, perfectly normal and there’s nothing wrong with it. He just wants you to understand that if your neighbor chooses a different type of relationship, that is also validated, perfectly normal and there’s nothing wrong with it.

“The simple biological truth that variation is good is often lost on us because modern society has trained us to be suspicious of differences. But this is a psychological tendency that we would do well to dismantle. It harkens back to the era when prehistoric humans lived in small bands that were often in competition with each other, fostering a sense of tribalism and prejudice that still vexes us today and underpins some of our greatest social problems.”

It is a travesty that this book has so few ratings. It deserves more attention and should be widely read. Highly recommended for fans of anthropology or sexual psychology as well as to animal lovers, people that enjoy broadening their perspective and to bigots needlessly suffering from their ignorance. Most importantly, this book will be well received by anyone that feels judged for rejecting the mainstream script, instead choosing to be their authentic selves rather than follow a crowd that ironically are the actual ones going against the natural and historical normatives.

“The three most foundational classical cultures of Western civilization - Egypt, Greece and Rome - were all less sexually restrictive and repressive than we are in the modern era.”
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First Sentence: One day in my high school biology class, the teacher announced that there would be a closed-book assessment at the end of the week covering the content of the chapter we’d been discussing.

Favorite Quote: Let me first say that being biological should never be a requirement for considering something real and valid. For example, race is primarily a social construct with almost no basis in biology, but that doesn’t mean it isn’t a real identity and a distinct lived experience with social, financial, psychological, and legal consequences. The same is true for being wealthy, substance or behavior addicted, having PTSD, being considered attractive (or not). While these may have a biological component, they are primarily psychosocial conditions, and denying their existence because they aren’t purely reducible to biology is as absurd as it is harmful.

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***Other quotes that I would like to add to Goodreads when able (currently receiving errors.)***


“Most animals have retained the default state of bi- or pansexuality, if you will. The animals that require an explanation are those that only pursue sex with members of the opposite sex. Therefore, it is not homosexual animals that require an explanation, but rather strictly heterosexual ones.”

“Sexual attraction based on the qualities of the other individual, both physical and behavioral, without regard for sex or gender, is the true default situation, and this is closer to what we see in most birds and mammals today. The anomaly that requires an explanation is being averse to having sex with an entire sex or gender because that is less likely, requires more assumptions, and comes with costs.”

“Sexual monogamy is exceedingly rare in nature - almost nonsexistent in fact. Social monogamy, on the other hand, is quite common. Also called economic monogamy, this is the tendency of animals, including humans, to pursue dyads that are enduring, marked by mutual aid and support, and that takes priority over all other social relationships. Simply put, social monogamy is two individuals building a life together, most often for the purpose of raising children. They share and defend a home (or a nest, territory, etc.); they share economic resources; they jointly contribute to the rearing of children, usually their own genetic offspring; and perhaps most important, they do so to the exclusion of others and for a substantial portion of their life history, if not perpetually.”

“The most recent survey found that 226 mammal species form socially monogamous dyads for at least a breeding season. That’s about 4.5 percent of all mammals, meaning that almost 95 percent of mammals are not monogamous in any way - the exact opposite of the situation in birds, in which 90 percent are seasonally or perpetually monogamous.”

“These monkeys have the distinction of being the eighth mammal species discovered to exhibit true sexual monogamy. That’s just eight out of more than five thousand species, or 0.0015 percent.”

“Sexual attraction on a fluid spectrum of pansexuality is the orientation that would most benefit animals, and this is precisely what we most often see. The more social an animal is, the broader their sexual attraction. The more antisocial an animal is, the more heterosexual they are, since the focus of their sex life is fertilization alone.”

“The stricture that marriage is a one-man, one-woman arrangement that has dominated much of the world for the last few centuries is a relatively recent social innovation that spread mostly by force, to be perfectly frank.”

“ “Imagine a society in which one woman in every three is raped, usually by a man she knows…where one-third of all women are beaten during pregnancy…where 20 percent of adult women…have experienced sexual abuse or assault [as children].” They weren’t talking about the Yanomami. Those statistics were taken from the American Medical Association and describe the situation for women in the United States of America. The Yanomami society would never tolerate such an awful state of affairs.”

“Anything and everything we examine in nature looks more diverse and more complex the closer we look. This is because the natural world is a constantly churning, diversity-generating machine, and the natural history of sex reflects that.”

“It seems that multiple marriage was only revolting when women did it, which is consistent with the patriarchal (misogynistic, if we’re being honest) flavor of Judeo-Christian Islamic morals and culture.”

“It is important to remember that the restrictive sexual morals of ancient Israel were at odds with those of basically all of its neighbors. According to the Old Testament, the Egyptians were joined by the Canaanites, Babylonians, Philistines, Phoenicians, Ammonites, and others in their ungodly approach to sex.”

“The primary concern of marriage was not love or even sex. It was inheritance.”

