Why Is Sex Fun?: The Evolution of Human Sexuality

by Jared Diamond

  Overall, it appears that monogamy has evolved independently at least seven times in higher primates: in us, in gibbons, and in at least five separate groups of monkeys.

  Family Tree of Mating Systems

  Figure 4.2

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  Harems must have evolved at least eight times, including in the Missing Link. Chimps and at least two monkeys must have reinvented promiscuity after their recent ancestors had given it up for harems.

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  Thus, we have reconstructed both the type of mating system and the type of ovulatory signal that probably existed in primates of the remote past, all along the primate family tree. We can now, finally, put both types of information together and ask: what mating system prevailed at each point in our family tree when concealed ovulation evolved?

  Here’s what one learns. Consider those ancestral species that gave signals of ovulation, and that then went on to lose those signals and evolve concealed ovulation. Only one of those ancestral species was monogamous. In contrast, eight, perhaps as many as eleven, of them were promiscuous or harem-holding species—one of them being the human ancestor that arose from the harem-holding Missing Link. We thus conclude that promiscuity or harems, not monogamy, is the mating system that leads to concealed ovulation (see figure 4.3). This is the conclusion predicted by the many-fathers theory. It doesn’t agree with the daddy-at-home theory.

  Conversely, we can also ask: what were the ovulatory signals prevailing at each point in our family tree when monogamy evolved? We find that monogamy never evolved in species with bold advertisement of ovulation. Instead, monogamy has usually arisen in species that already had concealed ovulation, and sometimes in species that already had slight ovulatory signals (see figure 4.4). This conclusion agrees with the predictions of the daddy-at-home theory.

  How can these two apparently opposite conclusions be reconciled? Recall that Sillén-Tullberg and Møller found, in step 3 of their analysis, that almost all monogamous primates have concealed ovulation. We now see that that result must have arisen in two steps. First, concealed ovulation arose, in a promiscuous or harem-holding species. Then, with concealed ovulation already present, the species switched to monogamy (see figure 4.4).

  Figure 4.3

  By combining facts about modern observed species with inferences about ancestral species, one can infer the mating system prevailing when ovulatory signals underwent evolutionary change. we infer that species 3 evolved concealed ovulation from a harem-holding ancestor with slight signs of ovulation, while species 1 and 2 preserved the ancestral mating system (harems) and slight ovulatory signs.

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  Figure 4.4

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  Perhaps by now you’re finding our sexual history confusing. We started out with an apparently simple question that deserved a simple answer: why do we hide our ovulations and have recreational sex on any day of the month? Instead of a simple answer, you’re being told that the answer is more complex and involves two steps.

  What it boils down to is that concealed ovulation has repeatedly changed, and actually reversed, its function during primate evolutionary history. It arose at a time when our ancestors were still promiscuous or living in harems. At such times, concealed ovulation let the ancestral ape-woman distribute her sexual favors to many males, none of which could swear that he was the father of her child but each of which knew that he might be. As a result, none of those potentially murderous males wanted to harm the ape-woman’s baby, and some may actually have protected or helped feed it. Once the ape-woman had evolved concealed ovulation for that purpose, she then used it to pick a good caveman, to entice or force him to stay at home with her, and to get him to provide lots of protection or help for her baby—secure in the knowledge that it was his baby too.

  On reflection, we shouldn’t be surprised at this shift of function for concealed ovulation. Such shifts are very common in evolutionary biology. That’s because natural selection doesn’t proceed consciously and in a straight line toward a distant perceived goal, in the way that an engineer consciously designs a new product. Instead, a feature that serves one function in an animal begins to serve another function as well, becomes modified as a result, and may even lose the original function. The consequence is frequent reinventions of similar adaptations, and frequent losses, shifts, or even reversals of function, as living things evolve.

  One of the most familiar examples involves vertebrate limbs. The fins of ancestral fishes, used for swimming, evolved into the legs of ancestral reptiles, birds, and mammals, which used them for running or hopping on land. The front legs of certain ancestral mammals and reptile-birds subsequently evolved into the wings, used for flying, of bats and modern birds, respectively. Bird wings and mammal legs then evolved independently into the flippers of penguins and whales, respectively, thereby reverting to a swimming function and effectively reinventing the fins of fish. At least three groups of fish descendants independently lost their limbs to become snakes, legless lizards, and the legless amphibians known as cecilians. In essentially the same way, features of reproductive biology—such as concealed ovulation, boldly advertised ovulation, monogamy, harems, and promiscuity—have repeatedly changed function and been transmuted into each other, reinvented, or lost.

