Narrator: From a dog's perspective, human sexual behavior is utterly baffling. A loyal canine might observe its owners, Barbara and John, having sex on any day of the month, even when Barbara is pregnant. It might hear John’s parents, long past their reproductive years, doing the same. To an animal whose entire existence is oriented around clear, cyclical fertility signals, this constant, private, and seemingly pointless activity is disgusting and bizarre. Why would any animal waste so much time and energy on sex that has no chance of producing offspring? This strange scenario, imagined from an animal’s point of view, sits at the heart of a profound evolutionary puzzle.

This is the central question explored in Jared Diamond's groundbreaking book, Why is Sex Fun?. Diamond argues that the features of human sexuality we consider normal—private relationships, sex for pleasure, and female menopause—are, in fact, some of the most unusual behaviors in the entire animal kingdom. The book provides an evolutionary detective story, piecing together clues to explain how and why our species developed the weirdest sex life on the planet.

Narrator: Compared to the vast majority of the world's thirty million animal species, human sexuality is an anomaly. Most animals restrict sex to very specific, brief periods of fertility. A female baboon, for instance, advertises her ovulation with a bright red, swollen rump, and sex is a public, transactional affair confined to this fertile window. For humans, however, the script is completely different.

Three key features make human sexuality unique. First is concealed ovulation. Unlike the baboon, a human female gives off no obvious external signal that she is ovulating. Neither she nor her partner knows the precise moment of fertility without modern technology like an Ovu-stick. Second, this leads to recreational sex. Because ovulation is hidden, sex is not tied to the biological imperative of conception. Humans have sex at any time of the menstrual cycle, during pregnancy, and long after menopause, primarily for pleasure and bonding. Third is female menopause, a complete shutdown of fertility a decade or more before the end of a woman’s natural lifespan. This is virtually unheard of in the animal world, where the ability to reproduce typically lasts until death. These oddities are not random quirks; Diamond argues they are essential adaptations that were crucial for the evolution of human culture, family structures, and even our large brains.

Narrator: At the core of sexual reproduction lies an inherent conflict of interest between males and females. Diamond explains this as a "battle of the sexes," where each partner’s evolutionary strategy is to maximize the transmission of their own genes, sometimes at the expense of the other. This conflict is most apparent in the decision of who cares for the offspring.

Diamond uses the metaphor of a "game of chicken" played at the moment of fertilization. Both parents might benefit if the other one stays to raise the child, freeing them up to find new mates and produce more offspring. Who "loses" this game and stays to provide care depends on three factors: their relative investment in the embryo, their alternative reproductive opportunities, and their confidence in parenthood.

The behavior of the Pied Flycatcher, a European bird, perfectly illustrates this conflict. A male flycatcher will woo a "primary" female and, once she lays her eggs, will fly to a new territory to deceive a "secondary" female into mating with him. He then devotes most of his energy to feeding the primary female's chicks, as they hatched first and have a better chance of survival. The secondary female is left to raise her chicks with little to no help, and her reproductive success plummets. The male wins the evolutionary game by fathering two broods, while the secondary female loses. This dynamic of deception, investment, and conflicting interests is a fundamental force shaping sexual strategies across the animal kingdom, including our own.

Narrator: Given that sex is costly in terms of energy, time, and risk, why did humans evolve to engage in it recreationally? Diamond explores two competing theories. The first is the "daddy-at-home" theory. This theory posits that concealed ovulation and constant sexual receptivity evolved to promote monogamy and ensure paternal investment. By hiding her fertile period, a woman encourages her male partner to stay with her and have sex frequently to increase his chances of fathering a child. This constant proximity cements their pair-bond and gives the man confidence in his paternity, making him more likely to invest resources in raising their uniquely helpless and demanding child. In this view, recreational sex is the "glue" that holds the nuclear family together.

However, a second, contrasting theory offers a different explanation: the "many-fathers" theory. This theory, championed by biologist Sarah Hrdy, argues that concealed ovulation evolved not to clarify paternity, but to confuse it. In many primate species, males who take over a troop will kill the infants of other males to bring the females back into estrus. By mating with multiple males, a female creates paternity confusion. No male can be sure he isn't the father, so all males in the troop are less likely to harm her infant. Evidence from vervet monkeys, who also practice concealed ovulation and mate with many males, supports this idea. Diamond suggests that both theories may be part of the story. Concealed ovulation may have first evolved to prevent infanticide in our more promiscuous ancestors, and its function later shifted to promote pair-bonding as human societies became more monogamous.

Narrator: The traditional image of early human society often features "man the provider," a hunter who bravely brings home meat to feed his loyal wife and children. Diamond challenges this romanticized view with compelling evidence from modern hunter-gatherer societies. Studies of the Ache people in Paraguay, for example, reveal a more complex reality. While women provide a steady, reliable source of calories through gathering plants, men pursue a high-risk, high-reward strategy of hunting large game.

On any given day, a hunter is more likely to come home empty-handed than a gatherer. Furthermore, when a hunter does make a big kill, the meat is not reserved for his own family but is shared widely throughout the entire camp. Anthropologist Kristen Hawkes argues that this behavior makes little sense as a family provisioning strategy. Instead, she suggests that hunting large game is a form of "showing off." A successful hunter gains immense social status, prestige, and, crucially, more opportunities for extramarital affairs. Women are more likely to have affairs with top hunters, who in turn father more children. In this light, men's hunting is less about feeding their family and more about winning the "battle of the sexes" by maximizing their own genetic legacy, even if it conflicts with the best interests of their primary partner.

Narrator: Female menopause presents a major evolutionary puzzle: why would natural selection favor a trait that ends a female's ability to reproduce? The answer likely lies in the "grandmother hypothesis." For a human, the risks of late-life childbirth are high, and the death of a mother could doom her existing dependent children. Diamond argues that an older woman can pass on more of her genes by ceasing to reproduce herself and instead investing her time and energy in helping her existing children and grandchildren. In preliterate societies, older individuals are also vital repositories of knowledge, like the old woman on Rennell Island who was the only person who remembered which wild plants were safe to eat after a devastating cyclone.

This logic of evolutionary trade-offs also extends to body signals. Diamond proposes that many of our physical traits evolved as a form of "truth in advertising." For example, the human penis is exceptionally large compared to that of other primates. He suggests this may be a costly signal of male quality, akin to a peacock's tail. The argument is that a male who can "afford" to grow a large, biologically expensive organ that isn't strictly necessary for reproduction must be genetically superior. Interestingly, observations of men's locker rooms suggest this signal may be directed as much at other men, for establishing dominance, as it is at women.

Narrator: The single most important takeaway from Why is Sex Fun? is that human sexuality, in all its strangeness, is a brilliant evolutionary compromise. Our unique traits—recreational sex, concealed ovulation, pair-bonding, and menopause—are not isolated quirks but interconnected solutions to the fundamental challenge of raising the most helpless, slow-growing, and big-brained offspring in the animal kingdom. Our sexuality evolved not just to be "fun," but to foster the intense, long-term, cooperative parental investment necessary for our species to thrive.

Ultimately, understanding the deep evolutionary roots of our desires and social structures does not provide an excuse for our behavior, but rather a powerful lens for self-awareness. It challenges us to look at our own relationships, the conflicts between men and women, and our societal norms not as arbitrary constructs, but as the modern-day echoes of an ancient evolutionary drama. The question it leaves us with is a profound one: now that we understand these ancient scripts, can we choose when to follow them and when to write our own?