Among animals, external fertilization favors, and internal fertilization mitigates against, the evolution of male parental investment. That fact has discouraged male parental investment by other mammal species but now uniquely favors it in humans, because in-vitro external fertilization techniques have become a reality for humans within the past two decades. Of course, the vast majority of the world’s babies are still conceived internally by natural methods. But the increasing number of older women and men who wish to conceive but have difficulty doing so, and the reported modern decline in human fertility (if it is real), combine to ensure that more and more human babies will be products of external fertilization, like most fish and frogs.
All these features make the human species a leading candidate for male lactation. While that candidacy may take millions of years to perfect through natural selection, we have it in our power to short-circuit that evolutionary process by technology. Some combination of manual nipple stimulation and hormone injections may soon develop the latent potential of the expectant father—his confidence in paternity buttressed by DNA testing—to make milk, without the need to await genetic changes. The potential advantages of male lactation are numerous. It would promote a type of emotional bonding of father to child now available only to women. Many men, in fact, are jealous of the special bond arising from breast-feeding, whose traditional restriction to mothers makes men feel excluded. Today, many or most mothers in first-world societies have already become unavailable for breast-feeding, whether because of jobs, illness, or lactational failure. Yet not only parents but also babies derive many benefits from breast-feeding. Breast-fed babies acquire stronger immune defenses and are less susceptible to numerous diseases, including diarrhea, ear infections, early-age-onset diabetes, influenza, necrotizing enterocolitis, and SIDS (Sudden Infant Death Syndrome). Male lactation could provide those benefits to babies if the mother is unavailable for any reason.
It must be acknowledged, however, that the obstacles to male lactation are not only physiological ones, which can evidently be overcome, but also psychological ones. Men have traditionally regarded breast-feeding as a woman’s job, and the first men to breast-feed their infants will undoubtedly be ridiculed by many other men. Nevertheless, human reproduction already involves increasing use of other procedures that would have seemed ridiculous until a few decades ago: procedures such as external fertilization without intercourse, fertilization of women over the age of fifty, gestation of one woman’s fetus inside another woman’s womb, and survival of prematurely delivered one-kilogram fetuses by high-tech incubator methods. We now know that our evolutionary commitment to female lactation is physiologically labile; it may prove psychologically labile as well. Perhaps our greatest distinction as a species is our capacity, unique among animals, to make counter-evolutionary choices. Most of us choose to renounce murder, rape, and genocide, despite their advantages as a means for transmitting our genes, and despite their widespread occurrence among other animal species and earlier human societies. Will male lactation become another such counter-evolutionary choice?
CHAPTER 4
WRONG TIME FOR LOVE
The Evolution of Recreational Sex
First scene: a dimly lit bedroom, with a handsome man lying in bed. A beautiful young woman in a nightgown runs to the bed. A diamond wedding ring flashes virtuously on her left hand, while her right hand clutches a small blue strip of paper. She bends down and kisses the man’s ear.
She: “Darling! It’s exactly the right time!”
Next scene: same bedroom, same couple, evidently making love, but details tastefully obscured by the dim lighting. Then the camera shifts to a calendar slowly being flipped (to indicate the passage of time) by a graceful hand wearing the same diamond wedding ring.
Next scene: the same beautiful couple, blissfully holding a clean smiling baby.
He: “Darling! I’m so glad that Ovu-stick told us when it was exactly the right time!”
Last frame: close-up of the same graceful hand, clutching the small blue strip of paper. Caption reads: “Ovu-stick. Home urine test to detect ovulation.”
If baboons could understand our TV ads, they’d find that one especially hilarious. Neither a male nor female baboon needs a hormonal test kit to detect the female’s ovulation, the sole time when her ovary releases an egg and when she can be fertilized. Instead, the skin around the female’s vagina swells and turns a bright red color visible at a distance. She also gives off a distinctive smell. In case a dumb male still misses the point, she crouches in front of him and presents her hindquarters. Most other female animals are equally aware of their own ovulation and advertise it to males with equally bold visual signals, odors, or behaviors.
We consider female baboons with bright red hindquarters bizarre. In fact, we humans are the ones whose scarcely detectable ovulations make us members of a small minority in the animal world. Men have no reliable means of detecting when their partners can be fertilized, nor did women in traditional societies. I grant that many women experience headaches or other sensations around the midpoint of a menstrual cycle. However, they wouldn’t know that these are signs of ovulation if they hadn’t been told so by scientists—and even scientists didn’t figure that out until around 1930. Similarly, women can be taught to detect ovulation by monitoring their body temperature or mucus, but that’s very different from the instinctive knowledge possessed by female animals. If we too had such instinctive knowledge, manufacturers of ovulation test kits and contraceptives wouldn’t be doing such a booming business.
