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Lixing Sun, Profile Books

As children, many of us learn the facts of life with examples from the natural world. The thinking is that it is more straightforward (and certainly less uncomfortable) to illustrate 鈥渢he talk鈥� with images of birds laying eggs and bees pollinating flowers than it is to focus on humans or, heaven forbid, your parents.

But this is a comforting fiction, as evolutionary and behavioural biologist Lixing Sun sets out in his engaging and frequently mind-boggling book, On the Origin of Sex: The weird and wonderful science of how our planet is populated. The 鈥渂irds and the bees鈥� analogy just doesn鈥檛 capture the diversity, drama and strangeness of animal reproduction.

Even animals that might strike us as dull have surprising and frequently cut-throat approaches to reproduction: barnyard hens can eject most of the sperm after mating with a low-ranking male; male ducks have evolved a corkscrew-shaped penis; and female ducks have evolved vaginas that spiral in the opposite direction, giving them greater control over paternity.

On the Origin of Sex is stuffed with such examples, enlivening the biological nuts and bolts. The range of reproduction makes the most adventurous of human activities seem tame, but, as the title suggests, Sun鈥檚 scope is far more ambitious. Instead, he grapples with the big questions puzzling us about reproduction, starting with why there is such a thing as sex at all.

The strict male-female binary starts to feel not just limited, but downright outdated

For readers who aren鈥檛 well-versed in evolutionary biology, this might seem an odd line of enquiry, but from an evolutionary perspective, sexual reproduction is more difficult and costly than asexual reproduction. This is because it requires two partners, one producing male gametes and the other female gametes. On the face of it, asexual reproduction is far more efficient and is still used by many species, such as bacteria and fungi. Parthenogenesis, a form of asexual reproduction in which the embryo develops without need for fertilisation, has been found in more than 80 species of vertebrates, including California condors, bonnethead sharks and Komodo dragons.

So why did a 鈥渢wo-sex system鈥� take off? Sun devotes the book鈥檚 first section to that question with a brisk tour through key ideas, such as Muller鈥檚 ratchet, proposed in 1964 by geneticist Hermann Muller. This explains that the lack of genetic mixing in asexual reproduction leads to the gradual accumulation of irreversible 鈥渂ad鈥� mutations.

Then there is the 鈥淩ed Queen hypothesis鈥�, which sees evolution as a perpetual arms race in which species 鈥渉ave to hustle at full throttle just to keep up with their adversaries and stay in the game鈥�, writes Sun. This effectively reinforces the stakes of sex as quite literally a matter of life and death.

A distinguished research professor at Central Washington University, Sun is well-connected to many of the scientists whose work he cites. On The Origin of Sex is his fourth book for a lay audience, following works on cheating and fairness in the natural world 鈥� and a natural history of the beaver.

By Sun鈥檚 own admission, On the Origin of Sex is a 鈥渇ast, selective ride鈥�, but his imagined reader isn鈥檛 always obvious. He can assume considerable knowledge of evolutionary biology. At other times, his asides and flourishes point to a more accessible title. But the book remains interesting because the many examples of kooky animal sex make it a pacy read, even if you struggle with some of the more technical scientific discussion.

Sun鈥檚 enthusiasm for his subject, and his wonder at the adaptive strategies of the animal kingdom, is also infectious. His chapters on gender adaptations and sex variations are especially refreshing. Sun argues that we need to take a more expansive view of gender, writing that our current definition is 鈥渉opelessly homocentric, cut off from our evolutionary connections鈥�. He uses the term to describe an animal鈥檚 roles and functions beyond gamete production, and his patient, step-by-step explanation of the science underpinning both sex and gender not only clarifies, but highlights how much humans have invested in the subject.

He writes that in 鈥減opular discourse, gender is often treated as a social construct. What鈥檚 missing, though, is a more basic understanding of gender as a biological construct 鈥� something evolutionary biology can help uncover. That鈥檚 where I come in.鈥�

He is careful not to wade into culture wars, stressing his concern is evolutionary biology and, mostly, non-human animals. But it becomes clear that nature isn鈥檛 binary. 鈥淏iological sex isn鈥檛 just a matter of splitting the world into sperm-producers and egg-makers,鈥� Sun writes. For example, swamp eels, clownfish, wrasses and other fish change sex as a matter of course. Even in birds and mammals (including humans), there is broad variation, including intersex individuals.

Gender, too 鈥� taking in not just reproductive roles but behaviour and social dynamics 鈥� is far more fluid among animals than we thought, and is also related to sex. For example, hyenas don鈥檛 just live in matriarchal packs; females also possess a 鈥減seudo-penis鈥� (and a 鈥減seudo-scrotal sac鈥�), evolved to give them an edge in the competition to get at food after a kill, but complicating pregnancy.

Sun points to this kaleidoscopic range to argue for fresh thinking on sex and gender, to be inclusive of differences as 鈥渘atural, adaptive parts of life鈥檚 diversity鈥�. As he concludes, the 鈥渟trict male-female binary starts to feel not just limited, but downright outdated鈥�.

Elsewhere, Sun has said his early ambition with On the Origin of Sex was to inject science into discussions of diversity and social justice. By sticking to science and giving examples from nature, he succeeds. Indeed, the dizzying diversity among non-human life and the many questions about sex and reproduction still awaiting answers should challenge us to embrace complexity and keep an open mind, he suggests.

We might think we know about the facts of life, but even Sun concludes his book feeling awed by 鈥渉ow little we truly understand鈥�.

Elle Hunt is a writer based in Norwich, UK

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Three more great reads on sex and evolution

by Matt Ridley

Check out this 1993 pop-sci classic by author Matt Ridley. Lixing Sun namechecks it for shaping our understanding of sex and gender 鈥� and for its 鈥渞azor-sharp wit and punchy prose鈥�.

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by Nathan H. Lents

Packed with examples of animal sexual diversity, this entertaining and witty book by Nathan H. Lents makes light work of his rigorous research and contains important points about sexual diversity in humans, too.

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by Justin Garcia

This 2026 book sheds light on the science of human attraction and connection. Justin Garcia is superqualified as an evolutionary biologist, executive director of the Kinsey Institute and scientific advisor to dating site .

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