Diseases, Sex and Reproduction – The essence of Maleness and Femaleness
Because it is crucial to fitness, you might think that natural selection has smoothly polished the path of sex and reproduction, from the first romantic longings of adolescence to love, marriage, sex, pregnancy, childbirth, and child-rearing. Alas, we all know the truth too well. From unrequited love to lover’s spats, premature ejaculation, impotence, lack of orgasm, menstrual problems, the complications of birth, the special vulnerabilities and demands of infants, and the inevitable conflicts between parents, and between parents and their children, reproduction is fraught with strife and suffering. Why does reproduction entail so much conflict and misery? Precisely because it is so crucial to Darwinian fitness. It is at the very core of intense competition and thus causes many problems.
While our main focus on this website is on how evolutionary ideas can help to explain and prevent or cure specific medical diseases, here and in the next post we broaden our view somewhat to encompass emotional and behavioral problems that may or may not be considered medical disorders. Some problems associated with reproduction, such as diabetes during pregnancy or sudden infant death syndrome, are clearly diseases, while others, such as jealousy, child abuse, and sexual problems, involve behavior and emotions. However we categorize them, they cause much suffering and make more sense in the light of evolution. Help from Darwinism does not end at the boundary between the medical and the social or educational. Darwinism is relevant to all aspects of human life, not just medicine.
WHY IS THERE SEX?
We begin with a fundamental enigma, one of those wonderful questions that is easy to overlook until you take an evolutionary view of life. Why does sex exist at all? It is costly to fitness in important ways, and many organisms do nicely without it, reproducing either by dividing, like amoebae, or by having females that can lay eggs that develop without fertilization, like aphids. Such creatures have a huge short-term fitness advantage over those who reproduce sexually. Imagine what would happen if a mutation produced a female robin that was perfectly standard in every other respect except that she laid eggs that carried all of her genes but none of her mate’s and developed normally without needing to be fertilized. In every generation, all the offspring would be identical females. Compared to a normal female, who can pass only half her genes on to each offspring and who has half male and half female offspring, this mutant strain would increase twice as fast.
So why didn’t some parthenogenetic woman, ages ago, flood the world with her progeny and drive us sexual beings to extinction? And why did sex evolve in the first place? Surprising as it may seem, biologists don’t yet fully agree about how to answer these questions. Most believe that the function of sex is to introduce variation in offspring, but it remains hard to understand how this variation can be useful enough to outweigh the enormous evolutionary costs of sexual reproduction. Biologists also realize that, in the long run, the recombination of genes during sexual reproduction may prevent an otherwise steady accumulation of deleterious mutations, but this does not answer the question of why asexual reproduction does not continually increase in the short run.
Recently, some scientists have proposed that sexual reproduction is maintained by the selective force of the arms race with pathogens. An individual who is genetically identical to many others is vulnerable to any pathogen that discovers the key to exploiting this bonanza of susceptible individuals. If a clone of ten thousand parthenogenetic women are all vulnerable to influenza, they might all be wiped out by the next epidemic, which would claim only some of their genetically diverse competitors. There is growing support for this hypothesis, including several studies that have found asexual reproduction more frequent in species and in habitats with fewer parasites.
THE ESSENCE OF MALENESS AND FEMALENESS
Imagine a time hundreds of millions of years ago, when cells had begun to exchange genetic material to provide variation but before the development of recognizable eggs and sperm. Such haphazard exchange of genetic material is fraught with conflict. A gene that can get itself donated to many other cells has a major fitness advantage, while one that allows itself to be replaced by genes from other cells is at a substantial disadvantage. The successful gene must get itself into new cells, yet not be displaced by incoming genes. In all organisms above the bacterial level, genes from different individuals are rarely allowed to enter. Genetic recombination is instead accomplished by the production of specialized sex cells (gametes) that can be sent off with half the genes needed for the initiation of a new individual. When two such cells find each other, they unite to produce a new organism with equal genetic contributions from each parent.
Gametes face two difficulties. First, they must have sufficient energy stores both to endure until they merge with another gamete and to nourish a developing embryo. Second, they must find another gamete. Large gametes may have abundant energy stores, but they are expensive to make. Small gametes can be produced in enormous numbers at moderate cost, but they can’t survive for long and have nothing to spare for nourishing an embryo. Middle-size gametes sacrifice numbers for larger but still inadequate nutrient supplies and are eliminated by natural selection. Multicellular organisms thus produce only large gametes, which we call eggs, and small ones, which we call sperm.
The next difficulty in understanding human sexuality is why there should be not only two kinds of gametes but two sexes. In other words, why should there be males that produce sperm and females that produce eggs, rather than hermaphrodites that produce both? Many animals and most plants are hermaphrodites, with both eggs and sperm produced by the same individual. The consensus among biologists is that hermaphroditism can be expected when the same adaptations can serve both sexual functions. Big, bright petals on a flower, for instance, may attract an insect that both brings pollen that fertilizes the plant’s eggs and picks up pollen to fertilize other plants’ eggs. As expected, most flowering plants are hermaphrodites. In mammals, there is a dearth of double-duty adaptations. A penis and secondary characteristics such as antlers serve male functions only. A uterus and milk glands serve only female functions. An individual that invested its limited resources in both male and female strategies would not be much good as either. No species of mammal is hermaphroditic.
The investment a female makes in an egg is many times what a male makes in a sperm. Even when the egg is microscopic, as it is in humans, it is still thousands of times bigger than a sperm, and two hundred million sperm cells are released in a single ejaculate to compete to fertilize a single egg. This initial difference in gamete expense is perpetuated and magnified. If most of the eggs produced are fertilized, most of the nutrients put into them will go to the resulting young. If most of the more numerous sperm die from not being able to fertilize an egg, nutrients put into them will seldom benefit an offspring. Extra nutrients in a sperm would be more likely to retard its swimming and be a handicap in competing for the limited number of eggs.