We’re also bizarre in our nearly continuous practice of sex, a behavior that is a direct consequence of our concealed ovulations. Most other animal species confine sex to a brief estrous period around the advertised time of ovulation. (The noun estrus and adjective estrous are derived from the Greek word for “gadfly,” an insect that pursues cattle and drives them into a frenzy.) At estrus, a female baboon emerges from a month of sexual abstinence to copulate up to one hundred times, while a female Barbary macaque does it on the average every seventeen minutes, distributing her favors at least once to every adult male in her troop. Monogamous gibbon couples go several years without sex, until the female weans her most recent infant and comes into estrus again. The gibbons relapse once more into abstinence as soon as the female becomes pregnant.
We humans, though, practice sex on any day of the estrus cycle. Women solicit it on any day, and men perform without being choosy about whether their partner is fertile or ovulating. After decades of scientific inquiry, it isn’t even certain at what stage in the cycle a woman is most interested in men’s sexual advances—if indeed her interest shows any cyclical variation. Hence most human copulations involve women who are unable to conceive at that moment. Not only do we have sex at the “wrong” time of the cycle, but we continue to have sex during pregnancy and after menopause, when we know for sure that fertilization is impossible. Many of my New Guinea friends feel obliged to have regular sex right up to the end of pregnancy, because they believe that repeated infusions of semen furnish the material to build the fetus’s body.
Human sex does seem a monumental waste of effort from a “biological” point of view—if one follows Catholic dogma in equating sex’s biological function with fertilization. Why don’t women give clear ovulatory signals, like most other female animals, so that we can restrict sex to moments when it could do us some good? This chapter seeks to understand the evolution of concealed ovulation, nearly constant female sexual receptivity, and recreational sex—a trinity of bizarre reproductive behaviors that is central to human sexuality.
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By now, you may have decided that I’m the prime example of an ivory tower scientist searching unnecessarily for problems to explain. I can hear several billion of the world’s people protesting, “There’s no problem to explain, except why Jared Diamond is such an idiot. You don’t understand why we have sex all the time? Because it’s fun, of course!”
Unfor
tunately, that answer doesn’t satisfy scientists. While animals are engaged in sex, they too look as if they’re having fun, to judge by their intense involvement. Marsupial mice even seem to be having lots more fun than we do, if the duration of their copulations (up to twelve hours) is any indication. Then why do most animals consider sex fun only when the female can be fertilized? Behavior evolves through natural selection, just as anatomy does. 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. Natural selection favors those individuals whose behavior lets them pass their genes to the most babies. How does it help you make more babies if you are crazy enough to enjoy sex at a time when you couldn’t possibly make a baby?
A simple example illustrating the goal-directed nature of sexual activity in most animal species is provided by Pied Flycatchers, the bird species I discussed in chapter 2. Normally, a female Pied Flycatcher solicits copulation only when her eggs are ready to be fertilized, a few days before laying. Once she begins egg laying, her interest in sex vanishes and she resists propositions from males or behaves indifferently toward them. But in an experiment in which a team of ornithologists made twenty female Pied Flycatchers into widows after completion of egg laying by removing their mates, six of the twenty experimental widows were seen to solicit copulation from new males within two days, three were seen actually to copulate, and more may have done so unobserved. Evidently, the females were attempting to trick the males into believing them to be fertile and available. When the eggs eventually hatched, the males would have no way of realizing that some other male had actually fathered the clutches. In at least a few cases, the trick worked, and the males proceeded to feed the hatchlings as a biological father would have. There was thus not the slightest indication that any of the females was a merry widow, pursuing sex for mere pleasure.
Since we humans are exceptional in our concealed ovulations, unceasing receptivity, and recreational sex, it can only be because we evolved to be that way. It’s especially paradoxical that in Homo sapiens, the species unique in its self-consciousness, females should be unconscious of their own ovulation, when female animals as dumb as cows are aware of it. Something special was required to conceal ovulation from a female as smart and aware as a woman. As we’ll discover, it has proven unexpectedly difficult for scientists to figure out what that special something was.
There’s a simple reason why most other animals are sensibly stingy about copulatory effort: sex is costly in energy, time, and risk of injury or death. Let me count the reasons why you should not love your beloved unnecessarily:
1: Sperm production is sufficiently costly for males that worms with a mutation that reduces sperm production live longer than normal worms.
2: Sex takes time that could otherwise be devoted to finding food.
3: Couples locked in embrace risk being surprised and killed by a predator or enemy.
4: 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.
5: Fights between male animals competing for an estrous female often result in serious injury to the female as well as to the males.
6: Being caught at extramarital sex is risky for many animal species, including (most notoriously) humans.
Thus, we would reap a big advantage by being as sexually efficient as other animals. What compensating advantage do we get from our apparent inefficiency?