  The implications of these evolutionary shifts can lend zest to our love lives. For example, in the last novel by the great German writer Thomas Mann, Confessions of Felix Krull, Confidence Man, Felix shares a compartment on a train journey with a paleontologist, who regales him with an account of vertebrate limb evolution. Felix, an accomplished and imaginative ladies’ man, is delighted by the implications. “Human arms and legs retain the bones of the most primitive land animals! . . . It’s thrilling! . . . A woman’s shapely charming arm, which embraces us if we find favor . . . it’s no different from the primordial bird’s clawed wing, and the fish’s pectoral fin. . . . I’ll think of that, next time. . . . Dream of that shapely arm, with its ancient scaffolding of bones!”

  Now that Sillén-Tullberg and Møller have unraveled the evolution of concealed ovulation, you can nourish your own fantasy with its implications, just as Felix Krull nourished his fantasy with the implications of vertebrate limb evolution. Wait until the next time that you are having sex for fun, at a nonfertile time of the ovulatory cycle, while enjoying the security of a lasting monogamous relationship. At such a time, reflect on how your bliss is made paradoxically possible by precisely those features of your physiology that distinguished your remote ancestors as they languished in harems, or as they rotated among promiscuously shared sex partners. Ironically, those wretched ancestors had sex only on rare days of ovulation, when they perfunctorily discharged the biological imperative to fertilize, robbed of your leisurely pleasure by their desperate need for swift results.

  CHAPTER 5

  WHAT ARE MEN GOOD FOR?

  The Evolution of Men’s Roles

  Last year I received a remarkable letter from a professor at a university in a distant city, inviting me to an academic conference. I did not know the writer, and I couldn’t even figure out from the name whether the writer was a man or a woman. The conference would involve long plane flights and a week away from home. However, the letter of invitation was beautifully written. If the conference was going to be as beautifully organized, it might be exceptionally interesting. With some ambivalence because of the time commitment, I accepted.

  My ambivalence vanished when I arrived at the conference, which turned out to be every bit as interesting as I had anticipated. In addition, much effort had been made to arrange outside activities for me, including shopping, bird-watching, banquets, and tours of archaeological sites. The professor behind this masterpiece of organization and the original virtuoso letter proved to be a woman. In addition to giving a brilliant lecture at the conference and being a very pleasant person, she was among the most stunningly beautiful women I had ever met.

  On one of the
shopping trips that my hostess arranged, I bought several presents for my wife. The student who had been sent along as my guide evidently reported these purchases to my hostess, because she commented on them when I sat next to her at the conference banquet. To my astonishment, she told me, “My husband never buys me any presents!” She had formerly bought presents for him but eventually stopped when he never reciprocated.

  Someone across the table then asked me about my field-work on birds of paradise in New Guinea. I explained that male birds of paradise provide no help in rearing the nestlings but instead devote their time to trying to seduce as many females as possible. Surprising me again, my hostess burst out, “Just like men!” She explained that her own husband was much better than most men, because he encouraged her career aspirations. However, he spent most evenings with other men from his office, watched television while at home on the weekend, and avoided helping with the household and with their two children. She had repeatedly asked him to help; she finally gave up and hired a housekeeper. There is, of course, nothing unusual about this story. It stands out in my mind only because this woman was so beautiful, nice, and talented that one might naively have expected the man who chose to marry her to have remained interested in spending time with her.

  My hostess nevertheless enjoys much better domestic conditions than do many other wives. When I first began to work in the New Guinea highlands, I often felt enraged at the sight of gross abuse of women. Married couples whom I encountered along jungle trails typically consisted of a woman bent under an enormous load of firewood, vegetables, and an infant, while her husband sauntered along upright, bearing nothing more than his bow and arrow. Men’s hunting trips seemed to yield little more than male bonding opportunities, plus some prey animals immediately consumed in the jungle by the men. Wives were bought, sold, and discarded without their consent.

  Later, though, when I had children of my own and sensed my feelings as I shepherded my family on walks, I thought that I could better understand the New Guinea men striding beside their families. I found myself striding next to my own children, devoting all my attention to making sure that they did not get run over, fall, wander off, or suffer some other mishap. Traditional New Guinea men had to be even more attentive because of the greater risks facing their children and wives. Those seemingly carefree men strolling along beside a heavily burdened wife were actually functioning as lookouts and protectors, keeping their hands free so that they could quickly deploy their bow and arrow in the event of ambush by men of another tribe. But the men’s hunting trips, and the sale of women as wives, continue to trouble me.

  To ask what men are good for may sound like a flip one-liner. In fact, the question touches a raw nerve in our society. Women are becoming intolerant of men’s self-ascribed status and are criticizing those men who provide better for themselves than for their wives and children. The question also poses a big theoretical problem for anthropologists. By the criterion of services offered to mates and children, males of most mammal species are good for nothing except injecting sperm. They part from the female after copulation, leaving her to bear the entire burden of feeding, protecting, and training the offspring. But human males differ by (usually or often) remaining with their mate and offspring after copulation. Anthropologists widely assume that men’s resulting added roles contributed crucially to the evolution of our species’ most distinctive features. The reasoning goes as follows.