If an animal releases eggs into the water, it becomes advantageous for the female to postpone their release until conditions are ideal and abundant sperm are nearby. If she can wait to pick a specific male, so much the better. Genes from a robust, healthy male may give her offspring an advantage. If she can induce males to fight over her or otherwise display their prowess, she will better her odds of picking the best possible mate. By retaining the eggs inside her until they are fertilized, she maximizes control over who fertilizes them, wastes fewer eggs that are never fertilized, and can protect the eggs to a later stage of development after fertilization. People automatically think of internal fertilization as meaning internal to the female, but logically this need not be. When seahorses copulate, a female lays eggs into a male’s brood pouch, analogous to a mammalian uterus, where the young develop to an advanced stage. This sort of development inside the male is exceptional in the animal kingdom. The small size and mobility of sperm cells make it easier for evolution to produce adaptations for getting sperm into a female rather than eggs into a male.
Since the fertilization of a human egg takes place inside the mother, this puts her in charge of the process. It also increases her control over which male will fertilize her eggs. As with females of other species, it is in her reproductive interest to look for males with demonstrable evidence of health and vigor. If females start selecting males with a particular characteristic, such as the huge, colorful feathers of the peacock or the large antlers of an Irish elk, a process of runaway selection may ensue. Males with the characteristic have an advantage simply because females choose them, so females prefer them in order to have sons that the next generation of females will prefer, thus selecting for still more of the characteristic and giving well-endowed males a still greater advantage and a still greater desirability to females. This positive feedback loop elaborates the trait to the point where it may be severely detrimental to the everyday functioning of the males. The poor peacock can hardly fly, and the Irish elk’s antlers became so heavy and unwieldy they have been thought responsible for the species’ extinction. This is a fine example of how natural selection may create traits that are by no means helpful to the individual or its species, only to the individual’s genes. Helena Cronin, in The Ant and the Peacock, gives an exquisite history of this idea and of the reluctance of male scientists to acknowledge the power of female choice and its burdensome effects on males.
If there is internal fertilization, the young can presumably be released at the optimal stage. Optimal for whom? Mother? Baby? Father? We’ll come to that soon. Exactly how long the young are retained is a life history feature very much subject to natural selection. With the nine-month human pregnancy, in which an offspring grows from a microscopic mite to an infant of several kilograms, a mother’s investment in each baby is vastly larger than that of the father. On the other hand, she is sure the baby is hers, while her mate may well be uncertain. This uncertainty means that male expenditures of time and energy caring for the offspring will generally have a more dubious payoff than similar investments by females. The initial tiny difference in the cost of sperm versus the cost of an egg is greatly amplified by human reproductive physiology and leads, as we will see, to different reproductive strategies for males and females.
Girls and boys are born in nearly equal numbers, as we explained in this post, because individuals of whichever sex is in excess will have lower reproductive success, on average. Selection therefore constantly shapes parents who have offspring of the scarcer sex, thereby equalizing the sex ratio in the long run. From the standpoint of maximizing collective reproduction, this is inefficient. It takes only a few men to keep a large number of women reproducing at whatever rate would maximize the women’s reproductive success. This is a clear illustration of the greater importance of lower levels of selection relative to higher (group) levels. If selection at the group level were at all important, the sex ratio would be biased toward females.
This is not a matter of merely academic interest. In India, a cultural preference for males has combined with a proliferation of ultra-sound imaging machines, which allow the determination of the sex of a fetus, to severely distort the sex ratio. More than 90 percent of abortions in India are now of female fetuses, and the sex ratio in the general population is beginning to show an imbalance. Similarly, in many areas of China, where population limitation campaigns restrict a couple to one child, that child is a boy more than 60 percent of the time. In the long run such imbalances will be tempered by natural selection, but in the coming generation they will have unpredictable political and social consequences. Our guess is that the excess men will compete vigorously and the scarce women will gain social power with remarkable speed.
CONFLICT AND COOPERATION BETWEEN THE SEXES
Conflict between the sexes is not continuous. Men and women can get along, sometimes for whole days at time, even weeks. This harmony is, however, inevitably disrupted by conflicts that originate in the differing reproductive interests and strategies of men and women. From the original difference between the tiny sperm and the larger egg, whole separate worlds of conflicting strategies have emerged to ensnarl our lives. Women can have a limited number of babies, usually four to six, rarely even as many as twenty according to the record books. Men can, however, have hundreds of children and have done so in cultures where a combination of surplus resources and social stratification made it possible for some men to have harems of hundreds of women while many others lacked even a single mate. These exceptional cases are extreme examples of the principle that the number of offspring may vary more widely for men than women. This difference arises from a woman’s unavoidably high investment in both time and calories for a single baby, compared to a man’s minimal expense of a few minutes and a single ejaculate.
These differences mean that men and women can and do use different kinds of strategies to maximize their Darwinian fitness. A woman can maximize the number of her genes in future generations by finding and keeping a man who will care and provide well for her and her children and who is disinclined to invest in other women. Men can use a similar strategy by finding and keeping a woman who is fertile, inclined to take good care of her children, and disinclined to mate with other men. Men also have another strategy not available to women, that of inseminating many women while providing little or no support for them and their babies. None of this implies that men and women think through their options in order to arrive at conscious strategies to maximize their reproductive success, and it certainly implies nothing about how people ought to act. Nonetheless, natural selection has inevitably shaped our emotional machinery in ways that maximize our reproduction—or that would have in Stone Age circumstances.