“Many reactionaries are invoking the term social contagion to describe the increase in children identifying as trans. The idea is that if being trans is seen as cool and trendy, it can spread like an infection. This is an odd position to take when it is crystal clear that being trans is much more likely to get a child bullied at school, rejected by family, and ostracized from their church and other social groups.”

“How are we to know how large the transgender population truly is without letting people decide for themselves, free of the social pressures to conform?”

“If we’re going to ignore everything else, then this binary should be called gametic sex, not biological sex, since the only biology it considers is that of the gametes.”

“To anyone who would accuse me of wishing to destroy the concept of marriage and family: Why would I want to tear down the very institution for which I fought so hard to access?”

[Karianne · Rating 3 out of 5]

Thank you to NetGalley for a copy of this book!
This book contains many great examples, both in the greater animal kingdom and in humans of sexual differentiation and varying expressions/interactions among these individuals.
The presentation left a little lacking for me. When referencing some controversial or outdated studies, the author spoke in absolutes about the lack of value in these studies, often using language that felt very condescending (like “eye roll!”, among other sarcastic comments). It was unclear in these moments who the author’s audience was. The book started out sounding as if the purpose was to show the biological backing for non-heterosexual/non-monogamous relationships, but these comments seemed to judge or ostracize people who weren’t already fully onboard with the author. I would have preferred a stronger argument and less asides.
There are also a few times where a single example is provided, but sweeping generalizations are made. There is discussion about what “true monogamy” is, but only uses the example of the Roosevelts. While it’s possible the author looked at multiple scenarios to come to his definition of monogamy in humans, this is unclear and feels like a jump.
Overall, a decent read, but I was left wishing the author would defend his views a bit more solidly at times.

[Meagan · Rating 2 out of 5]

somehow boring

[William Adams · Rating 2 out of 5]

The subtitle is revealing: “How 500 million years of sex, gender, and mating shape modern relationships.” That tells you the book will be yet another tedious exercise in sociobiology and psychobiology. Those ideologies presuppose tacitly, without examination or question, that we are nothing but biological beasts determined by evolution.

It is a premise I reject, as would anyone not in thrall to doctrine. We are not hairless cave-persons mindlessly grunting at each other because of our evolutionary past. Nor are our lives and choices determined by hormones, neurology, and genetics. We are instead, self-reflecting beings, the authors of intentionality, agents of our actions—constrained by our biological vehicles of self-expression—and intersubjective collaborators in construction of values and meaning. In that view, how has evolution “shaped” modern relationships? Not much.

The whole book is based on an error in thinking known as the Naturalistic Fallacy. The error occurs when one fails to appreciate that there is no reasoned pathway from “is” to “ought.” Just because a natural phenomenon is this way or that, it does not justify the conclusion that it, or anything else ought to be so. But this whole book carries the opposite, erroneous message that we humans ought to be more diverse in our sexual attitudes and relationships because in nature, many animals are diverse.

The author is apparently aware of this fallacy but dismisses it, saying “the lines are blurry” between is and ought. He doesn’t say how or why, or to whom, or therefore what (p. 11).

Another egregious problem that plagues the book is poor definitions. “Sex” (the main topic) is defined as 1) “engaging” the genitals and physical arousal of one or more parties, also called “having sex,” and as 2) gender, “maleness and femaleness,” terms defined only as “biological sex,” and as 3) generation of sperm or egg cells, “gametic sex.”

Those are weak definitions (except the last one), but worse, all three are used interchangeably throughout the book, making it very hard to understand what the author is trying to say. For example, try to make sense of this (p. 34): “If sex is determined by our chromosomes, how can an animal possibly change its sex?” The book has no glossary, and the index is perfunctory.

To make matters worse, the author imagines himself a humorist, using undefined slang throughout the book, not an appropriate tone for a scientific book and more seriously, one that obscures understanding. We are left to guess the meaning and importance of terms like “avian hanky-panky,” and “straight” versus “gay” swans. We are informed that King James I of England “enjoyed” sex with women on occasion but “was clearly drawn to” sexual and romantic relationships with other men. What enjoyment of sex (any definition) has to do with evolution is unstated, as is the definition of “romance” in a relationship, and the point of the whole anecdote.

The main conclusion of the book is that more diversity in human sexual relationships is desirable because plenty of sexual diversity is observed in the animal kingdom. However that conclusion is a non-sequitur, rendering the book no more than a mildly interesting opinion piece.

Lents, N. H. (2025). The Sexual Evolution. New York: Mariner Books, 321 pp.

[Ja · Rating 5 out of 5]

In The Sexual Evolution, Lents examines the biological and social history of various aspects of sexuality in the animal kingdom. From concepts of gender, homosexuality, monogamy, and even family values, there's a lot to be learned from what we see in the animal kingdom. Some may argue that the only purpose of sex is for reproduction, and therefore there's only one way to do it. But all other species will prove this wrong, and you'll see from the multitude of examples and studies that Lents offers in this fascinating book. At its heart, a lot of our notions about sex, gender, and relationship structures is entirely a social construct, but that's not to say that social constructs are wrong. Being able to recognize the difference is what Lents helps the reader understand.

