Biologist Lixing Sun of Central Washington University authored the book "On the Origin of Sex: The Weird and Wonderful Science of Reproduction." The book, published by Basic in 2024, explores the evolutionary aspects of sexual reproduction across various species.

Sun's book examines organisms with complex mating systems, such as the protozoan Tetrahymena thermophila. These organisms, approximately one-twentieth of a millimeter long, can reproduce by splitting in half or through conjugation. During conjugation, which occurs when they are starving, two cells temporarily fuse and exchange genetic material. Tetrahymena thermophila have seven distinct mating types, leading to twenty-one possible pairings. "That's one wildly complicated dating pool for a single-celled organism!" Sun stated.

Other species also exhibit numerous mating types. The common white-button mushroom has eighteen mating types, while the fairy inkcap mushroom has one hundred and forty-three. The split-gill mushroom possesses twenty-three thousand three hundred and twenty-eight mating types. The book discusses isogamy, an arrangement where partners are functionally equivalent and genetic material exchange is symmetrical, as seen in yeasts and slime molds. Reproductive disparities are also noted; in humans, an egg has approximately ten million times the volume of a sperm, a ratio that can reach hundreds of billions in large birds like ostriches.

Sun details various reproductive strategies and mate competition. Male cardinals have bright-red feathers, and male three-spined sticklebacks have bright-red bellies. In elephant seals, red deer, and rattlesnakes, fighting often determines mating privilege. Among lion species, typically only one male and his closest associates sire offspring. When a new male takes charge in lion species or mountain gorillas, he frequently kills the offspring fathered by the previous male. Sun wrote that infanticide is "surprisingly common, especially among carnivores and primates, often driven by the brutal logic of mate competition."

The tropical wader jacana presents another reproductive example. In most jacana species, males sit on the eggs. Females may kill other birds' chicks, mate with the male tending them, and then leave him to raise a new clutch. The book also highlights the genus Neotrogla, a type of bark louse native to Brazil that lives in dry caves and feeds predominantly on bat guano. Male Neotrogla produce spermatophores, capsules of sperm containing a protein-rich fluid. Female Neotrogla have evolved a penis-like appendage that they use to extract these spermatophores from mates.