The problems that result from these divergent strategies are evident in courtship choices. Females of all species do best if they can find a male who offers good genes and abundant resources. Thus, when females can choose, males compete to prove their abilities in contests that range from the familiar butting contests of deer and sheep to the deep braggadocio of the bullfrog. In other species the female mates with the male with the biggest nuptial gift, usually an insect or other source of protein, sometimes the male himself, as when the preying mantis male is eaten by the female even as he copulates with her. The male mantis might try harder to escape, but since he is unlikely to find another mate, he probably maximizes his own reproductive success by donating his bodily protein to the female, who can use it to give more to their offspring.
Men, while notoriously less choosy than women, still have strong preferences. A man maximizes his reproductive success by mating with a woman who has been healthy and successful (indicating good genes) and who is maximally fertile (indicated mainly by being in the peak reproductive years), uncommitted (indicated by lack of prior offspring), and able and motivated for mothering. As University of Michigan psychologist David Buss puts it:
Imagine a state in which human males had no mate preferences aside from species recognition and instead mated with females randomly. Under these conditions, males who happened to mate with females of ages falling outside the reproductive years would become no one’s ancestors. Males who happened to mate with females of peak fertility, in contrast, would enjoy relatively high reproductive success. Over thousands of generations, this selection pressure would, unless constrained, fashion a psychological mechanism that inclined males to mate with females of high fertility over those of low fertility.
So both sexes can increase their fitness by choosing their mates carefully, but they choose different characteristics. Males are relatively more interested in fertility and sexual loyalty, females in good genes and resources. In a study of 10,047 people from diverse cultures and religions in thirty-seven countries, Buss has confirmed these generalizations. Earning capacity was significantly more important to women than to men in all but one of the thirty-seven samples. Youth and appearance were relatively more important to men, and in twenty-three of the thirty-seven samples, men valued chastity significantly more highly than women did, while there was no culture in which the reverse was true.
Mate choice is especially complicated in the human species, where parents mate repeatedly and both provide care for the young. These circumstances mean that a woman faces the risk of being deserted and so must not only assess the current status of her mate but must also try to predict his ability and willingness to stay and provide for her and their offspring. An enduring bond and continuing investment by the man mean that he now also runs a new risk compared to most other primates, that of being cuckolded. He therefore must assess the likelihood that his prospective mate will mate with other men, thus exposing him to the possibility of unwittingly investing in a woman who may be carrying another man’s baby and, later, in the offspring of another man.
To succeed, an individual must predict the prospective mate’s future behavior, an iffy task at best. Both sexes look for indicators of loyalty and willingness to invest in offspring. Amotz Zahavi, an Israeli biologist, has suggested that these pressures might explain some otherwise mysterious conflicts by a mechanism he has called “testing of the bond.” By provoking the prospective partner, he suggests, one can assess his or her willingness to continue to deliver resources and loyalty in the face of future difficulties. Do lovers have spats to test each other? Zahavi provides examples from the world of courting birds to support his theory. Female cardinals, for example, peck and chase wooing males and allow mating only after long persecution. Their subsequent bond lasts for season after season. No one has yet looked in detail at human courtship to see whether we do the same.
Now we return to look at the strongest finding in the Buss study. Despite their differences, both sexes from cultures across the globe consistently agreed on the two most important characteristics they would look for in a mate: (1) kindness and understanding and (2) intelligence. Why do both sexes most of all want a caring and capable partner? For an answer we need to understand why there is such an institution as marriage. Why do men and women in every culture form long-lasting sexual and parenting associations while most other primates have very different kinds of mating systems? This question cannot be answered with certainty, but human patterns of food gathering and child rearing are certainly important parts of the explanation. In the natural environment, one caretaker cannot easily raise a child. Children are, for many years, too helpless and heavy to be taken on long foraging trips. In order to succeed, they need instruction in the ways of their culture and help in negotiating the group hierarchy. In short, each child is so expensive that it may take more than one individual to raise it. To the extent that both parents have all their children in common, they should have minimal conflicts of interest—except, that is, those conflicts that arise from obligations to other relatives. Problems with in-laws are entirely expectable, because helping in-laws directly benefits the genes only of the spouse, not one’s own.
DECEPTIVE MATING STRATEGIES
Mating without caring for the offspring benefits men’s reproductive interests more than women’s. This is consistent with some other aspects of human sexual patterns. First, prostitution is mainly a female profession. While erotic pleasures are possible for both sexes, the balance of supply and demand is such that everywhere men are willing to pay for sex while women rarely have difficulty finding willing sex partners. Second, the strategies that characterize the singles bar scene begin to make sense. In order to get women into bed, men brag about their ability to protect and provide, exaggerating their exploits and flashing their fake Rolex watches as they swear that they are in love forever. Experienced women are rarely completely taken in by this charade, but these patterns of male deception nonetheless seem to work. Men often accuse women of using the converse deceptive strategy, receiving expensive gifts with excited sexual interest and then, later, indignantly expressing surprise that he could have imagined her to be “that kind of woman.” For thousands of years, physicians have called this kind of emotional behavior pattern “hysteria.” This name arose because commonly associated physical symptoms such as abdominal pain and psychogenic paralysis were thought to result from the wanderings of the womb through the body. Had physicians usually been women, they might never have invented the dubious diagnosis of “hysteria.” Instead, women doctors, observing the deceptive mating strategies of men, might have attributed the characteristics of cads to an overly mobile prostate gland and called it “prostateteria.”