There's a lot more nuance in nature, and we're starting to see that in human expression as well. It's only recent human history that we've tried to really stigmatize anything that goes outside of the typical dichotomy, and at the end of the day that's entirely all social construct.

If you're interested in learning more about the scientific history of sexual behavior, check this one out!

[William Nist · Rating 4 out of 5]

Lents central point is that diversity of sexual attraction/orientation/genderization is a feature of evolution and not a defect. He traces this theory from single cell beings to homo sapiens. This renders criticism of sexual behaviors moot. The animal kingdom is resplendent with multiple combinations of behaviors that should lead to broader acceptance of those combinations.

He surveys the current science on the genetic basis of sexual orientation, and claims that genes can account for no more that 25% of the basis of anyone's particular orientation. He looks at monogamy and polyamory, and concludes it the culture bound and not inherent in our makeup. He rejects the view that evolution has been only interested in species reproduction, and indicates that pleasure has also played a significant role. He is of the view that the full spectrum of sexual behavior will reoccur in every generation, and non-heterosexual divergences will not die out due to inability to have children.

[Peter Tillman]

A good review at The Guardian, https://www.theguardian.com/books/202...
Excerpt:
"an entertaining and informative romp through mating strategies in nature. From Komodo dragons’ virgin births to the bilateral sperm transfer of hermaphroditic slugs, The Sexual Evolution chronicles a “wondrous variety” of behaviour in the animal world. Garter snake orgies, gender-masking hyenas, lusty bonobos and the lesbian Laysan albatrosses of Hawaii – this book has it all. All that diversity is fascinating, and frequently funny...."

[Ryan Bingham · Rating 5 out of 5]

I accidently stumbled upon this book at the library. I am so glad I did!! This book is FULL of interesting and insane information. Lents makes numerous arguments that are backed up by scientific facts and uses animals as examples. If you ever wanted to know anything about sexual variance in the animal kingdom then this is the book for you. Monogamy and straight sex are things that you'd assume are a large part of our world, which isn't the case. Easily one of the best books I have read in a long time. There are drawings too, which are wonderful. I can't recommend this book enough. Go read it!

[Nikita Goncharov · Rating 4 out of 5]

It’s really good, insightful, and eyes-opening. Sometimes it’s hilarious and sometimes it’s horrifying. I’d enjoyed it even more if there were a little bit less jargon and a little bit more about people sexual evolution

[Kira · Rating 4 out of 5]

Ich habe mit diesem Buch unendlich viel gelernt. Biologie ist nicht gerade mein Fach, aber folgen konnte ich dem Inhalt fast immer :) besonders erwähnenswert: der Autor schreibt teilweise wirklich lustig und sassy. Ein großes Plus für mich, weil es die vielen Fakten regelmäßig auflockert🙂‍↕️

[Tutankhamun18 · Rating 5 out of 5]

This is an absolutely fantastic book about sex, gender and gametes! It explores gender expression and sexuality within the animal kingdom and then uses this to inform how we think and know about human sexuality. What is brilliant about this book is that it looks into both the animal kingdom and human sexuality and culture with the same amount of depth. Truly 5 stars!!!

Key facts:

• “When biologists consider the sex of an animal, we are usually referring to which gamete its body makes. Gamete is the word for the very specialized reproductive cells that come in two versions—large and small, casually called eggs and sperm, respectively.” -> Gametic sex clarifies which gamete they make

• “The thing to really understand here is that, although alternatives exist, our gamete system (sperm and egg) seems to be the best balance of the pros and
*cons, and that's why plants and animals have opted for it. Yes, it's slow and it also means that each of us is only reproductively compatible with 50 percent of the population, but it enforces sexual recombination, avoids the temptation of cloning, provides backup copies of genes to allow experimentation, and involves no more than the minimum number of gamete types-two—to allow all of this. The evidence that this is the best compromise is that so many successful lineages have settled on this system. All plants and animals are dip-loid, meaning they have two copies of all chromosomes, and they have two different kinds of gametes, a big one and a small one.”

• “But the bigger reason some biologists are now applying the word gender to animals is because we have discovered that, for many animals, there is more than one way to be a male or a female. There are different versions of "male behavior" and "female behavior" that animals can adopt. Importantly, I do not just mean that males or females can choose from a tool kit of different possible mating strategies. What I mean is that there are often different kinds of males or females in many species, resulting from real biological differences and destining them for different kinds of behaviors. While the notion of animals having genders is relatively new, animals have always had a variety of what we now call gendered behavior.”

• “The radical approach that these scientists engaged was simply dropping the assumption that sexual behaviors were originally oriented toward opposite-sex partners in carly animals, and that any deviations from that were evolutionary events that needed explanations. Instead, a different assumption was put in place: that there was no sexual "orientation" in the first place, and that specific preferences and orientations would have evolved later, if at all.
Making and testing assumptions is a key aspect of the scientific method and the results can be scored on their likelihood, with the general rule that the explanation that requires the fewest assumptions is the most likely. This logic is based on a principle called Occam's razor, the idea that the simplest explanation is usually the correct one. As Professor Monk and her colleagues tested various assumptions regarding sexual orientation in animals, they were on the lookout for the simplest and therefore most likely evolutionary scenario.
If we assume that the earliest animals were heterosexual, then we would have to explain how same-sex sexual activity evolved in each of the myriad groups of animals in which we see it. This does indeed seem unlikely. But if we assume that animals were drawn to engage in sex with any members of their species, the only behavior that requires an evolutionary explanation is an aversion to having sex with certain members of their species.”

