Why Is Sex Fun?: The Evolution of Human Sexuality

While paleontologists usually attribute the evolution of these features to our attainment of large brains and upright posture, I argue that our bizarre sexuality was equally essential for their evolution. (View Highlight)

Most mammals do not live as a nuclear family of a mated adult male and adult female, caring jointly for their offspring. Instead, in many mammal species both adult males and adult females are solitary, at least during the breeding season, and meet only to copulate. Hence, males do not provide paternal care; their sperm is their sole contribution to their offspring and to their temporary mate. (View Highlight)

Sex in social mammals is generally carried out in public, before the gazes of other members of the troop (View Highlight)

Thus, sex is emphatically not just for fun and is rarely divorced from its function of fertilization. This generalization too admits exceptions: sex is flagrantly separated from reproduction in a few species, including bonobos (pygmy chimpanzees) and dolphins (View Highlight)

While humans and most other mammal species breed repeatedly, salmon, octopus, and many other animal species practice what is termed big-bang reproduction, or semelparity: a single reproductive effort, followed by preprogrammed death. (View Highlight)

Closest of all are Africa’s chimpanzee and bonobo, from which we differ in only about 1.6 percent of our nuclear genetic material (DNA). Nearly as close are the gorilla (2.3 percent genetic difference from us) and the orangutan of Southeast Asia (3.6 percent different). Our ancestors diverged “only” about seven million years ago from the ancestors of chimpanzees and bonobos, nine million years ago from the ancestors of gorillas, and fourteen million years ago from the ancestors of orangutans. (View Highlight)

Tags: #evolution

The male of some species of spiders and mantises is routinely eaten by his mate just after or even while he is copulating with her. This cannibalism clearly involves the male’s consent, because the male of these species approaches the female, makes no attempt to escape, and may even bend his head and thorax toward the female’s mouth so that she may munch her way through most of his body while his abdomen remains to complete the job of injecting sperm into her. (View Highlight)

Instead, by encouraging the female to eat him, he enables her to produce more eggs bearing his genes (View Highlight)

But most fathers make some contribution to their children, even if it’s just food or defense or land rights. We take such contributions so much for granted that they’re written into law: divorced fathers owe child support, and even an unwed mother can sue a man for child support if genetic testing proves that he is her child’s father. (View Highlight)

In particular, while natural selection favors both males and females that leave many offspring, the best strategy for doing so may be different for fathers and mothers. That generates a built-in conflict between the parents, a conclusion that all too many humans don’t need scientists to reveal to them. We make jokes about the battle of the sexes, but the battle is neither a joke nor an aberrant accident of how individual father or mothers behave on particular occasions. It is indeed perfectly true that behavior that is in a male’s genetic interests may not necessarily be in the interests of his female co-parent, and vice versa. That cruel fact is one of the fundamental causes of human misery (View Highlight)

All of us know from experience that we are much more reluctant to walk away from an ongoing enterprise in which we have invested a lot than from one in which we have invested only a little. That’s true of our investments in human relationships, in business projects, or in the stock market. It’s true regardless of whether our investment is in the form of money, time, or effort. We lightly end a relationship that turns bad on the first date, and we stop trying to construct from parts a cheap toy when we hit a snag within a few minutes. But we agonize over ending a twenty-five-year marriage or an expensive house remodeling. (View Highlight)

Under certain conditions, a considerable fraction of men experience breast development and milk production even without having been treated hormonally (View Highlight)

For example, already in the eighth week of gestation the testes begin producing the steroid hormone testosterone, some of which gets converted into the closely related steroid dihydrotestosterone. These steroids (known as an-drogens) convert some all-purpose embryonic structures into the glans penis, penis shaft, and scrotum; the same structures would otherwise develop into the clitoris, labia minora, and labia majora. Embryos also start out bet-hedging with two sets of ducts, known as the Mullerian ducts and Wolffian ducts. In the absence of testes, the Wolffian ducts atrophy, while the Mullerian ducts grow into a female fetus’s uterus, fallopian tubes, and interior vagina. With testes present, the opposite happens: androgens stimulate the Wolffian ducts to grow into a male fetus’s seminal vesicles, vas deferens, and epididymis. At the same time, a testicular protein called Mullerian inhibiting hormone does what its name implies: it prevents the Mullerian ducts from developing into the internal female organs (View Highlight)

Each step involves the synthesis of one molecular ingredient, termed an enzyme, specified by one gene. Any enzyme can be defective or absent if its underlying gene is altered by a mutation. Thus, an enzyme defect may result in a male pseudohermaphrodite, defined as someone possessing some female structures as well as testes. In a male pseudoher-maphrodite with an enzyme defect, there is normal development of the male structures dependent on enzymes that act at the steps of the metabolic pathway before the defective enzyme. However, male structures dependent on the defective enzyme itself or on subsequent biochemical steps fail to develop and are replaced either by their female equivalent or by nothing at all (View Highlight)

Evidently, evolution just didn’t design men to utilize that physiological potential under normal conditions. In computing terminology, at least some males have the hardware; we merely haven’t been programmed by natural selection to use it. (View Highlight)

Hence if sex is enjoyable, natural selection must have been responsible for that outcome. Yes, sex is fun for dogs too, but only at the right time: dogs, like most other animals, have evolved the good sense to enjoy sex when it can do some good (View Highlight)

Sperm production is sufficiently costly for males that worms with a mutation that reduces sperm production live longer than normal worms. (View Highlight)

Older individuals may succumb to the strain of sex: France’s Emperor Napoleon the Third suffered a stroke while engaged in the act, and Nelson Rockefeller died during sex (View Highlight)

Conversely, one can point to males of numerous mammal species that require no such sexual bribes to induce them to remain with their mate and offspring. Gibbons, which actually often live as monogamous couples, go years without sex (View Highlight)