REPRODUCTIVE ANATOMY AND PHYSIOLOGY
The human female’s reproductive cycles are quite different from those of other primates. Many female primates advertise their fertile periods with odors, bright patches of skin, and changed behavior. These advertisements are useful communications that increase competition and courtship by males during the females’ fertile period and discourage sexual harassment at other times. In human females, ovulation is not only unadvertised, it seems to be carefully concealed. The scheduling in women is also different, with human ovulation regularly repeated at about twenty-eight-day intervals, while most primates ovulate only once or twice a year, often in synchrony with the cycles of other females they are associated with. At the end of the cycle, if there is no pregnancy, the human female loses a considerable amount of blood in the menstrual flow. Human sexual activity is not confined to brief fertile periods but occurs throughout the cycle, with substantial time and energy spent on frequent sexual intercourse. Female orgasm in most primates is either absent or brief and inconspicuous, but in humans it is common and may be intense.
While the details remain very much at issue, there is a growing consensus that all these facts fit together. The key is that the woman and her mate both benefit if he is frequently present instead of away for weeks and months at a time. If her cycles were obvious, he could maximize his reproduction by inseminating her only at fertile times, but because he cannot tell when she is fertile, he must stay nearby and copulate at frequent intervals. If early Stone Age women, with their enlarging mental capacities, could know when they were fertile and connect sex with the pain of childbirth, they might avoid their partners at those times and thus decrease their reproductive success. Here is a possibility, first suggested by ornithologist Nancy Burley, where not knowing something may be good for one’s fitness. Concealed ovulation also protects the woman somewhat from being impregnated by men more powerful than her mate since such men cannot know when she is fertile and take advantage of her only at that time.
The average frequency of human intercourse, every three days or so, is high enough to make it likely that an ovulation will result in a pregnancy. As we noted before, however, this continuous sexual activity could also mean that bacteria and viruses can hitch regular free rides deep into the woman’s reproductive tract. One defense against such infection is the plug of mucus at the cervix that blocks sperm from ascending except during two or three fertile days a month, when the fibrils in the mucus align to make channels just wide enough for the sperm to swim up into the uterus. As suggested by Margie Profet, menstruation may be another defense to kill pathogens and sweep away the beginnings of infection. In the natural environment, of course, most women would experience far fewer menstrual cycles, since they would not cycle while pregnant or lactating, which would be most of the time. Anemia from loss of menstrual blood is another of the many problems that result largely from novel aspects of our environment, such as celibacy and effective contraception.
Men are also different from some other male mammals in having testicles permanently lodged in a scrotal sac outside the body proper. This is a precarious location for organs of such vital importance, so there must be a good reason for it. One clue is the infertility that many men experience from wearing tight underwear, which increases the temperature of the testicles. Anatomic examination shows that the veins bringing blood back into the body from the testicles wrap around the artery in a way that provides an effective countercurrent heat exchange mechanism to keep the testes cool. Why sperm cannot be formed at regular body temperature is an unsolved mystery. Men must keep their testicles cool and functioning all the time because fertile women may be available at any time.
The testicles of different primates vary greatly in size, and much of this variation can be explained by differences in mating patterns. A female chimp mates with several males, while female gorillas and orangutans mate with only one male. Because the reproductive success of the male chimpanzee depends not only on inseminating many females but also on the success of his sperm in competing with other sperm to fertilize the egg, natural selection has increased the number of sperm chimp males make as well as the size of their testicles. Gorillas, despite their large size and fearsomeness, have testicles that are about one-fourth the weight of the average chimpanzee testicles. In general, the relative testis weight is high for species in which females often mate with multiple males and low in those with little sperm competition. Where do humans fall? In between but toward the side of less sperm competition. It appears that multiple matings have, however, occurred often enough during human evolution to select for testicles slightly larger than those of species with reliably monogamous mating patterns.
Two British researchers, Robin Baker and Robert Bellis, have taken this topic of sperm competition much further. They note that human sperm in a single ejaculate are of several different kinds, some of which are incapable of fertilizing an egg. Many of these sperm are designed, they argue, specifically to find and destroy any sperm from other men. They have also shown that the volume of ejaculate collected in condoms from monogamous couples increases not merely with the amount of time since the last ejaculation but also with the amount of time the couple have been apart. This suggests an adaptation to increase sperm output when it may be needed to compete with sperm from another man. If confirmed, this will demonstrate that selection has designed our sexual machinery to compete in many different ways and at very close quarters.
However understandable jealousy may be, either in the theory of natural selection or in our intuitions, it has surely been responsible for a large part of the world’s miseries. Perhaps the ill will and bloodshed caused by Helen’s desertion of Agamemnon for Paris, as described by Homer, need not be taken literally, but it is not an implausible account of the emotions such an event could arouse. Canadian psychologists Martin Daly and Margo Wilson have convincingly demonstrated that a large proportion of the murders of women arise out of male jealousy. Othello’s lethal frenzy and Desdemona’s tragic death have all too many parallels in real life. More commonly, jealousy merely fuels marital battles that stop short of murder but lead to traumatic divorces and all their tragic consequences. In a few individuals the extremity of these feelings and false beliefs that the partner is unfaithful justify the clinical diagnosis of pathological jealousy. To make sense of all this, we must understand the evolutionary origins and functions of the capacity for sexual jealousy.
Maternity is a certainty, but paternity is always a matter of opinion. A man runs the risk of spending years providing for a woman who is having other men’s children and of unwittingly caring for children not his own, while women always know who their children are. A man incapable of jealousy would have a greater risk for being cuckolded, with a resulting decrease in reproductive success. Men who threaten potential interlopers and try to prevent their wives from mating with other men have an evolutionary advantage. Genes that predispose to male sexual jealousy will thus be maintained in the gene pool.