• “It's not that biologists were wrong about pair-bonding in birds. We were wrong about what pair-bonding really means because we confused social monogamy with sexual monogamy. Sexual monogamy is just what it sounds like: a bilateral sexual relationship, usually preceded by some form of courtship and mate selection, in which sexual activity is mutually exclusive. This sex-tal exclusivity is what comes to mind for most people when they hear the term monogamy, but as we will see, sexual monogamy is exceedingly rare in nature-almost nonexistent, in fact.
Social monogamy, on the other hand, is quite common.”

• “ They do not form pair-bonds with sexual partners at all, while humans clearly do.
Our oxytocin/vasopressin attachment system is activated in a unique way by sexual activity. That intense feeling of being "in love" is clearly reserved for current or prospective sexual partners. In this way, we are unlike either chimp species and more like the gorillas who, despite being polygynous, form strong attachments between sexual partners with all of the associated emotions, including longing, jealousy, and profound grief when an attachment is lost.
It is worth noting that the oxytocin attachment system is still present and very strong in both chimp species. They form strong mother-child bonds, as well as social bonds— friendships-that are enduring and profound. The ar-tachment system just isn't activated by sex. So among the various species of apes, all of us form strong familiar and social attachments, but the way that sex activates these attachments, or doesn't, is different.”

• “Nonetheless, some benefits of masturbation have been proposed. For males in species that engage in sperm competition, there is a clear benefit: fresh semen seems to work better 3º Therefore, "cleaning out the pipes" may actually boost the quality of both sperm cells and the semen that activates them. Sperm competition occurs in species in which the females mate with many males. Within the female reproductive tract, sperm have a very long way to swim relative to their body size. I's the equivalent of a human running about thirty kilometers (nineteen miles) nonstop at high speed, but their actual path is longer than that because they don't exactly swim in a straight line directly to the egg. Sperm cells meander aimlessly and swim in a corkscrew. Very few sperm end up anywhere near the target. With such a long way to go (and such poor aim), the key to successful fertilization is to ejaculate a lot of sperm. In humans the average ejaculation releases hundreds of millions of spern cells… Masturbation helps also because it clears out old and defective sperm cells, as well as stale seminal fluid. This allows the next ejaculate to be as fresh and speedy as possible. Indeed, because sperm cells are stored with inhibitors to keep them inactive, the longer they are in storage, the more sluggish they are when they are released. It has been shown in humans and rhesus macaques, for example, that regular masturbation boosts sperm quality,"”

• “For females, masturbation can be essential to reproductive health. Some mammals employ what is called induced ovulation, meaning that eggs are not released on a regular schedule, as with the human menstrual cycle, but rather, eggs are only expelled from the ovary when a female is stimulated by sexual activity. This is especially useful for species that live in low population densities. If an ovulation comes and goes with no male around, it is wasted.
On the othet hand, mature eggs can hang out in the ovary for a lite while. but if they are not eventually ovulated, they can develop into a painful cyse.
Domestic cats are induced ovulators, and anyone with an intact female cat has probably observed the painful experience that occurs if she doesnt her tually have sex when she is in heat. If there are no males around to help her out, what can be done?”

• “The answer is power. The plant- and animal-based foods, clothing, and other materials that agriculture produces in great abundance are owned by someone. These resources are not taken freely from the environment but, rather, created through directed human effort, which can be organized, controlled, and monopolized. Suddenly, individuals could own land—an unheard-of concept among hunter-gatherers—and use it to become wealthy. And this is exactly what happened
As foraging societies became farming societies, wealth inequality emerged.
For the first time, there were haves and have-nots, and the relatively flat and egalitarian social structure gave way to a hierarchical system with power concentrated in the hands of a very few. The prevailing life goal quickly became the maintenance of wealth and power. And voilà, we got the enforcement of
"order" and social control. This doesn't necessarily mean a shift to monog amy. In fact, polygyny was even more common than monogamy in agrarian societies all the way through the premodern era. But the point is the same: concentrated power and control brought an end to the era where everyone was free to do as they pleased”

• “In summary, the three most foundational classical cultures of Western civilization —Egypt, Greece, and Rome—were all less sexually restrictive and repressive than we are in the modern era. There was no taboo, let alone crim-inalization, associated with homosexuality, premarital sex, or nonpenetrative sex. Those laws and taboos came much later. While there was a taboo against adultery, at times with criminal consequences, this was defined narrowly as a married woman having sex with a man who was not her husband. Tell-ingly, we encounter this misogynistic double standard —both officially and unofficially-throughout the world, but always as a function of the concept of legitimacy when it comes to inheritance.
Married men were always free to have sex with whomever they wanted, if quietly. But married women had no such freedom because of the doubt this would inject into the paternity of her issue. The primary concern of marriage was not love or even sex. It was inheritance.”