Scientific speculation tends to center on another of our unusual features: the helpless condition of human infants makes lots of parental care necessary for many years. The young of most mammals start to get their own food as soon as they’re weaned; they become fully independent soon afterwards. Hence most female mammals can and do rear their young with no assistance from the father, whom the mother sees only to copulate. For humans, though, most food is acquired by complex technologies far beyond the dexterity or mental ability of a toddler. As a result, our children have to have food brought to them for at least a decade after weaning, and that job is much easier for two parents than for one. Even today it’s hard for the single human mother to rear kids unassisted, and it used to be much harder in prehistoric days when we were hunter-gatherers.
Now consider the dilemma facing an ovulating cavewoman who has just been fertilized. In any other mammal species, the male who did it would promptly go off in search of another ovulating female to fertilize. For the cavewoman, though, the male’s departure would expose her eventual child to the likelihood of starvation or murder. What can she do to keep that man? Her brilliant solution: remain sexually receptive even after ovulating! Keep him satisfied by copulating whenever he wants! In that way, he’ll hang around, have no need to look for new sex partners, and will even share his daily hunting bag of meat. Recreational sex is thus supposed to function as the glue holding a human couple together while they cooperate in rearing their helpless baby. That in essence is the theory formerly accepted by anthropologists, and it seemed to have much to recommend it.
However, as we have learned more about animal behavior, we have come to realize that this sex-to-promote-family-values theory leaves many questions unanswered. Chimpanzees and especially bonobos have sex even more often than we do (as much as several times daily), yet they are promiscuous and have no pair-bond to maintain. 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. You can watch outside your window how male songbirds cooperate assiduously with their mates in feeding the nestlings, although sex ceased after fertilization. Even male gorillas with a harem of several females get only a few sexual opportunities each year; their mates are usually nursing or out of estrus. Why do women have to offer the sop of constant sex, when these other females don’t?
There’s a crucial difference between our human couples and those abstinent couples of other animal species. 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. To find an animal with parallel living arrangements, one has to go far beyond our mammalian relatives to densely packed colonies of nesting seabirds. Even seabird couples, though, aren’t as dependent on each other economically as we are.
The human sexual dilemma, then, is that a father and mother must work together for years to rear their helpless children, despite being frequently tempted by other fertile adults nearby. The specter of marital disruption by extramarital sex, with its potentially disastrous consequences for parental cooperation in child-rearing, is pervasive in human societies. Somehow, we evolved concealed ovulation and constant receptivity to make possible our unique combination of marriage, coparenting, and adulterous temptation. How does it all fit together?
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Scientists’ belated appreciation of these paradoxes has spawned an avalanche of competing theories, each of which tends to reflect the gender of its author. For instance, there’s the prostitution theory proposed by a male scientist: women evolved to trade sexual favors for donations of meat from male hunters. There’s also a male scientist’s better-genes-through-cuckoldry theory, which reasons that a cavewoman with the misfortune to have been married off by her clan to an ineffectual husband could use her constant receptivity to attract (and be extramaritally impregnated by) a neighboring caveman with superior genes.
Then again, there’s the anticontraceptive theory proposed by a woman scientist, who was well aware that childbirth is uniquely painful and dangerous in the human species
because of the large size of the newborn human infant relative to its mother as compared to that ratio in our ape relatives. A one-hundred-pound woman typically gives birth to a six-pound infant, while a female gorilla twice that size (two hundred pounds) gives birth to an infant only half as large (three pounds). As a result, human mothers often died in childbirth before the advent of modern medical care, and women are still attended at birth by helpers (obstetricians and nurses in modern first-world societies, midwives or older women in traditional societies), whereas female gorillas give birth unattended and have never been recorded as dying in childbirth. Hence according to the anticontraceptive theory, cavewomen aware of the pain and danger of childbirth, and also aware of their day of ovulation, misused that knowledge to avoid sex then. Such women failed to pass on their genes, leaving the world populated by women ignorant of their time of ovulation and thus unable to avoid having sex while fertile.
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.
Take first the daddy-at-home theory, developed by the biologists Richard Alexander and Katharine Noonan of the University of Michigan. To understand their theory, imagine what married life would be like if women did advertise their ovulations, like female baboons with bright red derrières. A husband would infallibly recognize, from the color of his wife’s derrière, the day on which she was ovulating. On that day he would stay home and assiduously make love in order to fertilize her and pass on his genes. On all other days, he would realize from his wife’s pallid derrière that lovemaking with her was useless. He would instead wander off in search of other, unguarded, red-hued ladies, so that he could fertilize them too and pass on even more of his genes. He’d feel secure in leaving his wife at home then, because he’d know that she wasn’t sexually receptive to men and couldn’t be fertilized anyway. That’s what male geese, seagulls, and Pied Flycatchers actually do.
Why Is Sex Fun?: The Evolution of Human Sexuality Page 7