While women do not face the same risk, they face others. A husband’s wandering affections can lead to a drain of resources and time, to potential loss of the husband, and to the risk of sexually transmitted diseases. Cross-cultural data show enormous diversity in sexual mores, from cultures where extramarital liaisons are tolerated to those where any infidelity is punished by death. However, sexual jealousy is consistently reported to be more intense for men than women.
Sexual jealousy is such a strong influence on human life that it is institutionalized and regulated by custom or formal law in almost all societies. In technologically advanced Western countries men often treat women as property and try to control their sexuality, but in many traditional societies the control may be even more blatant and institutionalized. In some Mediterranean societies, women must demonstrate their virginity with blood on the marital sheet and then are cloistered so they can associate with no men but their husband. In some Muslim societies women must wear robes and veils that make them unrecognizable by men outside the home. In China, women’s feet were bound from early childhood to discourage straying. In many areas of Africa it remains routine for girls at puberty to have the clitoris excised and the labia sewn shut. Everywhere men create social institutions to control female sexuality.
What would be the attitude, in our own society, toward a woman who is faithful to her husband 90 percent of the time, but who has another lover for the remaining 10 percent of her sex life? Her husband would have a 90 percent probability of being the father of her next child, and so, from a strictly evolutionary perspective, we would expect him to be 90 percent as good a father to that child as he would if his wife had been perfectly monogamous. Yet in many cultures a single instance of adultery by a woman may be legally considered a justification for total cancellation of the marriage and abandonment of any ensuing child by the woman’s husband. Many people seem to think that culture opposes such biological tendencies, but with jealousy, culture and the legal system exaggerate a biological tendency. People who think that laws should oppose our more destructive biological tendencies would presumably want to change the social system in ways that would discourage divorces based on infidelity. What do you think the world will be like if someone invents a pill that cures jealousy?
People are, to put it mildly, very interested in the quality of their sex lives. This is ultimately because genes that result in behaviors that increase reproduction have been selected for, while genes that make people uninterested in sex have been eliminated. But from this point on, sex becomes more problematic. The ubiquity of sexual problems is confirmed by a visit to any bookstore. The very existence of rows of sex therapy books documents the unfortunate truth. Sex is a problem not just for a few people some of the time, but for many people much of the time. The books contain strong hints that these problems are not genetic defects, not results of an abnormal environment, but direct products of evolution. Each website has a post about premature orgasm in men and another on delayed or absent orgasm in women. There are no posts about too-rapid orgasms in women or too-slow orgasms in men and no explanations for why men and women differ in this regard. There are posts on men with fetishes but no mention of similar problems in women, and again, no comment on why the sexes differ in this susceptibility. Some difficulties the sexes share: both are troubled, on occasion, by lack of sexual desire and difficulty getting aroused. And both sexes (but especially men) are troubled by boredom with the same sexual partner. Here, at the heart of reproduction, we find a biological system that seems haphazard at best. Why should men and women have so many and such different complaints?
At the very least, we might expect the evolved regulatory mechanisms to coordinate the orgasms of men and women. But orgasms are not only uncoordinated, they are systematically sooner for men than women. This bias is one of the more unfortunate illustrations of the principle that natural selection shapes us to maximize reproduction, not satisfaction. Imagine the reproductive success of a man who tends to come to orgasm very slowly. He might please his partner, but if the sex act is interrupted or his partner has been satisfied and does not want to continue, his sperm will sometimes not get to where they will do his genes any good. The same forces shape the timing of the female sexual response. A woman who rapidly has a single orgasm may, on occasion, stop intercourse before her partner ejaculates and thus will have fewer offspring than the woman with a more leisurely sexual response.
A closer look reveals that there may be a system to adjust male sexual timing according to the particular circumstances. Premature ejaculation is common mainly in young men, especially when they are in anxiety-provoking situations. According to anthropologists who study hunter-gatherer cultures, the liaisons of young men are often illicit and would be dangerous if discovered by older men. In such circumstances, brevity of the sexual act may be especially adaptive. These ideas are mere speculation now, but they deserve consideration.
Pregnancy would seem to be the ultimate in shared goals—a refuge from conflict, perfect unity of purpose between mother and fetus. And the relationship between mother and fetus is about as intimate and mutual as any relationship can be. Nonetheless, because mother and fetus share only half their genes, there is conflict aplenty. Whatever benefits go to the fetus help all its genes. The fetus maximizes its fitness by appropriating whatever maternal resources it can use short of jeopardizing the mother’s ability to care for it in the future and her ability to raise full or half brothers and sisters (all discounted by the one half or three quarters of genes they do not have in common).
From the mother’s point of view, benefits given to the fetus help only half of her genes, so that her optimum donation to the fetus is lower than the amount that is optimal for the fetus. She is also vulnerable to injury or death from the birth of too large a baby. The fitness interests of the fetus and the mother are therefore not identical, and we can predict that the fetus will have mechanisms to manipulate the mother to provide more nutrition and that the mother will have mechanisms to resist this manipulation.
People sometimes argue that there could be no net advantage to a gene that benefits an offspring at a cost to its mother, because its early advantage would be exactly reversed by the later cost. This is not the way things work out. Suppose, in a population in which maternal and fetal interests are served equitably, a gene arises that increases fetal nutrition slightly, at a slight cost to the mother. A fetus that enjoys that advantage can avoid the cost half the time when it grows up, because only half its offspring will carry the gene. Also, even more obviously, it will pay the cost only if it is female. So the cost would be paid in only about 25 percent of the pregnancies of the next generation. There are additional complexities—which we will not go into—but such quantitative considerations led Harvard biologist David Haig to expect conflict between parent and offspring, even though the ideal contribution from the mother’s perspective may be only slightly less than the ideal for the fetus.