• “Just about the only measure in which the sex distribution is binary, rather than bimodal, is in the production of sperm and eggs, and that's why it is the final retreat of those committed to a sex binary. If we're going to ignore everything else, then this binary should
be called called gametic sex, not biological sex, since the only biology it considers is that of the gametes.”

• “Yes, in this one very particular moment in time, same-sex attraction may, on average, reduce the likelihood of someone leaving genetic offspring. But we have no reason to believe that these conditions will persist very long and every reason to believe that this is a temporary anomaly.
First of all, sex and sexual attraction are not very directly tied to the production of offspring.”

• “ In fact, we should probably not expect homosexuality to disappear even if queers were to leave no children at all for thousands of years. Because there is some biological and genetic influence, it is possible that homosexuality would slowly wane if the negative selective force was strong enough and was applied for long enough. But if the centuries of queer oppression in Western and Eastern civilizations have taught us anything, it's that diverse sexualities are an integral feature of our
-species and can't be easily stamped out. And, given that the same is true for basically all other species, it probably couldn't be any other way.”

[Erin Markel · Rating 4 out of 5]

This book started with a lot of fun anecdotes about animal gender and sexuality throughout natural history. When it transitioned to the human section of the book, things felt kind of preachy, but when I got to the conclusion, I realized it worked; I now understood the importance and prolificacy of gender and sexual diversity. What a relief to have a more scientific understanding and justification of a concept that my community has taught me to love, but that I have struggled to appreciate about myself. The colloquial writing left something to be desired for me, but overall, I loved the message and learned a lot.

[Maureen Weiner · Rating 5 out of 5]

The Sexual Evolution: How 500 million years of sex, gender, and mating shaped modern relationships

By Nathan H. Lents

This book is a refreshing look at homosexuality, bisexuality, and transgenderism that shows how natural these behaviors are, how they are not at all detrimental to human reproduction, and how they can impart some benefit to human reproduction and survival.

Lents begins his book by stating that he is not in the business of determining what people ought to do, he is in the business of exploring what people, and other living organisms, actually do. He certainly has his own opinions on what people ought to be allowed to and prohibited from doing, but he generally seems to be a live-and-let-live type of person.

To see how homosexuality, bisexuality, and transgenderism fit into reproduction, Lents reminds us that most of life on earth reproduces asexually. An organism copies its entire genome and then splits into two; minus any copying errors, these two organisms are identical copies of the original. For asexually reproducing organisms, evolution occurs via copying errors and via gene transference via physical contact with another organism.

Sexual reproduction, on the other hand, occurs when two organisms of the same species each copy half of their genome, into what we call a gamete, and combine the two halves together to form a third organism. The offspring is never an identical copy of either parent. In addition, with the exception of identical multiples, no two siblings are ever copies of each other because each parent’s gametes are also unique from each other.

When organisms first began to reproduce sexually, there were no males or females; there weren’t even male or female gametes. Each parent had to provide not only half of their genome, but also half of the raw material their offspring would need to begin and sustain life. This particular method of providing offspring with the raw material to begin life (half from one parent and half from the other) seems to have been an evolutionary dead end because most sexually reproducing organisms evolved to have two types of gametes. We know them as the large gamete or egg and the small gamete or sperm. Eggs, alone, provide all the raw material the offspring needs to begin life plus half of that parent’s genome. We call this raw material from the egg mitochondrial DNA, the powerhouse of the cell. Sperm, on the other hand, only provide half of that parent’s genome and nothing else.

Even though most sexually reproducing species divide their gametes into what we call sexes (eggs are female and sperm are male), most sexually reproducing species are hermaphrodites. Sequential hermaphrodites are born one sex and then change sex at some point in their life cycle, usually due to environmental changes, such as being in a single-sex environment. Simultaneous hermaphrodites are organisms that make both eggs and sperm throughout their entire reproductive lives.

Simultaneous hermaphrodites are capable of sexually reproducing with themselves. However, unlike with asexual reproduction, these offspring do not result in identical copies of the parent. Because each egg and each sperm are a random generation of half of the parents genome, the offspring of self-fertilizing hermaphrodites have unique genomes. This practice does not lead to the best outcomes in terms of genetic diversity, so most simultaneous hermaphrodites have measures in place to prevent self-fertilization, except as a last resort. Flowering plants will release their eggs and sperms at different times to prevent self-fertilization or keep their eggs and sperm in separate parts of their flowers so that their pollinators will deposit the sperm in the next flower they visit instead of the flower they got the sperm from. Animals will transfer egg and/or sperm to another member of their species rather than to themselves.

A vital part of sexual reproduction is the sex drive. Without the sex drive, sexual reproduction ends in extinction. Since most sexually reproducing species are hermaphrodites and can sexually reproduce with any other member of their species, the sex drive strongly promotes animals to engage in sexual activity with any and all members of their own species. This drive or instinct did not end or diminish when hermaphroditic animals evolved into sexually dimorphic animals.