  The economic roles of men and women are differentiated in all surviving hunter-gatherer societies, a category that encompassed all human societies until the rise of agriculture ten thousand years ago. Men invariably spend more time hunting large animals, while women spend more time gathering plant foods and small animals and caring for children. Anthropologists traditionally view this ubiquitous differentiation as a division of labor that promotes the nuclear family’s joint interests and thereby represents a sound strategy of cooperation. Men are much better able than women to track and kill big animals, for the obvious reasons that men don’t have to carry infants around to nurse them and that men are on the average more muscular than women. In the view of anthropologists, men hunt in order to provide meat to their wives and children.

  A similar division of labor persists in modern industrial societies: many women still devote more time to child care than men do. While men no longer hunt as their main occupation, they still bring food to their spouse and children by holding money-paying jobs (as do a majority of American women as well). Thus, the expression “bringing home the bacon” has a profound and ancient meaning.

  Meat provisioning by traditional hunters is considered a distinctive function of human males, shared with only a few of our fellow mammal species such as wolves and African hunting dogs. It is commonly assumed to be linked to other universal features of human societies that distinguish us from our fellow mammals. In particular, it is linked to the fact that men and women remain associated in nuclear families after copulation, and that human children (unlike young apes) remain unable to obtain their own food for many years after weaning.

  This theory, which seems so obvious that its correctness is generally taken for granted, makes two straightforward predictions about men’s hunting. First, if the main purpose of hunting is to bring meat to the hunter’s family, men should pursue the hunting strategy that reliably yields the most meat. Hence we should observe that men are on the average bagging more pounds of meat per day by going after big animals than they would bring home by targeting small animals. Second, we should observe that a hunter brings his kill to his wife and kids, or at least shares it preferentially with them rather than with nonrelatives. Are these two predictions true?

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  Surprisingly for such basic assumptions of anthropology, these predictions have been little tested. Perhaps unsurprisingly, the lead in testing them has been taken by a woman anthropologist, Kristen Hawkes of the University of Utah. Hawkes’s tests have been based especially on quantitative measurements of foraging yields for Paraguay’s Northern Aché Indians, carried out jointly with Kim Hill, A. Magdalena Hurtado, and H. Kaplan. Hawkes performed other tests on Tanzania’s Hadza people in collaboration with Nicholas Blurton Jones and James O’Connell. Let’s consider first the evidence for the Aché.

  The Northern Aché used to be full-time hunter-gatherers and continued to spend much time foraging in the forest even after they began to settle at mission agricultural settlements in the 1970s. In accord with the usual human pattern, Aché men specialize in hunting large mammals, such as peccaries and deer, and they also collect masses of honey from bees’ nests. Women pound starch from palm trees, gather fruits and insect larvae, and care for children. An Aché man’s hunting bag varies greatly from day to day: he brings home food enough for many people if he kills a peccary or finds a beehive, but he gets nothing at all on one-quarter of the days he spends hunting. In contrast, women’s returns are predictable and vary little from day to day because palms are abundant; how much starch a woman gets is mainly a function of just how much time she spends pounding it. A woman can always count on getting enough for herself and her children, but she can never reap a bonanza big enough to feed many others.

  The first surprising result from the studies by Hawkes and her colleagues concerned the difference between the returns achieved by men’s and women’s strategies. Peak yields were, of course, much higher for men than for women, since a man’s daily bag topped 40,000 calories when he was lucky enough to kill a peccary. However, a man’s average daily return of 9,634 calories proved to be lower than that of a woman (10,356), and a man’s median return (4,663 calories per day) was much lower. The reason for this paradoxical result is that the glorious days when a man bagged a peccary were greatly outnumbered by the humiliating days when he returned empty-handed.

  Thus, Aché men would do better in the long run by sticking to the unheroic “woman’s job” of pounding palms than by their devotion to the excitement of the chase. Since men are stronger than women,
they could pound even more daily calories of palm starch than can women, if they chose to do so. In going for high but very unpredictable stakes, Aché men can be compared to gamblers who aim for the jackpot: in the long run, gamblers would do much better by putting their money in the bank and collecting the boringly predictable interest.

  The other surprise was that successful Aché hunters do not bring meat home mainly for their wives and kids but share it widely with anyone around. The same is true for men’s finds of honey. As a result of this widespread sharing, three-quarters of all the food that an Aché consumes is acquired by someone outside his or her nuclear family.

  It’s easy to understand why Aché women aren’t big-game hunters: they can’t spend the time away from their children, and they can’t afford the risk of going even a day with an empty bag, which would jeopardize lactation and pregnancy. But why does a man eschew palm starch, settle for the lower average return from hunting, and not bring home his catch to his wife and kids, as the traditional view of anthropologists predicts?