Unfortunately, these slight differences create major conflicts. The fetus may be striving mightily to glean an extra few percent of nutrient delivery from the mother, while the mother tries just as hard to prevent this. When the balance of power is disrupted because one participant’s efforts are seriously impaired, medical problems arise. For example, the fetus secretes a substance, human placental lactogen (hPL), that ties up maternal insulin so that blood glucose levels rise and provide more glucose to the fetus. The mother counters this fetal manipulation by secreting more insulin, and this makes the fetus secrete even more hPL. This hormone is normally present in all human bodies, but in a pregnant woman it can reach a thousand times the normal concentration. As Haig points out, these raised hormone levels, like raised voices, are a sign of conflict.
If the mother happens to be deficient in her production of insulin, this can cause gestational diabetes, possibly fatal to the mother, and therefore to the glucose-greedy fetus itself. The fetus would have been well advised to curtail its secretion of hPL, but all it can do is play the odds. The average mother is thoroughly competent to produce enough insulin to avoid diabetes, even when flooded by fetal hPL.
The evolutionary theory of parent-offspring conflict was worked out many years ago by Robert Trivers, but it was only in 1993 that David Haig applied it to the workings of human pregnancy. It is also only recently that an unexpected but highly relevant genetic phenomenon came to light. Experiments, mainly with mice, have shown that the genes need not rely on the lottery of sexual reproduction to avoid the later costs of special benefits in fetal development. They may resort to genetic imprinting, whereby a gene is somehow conditioned by its parent either to start acting immediately or to avoid acting in the offspring. Genes from a father may be imprinted so they side with a fetus in the conflict with the mother. These same genes, when they come from a mother, may be imprinted so they have no such effect. The relevance of this to human pregnancy remains to be determined, but in mice, genes imprinted by males produce a fetal growth factor and other genes imprinted by females produce a mechanism for destroying that growth factor. Such evidence suggests that it may not be farfetched to view the womb itself as the battleground on which genes play out their interests at the expense of our health.
Aside from diabetes, another scourge of pregnancy is high blood pressure. This is called preeclampsia when it gets severe enough to damage the kidneys so that protein is lost in the urine. Haig has suggested that this too may result from conflict between the fetus and the mother. In the early stages of pregnancy, the placental cells destroy the uterine nerves and arteriolar muscles that adjust blood flow, and this makes the mother unable to reduce the flow of blood to the placenta. If something constricts other arteries in the mother, her blood pressure will go up and more blood will therefore go to the placenta. The placenta makes several substances that can constrict arteries throughout the mother’s body. When the fetus perceives that it is receiving inadequate nutrition, the placenta releases these substances into the mother’s circulation. They can damage the mother’s tissues, but selection may have shaped a fetal mechanism that takes this risk in order to benefit itself even at the expense of the mother’s health. Data on thousands of pregnancies show that moderate increases in maternal blood pressure are associated with lower fetal mortality, and that women with preexisting high blood pressure have larger babies. Further support is provided by findings that preeclampsia is especially common when the blood supply to the fetus is restricted, and that the mother’s high blood pressure results from increased resistance in the arteries, not from increased pumping by the heart.
We wonder if the same mechanism may explain some adult high blood pressure. Low-birth-weight infants are especially likely to develop this condition as adults. If genes that are expressed in the fetus to make substances that increase the mother’s blood pressure continue to be active, this could cause high blood pressure later in life.
From a traditional medical perspective, these explanations for diabetes and high blood pressure in pregnancy are revolutionary, and unproven, but we suspect they may well prove correct. If so, they provide extraordinary evidence for the power of looking at life from the gene’s point of view, for the ubiquity of biological conflicts of interest, and for the practical utility of an adaptationist approach to disease.
Human chorionic gonadotropin (hCG) is another hormone made by the fetus and secreted into the mother’s bloodstream. It binds to the mother’s luteinizing hormone receptors and stimulates the continued release of progesterone from the mother’s ovaries. This hormone blocks menstruation and lets the fetus stay implanted. hCG seems to have originated in the contest between the fetus and the mother over whether the pregnancy should continue or not. Up to 78 percent of all fertilized eggs are never implanted or are aborted very early in pregnancy. The majority of these aborted embryos have chromosomal abnormalities. Mothers seem to have a mechanism that detects abnormal embryos and aborts them. This adaptation prevents continued investment in a baby that would die young or be unable to compete successfully in adult life. It is advantageous for the mother to cut her losses as early as possible and start over, even if this means culling a few normal embryos in order to avoid the risk of nurturing an abnormal one. The fetus, by contrast, does everything it can to implant itself and to stay implanted. Producing hCG is an important early strategy for the fetus to further this goal.
It seems likely that high hCG levels are somehow detected and interpreted by mothers’ bodies as a sign of a viable fetus—if it, can make enough hCG, it is probably normal. So the embryo, to demonstrate its fitness to the mother, must now make greater amounts of hCG, levels that say as loud as they can, “I am the makings of a great baby.” It is also conceivable, as Haig points out, that these high levels of hCG are a cause of nausea and vomiting in pregnancy. Do you think this an alternative to Profet’s morning-sickness theory, summarized in this post? Not if you understand the distinction between proximate and ultimate causes. The hCG effect could be part of the adaptive machinery that deters ingestion of toxins. Conversely, it may just be an incidental consequence of high hCG levels. Only a well-designed investigation can resolve this issue.