Lents cites that every sexually dimorphic animal species that scientists have studied engages in same-sex sexual behavior. He states that the evidence shows a standard bell curve in terms of animals who will sexually engage with members of the same sex and opposite sex with most individuals happy to engage with either. The exceptions, of course, are animals that are solitary with the more solitary species skewing towards heterosexuality and away from homosexuality.

Since humans are very social, Lents believes it is only natural that we will see our species returning to a standard bell curve once the social stigma against homosexuality is lifted from society. A stigma that has only been in existence for a few thousand years and only widespread in the past few hundred years.

When it comes to transgenderism, Lents spends considerable time describing the different genders seen amongst various sexually dimorphic animals. Most of his examples are different genders seen amongst these species’ males. Plus, most of these different genders amongst males are not only behaviorally different, but they are physiologically different as well. Lents theorizes that most of these different genders seen in sexually dimorphic animals exist in order to create diverse reproduction strategies. Having diverse reproductive strategies is a good way to stave off extinction if and when the environment changes.

One particular example is happening currently in Hawaii. Hawaiian field cricket males consist of two distinct genders -- the chirpers and the flatwings. The chirper males rub their wings together to create a song that attracts female crickets. The flatwing males are physically unable to produce these songs because their wings lack the necessary “teeth,” hence the name flatwing. Because the flatwing males are unable to attract females with songs, they usually have significantly lower reproductive success. One might suppose that the flatwing males would disappear from the field cricket population, but it never did. Why not? Probably because being unable to attract females with song, while being less reproductively successful, isn’t so unsuccessful as to be extinction-worthy. In fact, when the environment changed, and a new fly, Ormia ochracea, was introduced, the flatwing males suddenly had an advantage. Ormia ochracea are also attracted to the chirper males’ songs, but instead of mating, they deposit maggots onto the chirper males’ backs. These maggots burrow into the chirper males’ bodies, grow, and kill the chirper males. This environmental change could have resulted in the extinction of the entire Hawaiian field cricket species, but it hasn’t, because the Hawaiian field crickets had more than one reproductive strategy. And when the more successful strategy became a liability, the secondary strategy has picked up the slack and is saving the species from extinction.

A major theme in Lents’ book is the praise of diversity. Sexual reproduction increases diversity within those species that use it because it increases the speed of evolution. The more diverse a species is, the less likely it will succumb to extinction when the environment changes, and the environment is always changing. It’s just difficult for us, humans, to see environmental changes because our short lifespans limit our historic vision.

Lents’ opinion is that human societies should be more open to sexually diverse behavior, but not all sexual behavior. For example, it is natural for male gorillas and male lions to kill all the offspring when they take over a harem or pride. It is also natural for chimpanzees to engage in sexual behavior with sexually immature chimps, including infants. Rape is so prevelent in certain species of ducks that their genitals are evolving in a sort of arms race to counter and counterattack this behavior. However, Lents does not believe humans should consider adopting infanticide, pedophilia, or rape, despite the fact that they are natural, and I agree with him.

Personally, I understand how some people can feel uncomfortable with changing sexual norms. Change and unfamiliarity can be very scary, especially since our species begins sexually maturing while we’re still children and ill-equipped to make serious decisions that will affect the rest of our lives. That’s why I think this book and other like it should be read by as many adults as possible, so we can be as informed as possible, so that when our children need our guidance we are prepared to guide them proactively, instead of reactively, with an appreciation for diversity, instead of fear of diversity.

[Elisa · Rating 4 out of 5]

Out of all the books about non-human animals that I read (as many as possible), I find the ones about their sexual behavior the most fascinating. I’ve also enjoyed the author’s previous works, so I was really looking forward to this one. I was not disappointed. Much of the information here wasn’t new to me, but I still learned a lot about the subject. Written in an approachable way, it was easy to follow even the drier parts (especially genetics, which is something I don’t easily understand and usually find boring), and the many anecdotes were fun (and, sometimes, funny). The author adds a lot of footnotes, so that it’s easy to see the bibliography and the exhaustive research behind it. Whenever something is not known, he says so. The book includes information on many diverse species from all the animal kingdom, even including bacteria! Since I read these books to learn more about non-human animals, the last chapters, devoted to the ugliest ape, didn’t hold my attention as much, but readers looking for more insights into human sexuality will find them interesting. The author does his best to stay away from judging the behaviors described here and, since they sometimes involve children (animal and human), those parts were harder to read. Except for those sections, the rest of the book was catnip for me.
I chose to read this book and all opinions in this review are my own and completely unbiased. Thank you, NetGalley/Mariner Books.

[Miles Dawson · Rating 4 out of 5]

I liked the book and the ideas presented. It's led to some interesting conversations between friends. The only negative things I could say about this book is that in some sections, he explains concepts that were already thoroughly explained just a chapter earlier. Also, at times, it seems as if he is personally defending his own sexuality instead of simply explaining and defending why he believes non-heterosexuality is normal/natural and should not be stigmatized in modern society.