Gibbons, most songbirds, and gorillas live dispersed over the landscape, with each couple (or harem) occupying its separate territory. That pattern provides few encounters with potential extramarital sex partners. Perhaps the most distinctive feature of traditional human society is that mated couples live within large groups of other couples with whom they have to cooperate economically (View Highlight)

From this plethora of hypotheses to explain concealed ovulation, two, which I shall refer to as the “daddy-at-home” theory and the “many-fathers” theory, have survived as most plausible. Interestingly, the two hypotheses are virtually opposite. The daddy-at-home theory posits that concealed ovulation evolved to promote monogamy, to force the man to stay home, and thus to bolster his certainty about his paternity of his wife’s children. The many-fathers theory instead posits that concealed ovulation evolved to give the woman access to many sex partners and thus to leave many men uncertain as to whether they sired her children. (View Highlight)

In short, Hrdy considers concealed ovulation an evolutionary adaptation by females to minimize the big threat to their offsprings’ survival posed by adult males (View Highlight)

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. (View Highlight)

In any case, all that meat occasionally bagged by a Hadza or Ache hunter isn’t reserved for his family, so the question of whether big-game hunting yields higher or lower returns than alternative strategies is academic from his family’s point of view. Big-game hunting just isn’t the best way to feed a family. (View Highlight)

In reality, though, successful Ache and Hadza hunters share their catch with anyone around, whether he’s a good or hopeless hunter. That raises the question of why an Ache or Hadza man bothers to hunt at all, since he can claim a share of meat even if he never bags anything himself. Conversely, why should he hunt when any animal that he kills will be shared widely? Why doesn’t he just gather nuts and rats, which he can bring to his family and would not have to share with anyone else? There must be some ignoble motive for male hunting that I was overlooking in my efforts to find a noble motive. (View Highlight)

A woman is best off married to a provider, but a man is not best off being a provider. (View Highlight)

To evolutionary biologists, female menopause is thus among the most bizarre features of human sexuality. As I shall argue, it is also among the most important. Along with our big brains and upright posture (emphasized in every text of human evolution), and our concealed ovula-tions and penchant for recreational sex (to which texts devote less attention), I believe that female menopause was among the biological traits essential for making us distinctively human-a creature more than, and qualitatively different from, an ape. (View Highlight)

The aging (or senescence, as biologists call it) of the female reproductive tract cannot be profitably considered in isolation from other aging processes. Our eyes, kidneys, heart, and all other organs and tissues also senesce. But that aging of our organs is not physiologically inevitable-or at least it’s not inevitable that they senesce as rapidly as they do in the human species, because the organs of some turtles, clams, and other species remain in good condition much longer than ours do. (View Highlight)

Analogy to our cars again makes the reason obvious: the expense of repair and maintenance. Most of us have only limited amounts of money, which we are obliged to budget. We put just enough money into car repair to keep our car running as long as it makes economic sense to do so. When the repair bills get too high, we find it cheaper to let the old car die and buy a new one. Our genes face a similar tradeoff between repairing the old body that contains the genes and making new containers for the genes (that is, babies) (View Highlight)

Instead, the human female, but not the human male, has become specifically programmed by natural selection, at some time within the last few million years, to shut down reproduction prematurely. That premature senescence is all the more surprising because it goes against an overwhelming trend: in other respects, we humans have evolved delayed rather than premature senescence. (View Highlight)

At least three sets of human signals seem to me to conform to Kodric-Brown’s and Brown’s truth-in-advertising model: men’s body muscle, facial “beauty” in both sexes, and women’s body fat. (View Highlight)

Lactation and child care are a big energy drain on a mother, and lactation tends to fail in an undernourished mother. In traditional societies before the advent of infant formulas and before the domestication of milk-producing hoofed animals, a mother’s lactational failure would have been fatal to her infant. Hence a woman’s body fat would be a truthful signal to a man that she was capable of rearing his child. Naturally, men should prefer the correct amount of fat: too little could be a harbinger of lactational failure, but too much could signal difficulties in walking, poor food-gathering ability, or early death from diabetes. (View Highlight)

Men throughout the world tend to be interested in women’s breasts, hips, and buttocks, giving rise in modern societies to yet another surgical method of fake signals, breast enhancement (View Highlight)

Every garbage can has a lid, and there is usually a bad-looking man for every bad-looking woman. (View Highlight)

Tags: #savage

Although virtually any woman can get some man to marry her, only a few women can succeed in getting one of the few high-quality men, for whom women must compete intensely. Every woman knows that, even though some male scientists evidently don’t. (View Highlight)

The length of the erect penis is only about VA inches in gorillas and 1 1/2 inches in orangutans but 5 inches in humans, even though males of the two apes have much bigger bodies than men (View Highlight)

Penis evolution evidently illustrates the operation of runaway selection just as Fisher postulated. Starting from a 1/4-inch ancestral ape penis similar to the penis of a modern gorilla or orangutan, the human penis increased in length by a runaway process, conveying an advantage to its owner as an increasingly conspicuous signal of virility, until its length became limited by counterselection as difficulties fitting into a woman’s vagina became imminent. (View Highlight)

However, it is large enough that if the same quantity of tissue were instead devoted to extra cerebral cortex, that brainy redesigned man would gain a big advantage. Hence a large penis’s cost should be regarded as a lost-opportunity cost: because any man’s available biosyn-thetic energy is finite, the energy squandered on one structure comes at the expense of energy potentially available for another structure. (View Highlight)

However, women tend to report that they are more turned on by other features of a man, and that the sight of a penis is, if anything, unattractive. Instead, the ones really fascinated by the penis and its dimensions are men. In the showers in men’s locker rooms, men routinely size up each other’s endowment. (View Highlight)