The large brains and small pelvic openings of humans have combined to make birth especially stressful and risky. As we noted in this post, it would be far better if the baby could be born through an opening in the abdominal wall, as occurs artificially in a cesarean section, but historical constraints make that impossible, and the baby must still squeeze through the pelvis. The relative immaturity and helplessness of human babies compared to those of other primates are an unavoidable cost of being small enough to be born, but the dangers nonetheless remain for both baby and mother.
Wenda Trevathan, an anthropologist at New Mexico State University, notes that while other primates go off alone to give birth, human mothers often seek companionship and support. She suggests that this may in part be explained by the unusual birth orientation of human babies. In contrast to those of other primates, human babies normally emerge facing backward, so that if the mother were to try to finish a difficult labor by pulling on the baby, she might injure it. The presence of a helper at birth greatly decreases the risk. Even in modern times, the simple presence of a supportive woman during birth can reduce the rate of cesarean section by 66 percent and the use of forceps by 82 percent. Six weeks after birth, mothers who had a helper at birth are less anxious and breast-feed more easily than mothers who gave birth without a helper.
After the baby is born, a modern obstetrician or midwife helps extract the placenta and tries to minimize bleeding. Oxytocin is a natural hormone stimulated by nursing that constricts uterine blood vessels at birth, and injections of extra oxytocin have stopped excessive bleeding and saved thousands of lives. Doctors cannot always predict who will bleed excessively, and oxytocin administration is now part of the delivery routine. There has, however, been little research on the possibility that such routine administration of oxytocin might disrupt other mechanisms.
In some species, notably sheep, birth by cesarean section usually results in the mother not accepting the offspring as her own. A ewe will kick and butt her lamb born by cesarean section. During normal birth, pressure on her vaginal walls stimulates the release of oxytocin, which activates a brain mechanism that makes the mother bond to the first lamb she sees in the next few minutes. Administration of a dose of oxytocin enables a ewe to bond normally to a lamb delivered by cesarean section. We don’t know whether oxytocin plays a similar role in human bonding. Because human mothers seem to attach normally to cesarean babies born by cesarean section, it seems that oxytocin may not be necessary to bonding by human mothers. Need this mean it doesn’t help? Because the issue is so important, and because of the frequency of cesarean sections and the routine administration of large doses of extra oxytocin, further study of the positive and negative effects of this hormone is needed.
When the baby first nurses at the mother’s breasts, they secrete not milk but colostrum, a watery liquid full of substances that protect the baby from infection. In a few days, the real milk comes in, which also contains a variety of substances that protect the baby far better than anything in infant formula. Much has been said about the benefits of natural breast-feeding, and we will not belabor the point, except to note parenthetically how completely nonadaptive human behavior can be in the modern environment. For instance, four of Mozart’s six children died in the first three years of life—tragic but not surprising when we learn that they were fed mainly sugar water.
Many babies now spend a few extra days in the hospital because they are jaundiced. The yellow color results from high levels of bilirubin, a by-product of the breakdown of hemoglobin. At the time of birth, fetal hemoglobin, which is well suited to the intrauterine environment, is being replaced by the adult form, which is better suited to life outside the womb. If the liver gets behind in processing the great onslaught of hemoglobin derivatives, a certain amount of jaundice is both understandable and unremarkable.
Physicians first recognized the dangers of high levels of bilirubin in those babies whose blood cells had an Rh antigen that is attacked by their mother’s antibodies. The rapid breakdown of blood cells and resulting high bilirubin levels sometimes caused permanent brain damage. Today this can usually be prevented by administering substances that prevent the mother from developing Rh antibodies or by giving the baby an exchange transfusion at birth. But many babies who do not have Rh antigens also have visible jaundice at birth. To prevent any possibility of brain damage, such babies are often treated with exposure to bright light, which changes the bilirubin in the skin to a form that can be excreted in the urine, thus hastening the disappearance of jaundice.
So far it looks as if the high bilirubin levels at birth are simply a glitch in the mechanism, one we can fortunately circumvent by routine medical treatment. John Brett at the University of California at San Francisco and Susan Niermeyer at the Children’s Hospital in Denver have taken a more careful evolutionary look at this situation. They note that the first breakdown product of hemoglobin is biliverdin, a water-soluble chemical that is excreted directly in birds, amphibians, and reptiles. In mammals, however, biliverdin is converted to bilirubin, which is then transported throughout the body bound to the blood protein albumin. Furthermore, bilirubin levels at birth are under partial genetic control and therefore could be lowered by natural selection if that were beneficial. This led Brett and Niermeyer to suspect that high bilirubin levels at birth might be adaptive. As they put it, “Given that all babies will be jaundiced well above the adult level within the first postnatal week and over half will be visibly jaundiced, it seems difficult to imagine that something is wrong with all of these infants.” Further investigation revealed that bilirubin is an effective scavenger of the free radicals that damage tissues by oxidation. At birth, when the baby must suddenly start breathing, the arterial oxygen concentration becomes three times as great, with concordant increases in damage from free radicals. Adult levels of defenses against free radicals are only gradually implemented during the first weeks of life, as the bilirubin levels decrease. If Brett and Niermeyer are correct, we need to rethink our treatment of jaundice of the newborn, perhaps saving millions of dollars in unnecessary treatment each year.
The risks of light treatment have been inadequately investigated, but we know that color vision impairments can result from continuous bright light in the first few days after birth. We want to make it clear that the adaptive interpretation of Brett and Niermeyer has not been widely accepted and strongly caution parents against refusing to let their babies have light treatment if their doctors deem it necessary. It would be worthwhile, however, for parents to ask questions and to get second opinions, and for scientists to initiate studies to provide the decisive answers.