[Eset · Rating 5 out of 5]

I loved the whole book, but as someone who's polyamorous, I really appreciated the chapters regarding the different forms of relationships and sexuality throughout the animal kingdom, which are illustrated by a plethora of examples. I was particularly interested in the question of whether there's some way that we're more predisposed by our genes toward relationship structures with greater fidelity than you'd usually find in the animal kingdom (i.e., monogamy and polygyny, which are pretty common through known human history). I've always suspected we have a certain predisposition towards "high fidelity" type relationships given the uniquely problematic nature of human reproduction. To make allowances for our large brains, we have long gestation times, babies are born helpless, it takes a long time to grow into an adult, etc. — all of which puts a premium on stable parental relationships.

The author recognizes that monogamy and polygyny are more common over the course of human history, but he also points to certain "traditional" non-monogamous and non-polygynous cultural practices in many different areas across the globe, which seem to suggest the absence of any universals. However much we might have a predisposition towards relationship structures with fidelity, it doesn't seem to be a "given." A great deal of other evidence and examples are brought to bear on this question, all of which are handled thoughtfully and with a gift for storytelling.

On this point, I think the biggest insight is that "when resources are scarce or easily monopolized, there tends to be more social control over sexuality," but "when resources are abundant and accessible to all, freedom and diversity flourish." This observation certainly has a lot of explanatory power with regard to the many examples he discusses in the book. It explains why "lower fidelity" relationships (e.g., polyamory) have become more common now in our industrial era, which is more affluent and egalitarian, and in comparison were less common in the pre-industrial agrarian era, which was marked by starvation and oppression (notably, though, the aristocracy had a lot of sexual diversity, which is sort of an exception that proves the rule). It's also consistent with the evidence we have of hunter-gatherer societies, which seemed to have lived in relative abundance relative to agrarianism and had more sexual diversity. It's also consistent with the behavior of certain close cousins of humans (e.g., the sexual diversity of the bonobos seems to be due to their abundant environment). Through all these examples, the author demonstrates that his "sexual diversity" generalization holds up pretty well, which certainly suggests our genes are more mixed in terms of how much they push us toward or away from fidelity

Taking all of the evidence into consideration, the author declines to make any sweeping conclusions about whether we're "evolved" to be monogamous, polygynous, or something else. He instead concludes that "[h]umans are not hardwired for *any* specific type of sexual-romantic relationship. Instead, we have a flexible constitution in this regard and can adapt to various social environments.... [W]e evolved to be *the ultimate generalists.* ... Humans don't solve our environmental challenges with our bodies; we solve them with our minds. We solve them by adjusting our *behavior.*" I think that's the best way to put it and perfectly reflects the project undertaken by people like me in polyamorous relationships — one that is challenging and experimental, but also hopeful, humane, and, for some, necessary. Really, I think anyone in a polyamorous relationship should read this book, if only to understand themselves and their relationships a little better.

Having said that, I'm a bit of a history nerd, and I'm fascinated by the Roosevelts, so I did have quibble regarding the author's brief but glowing description of Eleanor and Franklin Roosevelt as a example of what we would now call a polyamorous marriage (it's one of many examples of human non-monogamy). This passage is well intentioned, and maybe it's unfair to nitpick one small example like this (it's just three pages). However, his assessment of their relationship gives insufficient weight to its darker aspects — which are actually quite interesting.

We're told Eleanor merely had an "initial unease with Franklin's affairs," but the reality is she was devastated after the first time she discovered his infidelity — specifically, his affair with her secretary Lucy Mercer in 1918. In her own words: "The bottom dropped out of my own particular world, and I faced myself, my surroundings, my world honestly for the first time." Eleanor very nearly divorced him despite the practical impossibly of doing so. She refused to share his bed and may have never had sex with him again. It was apparently only years later, in the late 1920s, after their shared struggle with Franklin's polio, that she sought her own lovers and approved of Franklin's subsequent relationship with his secretary Missy LeHand. To be fair to Franklin, they had serious differences on a number of levels that must have made it difficult to connect.

The author interprets Franklin and Eleanor's emotional distance as a sign of healthy independence, but it really seems that at some point along the way their marriage effectively came to an end in private, even while they were the "quintessential power couple" in public. Most tellingly, in 1942, as his health was failing, and with Missy paralyzed from a stroke, Franklin sought to renew his connection with Eleanor and share more of their life together, but she declined. Nor was Eleanor there for his death (though his old mistress Lucy was there — a fact kept hidden from her).

The author tries to use Franklin and Eleanor's marriage to show that "[s]exual fidelity is not a requirement for committed and permanent social monogamy." But if it is an example of polyamory, it is an unhappy one. In the words of their son, their marriage was really more of an "armed truce."

One wonders whether they could have solved their problems better if they had the "tools" of our own era for having a healthy polyamorous relationship — that is, our ethics (hopefully not just our lip service) of maintaining greater honesty, communication, and connection among polyamorous couples. To paraphrase the author, we solve the challenges of polyamory with our minds. And, if I may add, with our hearts.