CRYING AND COLIC
The baby is home now, and the wonderful joy is punctuated, regularly, day and night, by hours of wails that cannot be ignored. It is easy enough to understand how crying benefits the baby. If it is hungry, thirsty, hot, cold, frightened, or in pain, the baby cries and a parent comes to meet its needs. A baby unable to cry might be seriously neglected. How does the baby’s cry affect parents? It gets on their nerves, to put it mildly. Parents do whatever needs to be done to stop the crying, at any time of day or night. Genes that make the cry aversive to parents are selected for because those same genes are in the child, who benefits from the parent’s discomfort and resulting aid. The parent suffers, but its genes in the baby benefit—a fine example of the actions of kin selection.
If the baby cries for a good reason, all to the good. But is all crying a call for necessary help? Often it is impossible to find any cause at all, and yet nothing seems to stop the baby’s crying. This is the most common reason new mothers consult their pediatricians, who usually call the problem “colic” despite little evidence that gastrointestinal difficulties are responsible. Ronald Barr, a pediatrician at McGill University, has made an intensive study of infant crying. He finds that babies with supposed colic do not cry more often or at special times, just longer each time. This has led him to suggest that such crying is normal, although perhaps prolonged by modern practices such as long intervals between feedings. !Kung women in Africa carry their infants constantly and feed them whenever they cry, at least once and often three or four times per hour for two minutes at each feeding. By contrast, American mothers feed their two-month-old infants approximately seven times a day with an average of three hours between feedings. In an experimental study, Barr asked a group of mothers to carry their babies at least three hours per day. Mothers in that group reported that their babies cried only half as long as those whose mothers did not receive the special instructions.
Barr suggests that frequent crying increases fitness by promoting bonding with the mother and by encouraging frequent feeding, which maintains lactation and prevents any competing pregnancy. This last argument again illustrates the conflict of interests between the parent and the offspring. The frequency of babies “spitting up” may be another instance in which the baby manipulates the mother, in this case to make more milk than is in her interests. Or “spitting up” may be explained as a result of unnaturally infrequent but larger feedings. An examination of the phenomenon in hunter-gatherer societies could provide an answer, but it is not the kind of thing that anthropologists routinely report.
SUDDEN INFANT DEATH SYNDROME (SIDS)
Many a parent’s greatest fear is of going to wake the baby and finding it dead in the crib. Sudden infant death syndrome (SIDS) kills more babies than any other cause of death except accidents—1.5 per 1000 babies, or more than 5000 per year in the United States alone. The cause, however, remains unknown. James McKenna, an anthropologist from Pomona College, has investigated SIDS from an evolutionary and cross-cultural perspective and found that crib deaths are many times more frequent in modern societies than in tribal cultures. The SIDS rate is especially high, as much as ten times higher, in those cultures in which babies sleep apart from their parents instead of in the same bed. In a series of experiments that simultaneously measured the movements and brain waves of sleeping mothers and their babies, he found substantial relationships between the sleep cycles of mothers and babies who sleep together. He suggests that this coordination leads to intermittent arousals that sustain SIDS-vulnerable babies through periods when their breathing might otherwise cease. The more fundamental problem, cessation of breathing, may be related to the extreme immaturity of the human infant’s nervous system, the price of avoiding the danger of the birth of babies with too large a skull to fit through the pelvis. None of this is to say that SIDS is in any way normal, only that the tendencies that make some infants vulnerable to it may have been far less dangerous in a natural environment, where mothers usually sleep with their newborns.
WEANING AND BEYOND
Eventually the mother begins to discourage the baby from nursing. In industrial societies, this usually occurs sometime in the first year, while in hunter-gatherer cultures nursing lasts an average of three to four years. The interval between births is critical to maximizing reproduction. If it is too short, the first infant may still need so much milk and effort that the next infant will not survive. If the mother waits too long, she is wasting her reproductive potential. As you might expect from our discussions of parent-offspring conflict, this is yet another instance in which the interests of the mother and the infant diverge. There will come a time, usually when an infant is two to four years old, when it is in the mother’s genetic interests to conceive again but in the baby’s interests to keep nursing and prevent her from having another baby. This is the weaning conflict, discussed by biologist Robert Trivers in his classic paper that first outlined the divergent interests of parents and their offspring. He noted that weaning conflicts have a natural end point. Eventually, the baby can do well enough with solid foods and less aid from the mother that it too will benefit more from having a baby brother or sister (who shares half its genes) than from continuing to monopolize its mother.
During the period of weaning conflict, how can the infant manipulate its mother to continue nursing? Here again Trivers had a brilliant insight. The infant, unable to force the mother to keep nursing, can only use deception, and the best deception is to convince the mother that it is in her best interest to let nursing continue. How can the baby accomplish such deception? Simply by acting younger and more helpless than it really is. Psychologists have long recognized this pattern and named it regression, but we believe Trivers has offered the first evolutionary explanation, with implications that are just beginning to be explored.
Parent-offspring conflicts don’t end with weaning; they just change their form. For a long period in childhood, conflicts are relatively routine and mild, but come adolescence, all hell breaks loose. Teenagers may want to do everything their own way and insist that no help of any sort is needed. Then, at the least difficulty, they are back into the regression act, apparently helpless and needy and asking for more than the parents want to give. This isn’t so surprising, really. It is just the last major episode of parent-offspring conflict in the long drama of development. In a few years the adolescent really will be independent and beginning to look longingly at a potential partner with whom to raise a family and start a new episode in that ongoing drama of adaptively modulated conflict and cooperation called sexual reproduction.