[Will · Rating 4 out of 5]

A wonderful, sassy/quirky, simple yet highly informative book on so many useful subjects to help navigate the growing evolution of the human species. Explanations on transgender, queer, non-binary, bisexuality, intersex & even heterosexuals were very interesting. How animals exhibit behaviors that prove that evolution is a real thing, our evolution specifically as humans have put so many setbacks in place instead of embracing that growth. There were sections on dispelling mythologies of sexual orientation & gender as well as summarized pieces of monumental discoveries in the history of sexual research that I really enjoyed. I’d recommend this book to anyone who has ever sat back & thought deeply about our existence. Also really enjoyed how easy it was to understand even when the science terms really came into play.

[Dabi · Rating 3 out of 5]

Overall good. Very clear thesis. Manages to make his point with surprisingly few soapbox moments (though I’m sure if I disagreed with his thesis I’d be far more put off by the humor.)

Obviously I am not an expert and this book is aimed at the layperson so take this review with a pinch of salt, but the topics on which Dr. Lents is not an expert do suffer somewhat. Particularly the chapters that are more anthropology and sociology than biology (when he touches on hunter gatherer societies, his digressions on real historical and modern societies and cultures, what exactly Plato’s Symposium says about marriage and sex) did leave me thinking ‘I’m pretty sure the experts say it’s a bit more complicated than that.’

That said everything is more complicated than that and this is only the one book so. Not bad. 3.5 stars.

[Andrea Wenger · Rating 5 out of 5]

This book uses examples from the animal kingdom to demonstrate that diverse intimate behaviors are not a recent or uniquely human phenomenon, but a product of billions of years of evolution, challenging the notion that such diversity is unnatural.

This entertaining book is eye-opening, well-researched, and a pleasure to read. Its perspective on gender and reproduction is revolutionary. Highly recommended!

Thanks, NetGalley, for the ARC I received. This is my honest and voluntary review.

[Lauren Bruce · Rating 5 out of 5]

Thank you to NetGalley for The Sexual Evolution by Nathan H Lents.

This book was so cool. I took a neuroscience of gender course in college where we learned a lot of this information. But we covered strictly neurobiology, so most of the cultural history was new to me. The author made this very approachable for how much information was included and part of that was the occasional commentary. There was a perfect amount to keep it from being too dry, but without taking away from the facts.

I added everything else by Lents to my TBR! 5 stars!

[Meg Jacoby · Rating 4 out of 5]

4/5 ⭐️ super interesting book! i love how the author made the science really approachable for individuals who might not have a background in genetics but still have enough info to not dumb anything down. i’m the progression through different species manifestations of gender, sex, and sexuality finally culminating in discussion of people was a really fun and informative read.

[Maurice Chao · Rating 5 out of 5]

I thought I understood diversity until I finished this book.

This is very thought provoking. I don’t have any complaints.

It explores not only homosexuality (and bisexuality), but different expressions of gender, weird ways of mating (including penis fencing and semen stabbing??) and moreover, explains the functionality of diversity.

[Shane · Rating 5 out of 5]

Highly recommend!

[Owen Farrell · Rating 5 out of 5]

excellent scientific book. highly reccomend

[Matt Easton · Rating 5 out of 5]

Really fun! Interesting and funny, great pop science book about animal behavior and biology. A great read for the queer science enthusiast. Will make you say “whoa I didn’t know that” many times!

[Jennie · Rating 5 out of 5]

I enjoyed every bit of this book from the start to the book recommendations at the end (he gives enough info about each book so you know which will appeal to you).

As a non-scientist I found his writing easy to understand and entertaining. His occasional personal notes were also appreciated. The topic is very timely and giving a lay person a better understanding of the science is very helpful.

The best part is he is looking at the world without the "patriarchal glasses" that science has almost always been seen through.

[nic mistry · Rating 4 out of 5]

This book was incredibly informative and thought provoking. As someone drawn to animal behavior, Chapters 1-4 were full of intriguing facts, but I found Chapters 5-8 particularly inspiring and informative. I loved the message that "if you assume a bimodal model, you ignore all of the points in between" because it highlights a lot of the issues both with past research and with societal bias today. As it is a more scientific read, there are lot of examples/evidence which can make it seem slightly dense if that isn't what you're looking for, but all of the claims and theories are explained very clearly, and Lents tied together biology, sociology, psychology, and history together seamlessly, making this book accessible to just about anyone.

[Ashlie Miller · Rating 4 out of 5]

Thank you to Mariner Books for this free copy in exchange for an honest review!

This book taught me so much, and learning about the sexual behaviors of animals brought me so much joy (but not in a weird bestiality way). Animals are truly fascinating, and I loved sharing facts across my social media accounts.

The biggest takeaway I got from this book is that DIVERSITY WINS! Transgender and gay animals (including humans) have and will always exist. They are NOT unnatural or abnormal and should not be treated as such.