Why is Sex Fun?: the evolution of human sexuality (6 page)

In a last effort to detect family values in men's hunting, I reflected on hunting's relevance to the role of men as protectors. The males of many territorial animal species, such as songbirds, lions, and chimpanzees, spend much time patrolling their territories. Such patrols serve multiple purposes: to detect and expel intruding rival males from adjacent territories; to observe whether adjacent territories are in turn ripe for intrusion; to detect predators that could endanger the male's mate and offspring; and to monitor seasonal changes in abundance of foods and other resources. Similarly, at the same time as human hunters are looking for game, they too are attentive to potential dangers and opportunities for the rest of the tribe. In addition, hunting provides a chance to practice the fighting skills that men employ in defending their tribe against enemies.

This role of hunting is undoubtedly an important one. Nevertheless, one has to ask what specific dangers the hunters are trying to detect, and whose interests they are thereby trying to advance. While lions and other big carnivores do pose dangers to people in some parts of the world, by far the greatest danger to traditional hunter-gatherer human societies everywhere has been posed by hunters from rival tribes. Men of such societies were involved in intermittent wars, the purpose of which was to kill men of other tribes. Captured women and children of defeated rival tribes were either killed or else spared and acquired as wives and slaves, respectively. At worst, patrolling groups of male hunters could thus be viewed as advancing their own genetic self-interest at the expense of rival groups of men. At best, they could be viewed as protecting their wives and children, but mainly against the dangers pound by other men. Even in the latter case, the harm and the good that adult men bring to the rest of society by their patrolling activities would be nearly equally balanced.

Thus, all five of my efforts to rescue Ache big-game hunting as a sensible way for men to contribute nobly to the best interests of their wives and children collapsed. Kris-ten Hawkes then reminded me of some painful truths about how an Ache man himself (as opposed to his wife and kids) gets big benefits from his kills besides the food entering his stomach.

To begin with, among the Ache, as among other peoples, extramarital sex is not uncommon. Dozens of Ache women, asked to name the potential fathers (their sex partners around the time of conception) of 66 of their children, named an average of 2.1 men per child. Among a sample of 28 Ache men, women named good hunters more often than poor hunters as their lovers, and they named good hunters as potential fathers of more children.

To understand the biological significance of adultery, recall that the facts of reproductive biology discussed in chapter 2 introduce a fundamental asymmetry into the interests of men and women. Having multiple sex partners contributes nothing directly to a woman's reproductive output. Once a woman has been fertilized by one man, having sex with another man cannot lead to another baby for at least nine months, and probably for at least several years under hunter-gatherer conditions of extended lacta-tional amenorrhea. In just a few minutes of adultery, though, an otherwise faithful man can double the number of his own offspring.

Now compare the reproductive outputs of men pursuing the two different hunting strategies that Hawkes terms the “provider” strategy and the “show-off” strategy. The provider hunts for foods yielding moderately high returns with high predictability, such as palm starch and rats. The show-off hunts for big animals; by scoring only occasional bonanzas amid many more days of empty bags, his mean return is lower. The provider brings home on the average the most food for his wife and kids, although he never acquires enough of a surplus to feed anyone else. The show-off on the average brings less food to his wife and kids but does occasionally have lots of meat to share with others.

Obviously, if a woman gauges her genetic interests by the number of children whom she can rear to maturity, that's a function of how much food she can provide them, so she is best off marrying a provider. But she is further well served by having show-offs as neighbors, with whom she can trade occasional adulterous sex for extra meat supplies for herself and her kids. The whole tribe also likes a show-off because of the occasional bonanzas that he brings home for sharing.

As for how a man can best advance his own genetic interests, the show-off enjoys advantages as well as disadvantages. One advantage is the extra kids he sires adultorously. The show-off also gains some advantages apart from adultery, such as prestige in his tribe's eyes. Others in the tribe want him as a neighbor because of his gifts of meat, and they may reward him with their daughters as mates. For the same reason, the tribe is likely to give favored treatment to the show-off's children. Among the disadvantages to the show-off are that he brings home on the average less food to his own wife and kids; this means that fewer of his legitimate children may survive to maturity. His wife may also philander while he is doing so, with the result that a lower percentage of her children are actually his. Is the show-off better off giving up the provider's certainty of paternity of a few kids, in return for the possibility of paternity of many kids?

The answer depends on several numbers, such as how many extra legitimate kids a provider's wife can rear, the percentage of a provider's wife's kids that are illegitimate), and how much a show-offs kids find their chances of survival increased by their favored status. The values of these numbers must differ among tribes, depending on the local ecology. When Hawkes estimated the values for the Ache, she concluded that, over a wide range of likely conditions, show-offs can expect to pass on their genes to more surviving children than can providers. This purpose, rather than the traditionally accepted purpose of bringing home the bacon to wife and kids, may be the real reason behind big-game hunting. Ache men thereby do good for themselves rather than for their families.

Thus, it is not the case that men hunters and women gatherers constitute a division of labor whereby the nuclear family as a unit most effectively promotes its joint interests, and whereby the work force is selectively deployed for the good of the group. Instead, the hunter-gatherer lifestyle involves a classic conflict of interest. As I discussed in chapter 2, what's best for a man's genetic interests isn't necessarily best for a woman's, and vice versa. Spouses share interests, but they also have divergent interests. A woman is best off married to a provider, but a man is not best off being a provider.

Biological studies of recent decades have demonstrated numerous such conflicts of interest in animals and humans-not only conflicts between husbands and wives (or between mated animals), but also between parents and children, between a pregnant woman and her fetus, and between siblings. Parents share genes with their offspring, and siblings share genes with each other. However, siblings are also potentially each other's closest competitors, and parents and offspring also potentially compete. Many animal studies have shown that rearing offspring reduces the parent's life expectancy because of the energy drain and risks that the parent incurs. To a parent, an offspring represents one opportunity to pass on genes, but the parent may have other such opportunities. The parent's interests may be better served by abandoning one offspring and devoting resources to other offspring, whereas the offspring's inter-ests may be best served by surviving at the expense) of its parents. In the animal world as in the human world, such conflicts not infrequently lead to infanticide, parricide (the murder of parents by an offspring), and siblicide (the murder of one sibling by another). While biologists explain the conflicts by theoretical calculations based on genetics and foraging ecology, all of us recognize them from experience, without doing any calculations. Conflicts of interest between people closely related by blood or marriage are the commonest, most gut-wrenching tragedies of our lives.

What general validity do these conclusions possess? Hawkes and her colleagues studied just two hunter-gatherer peoples, the Ache and the Hadza. The resulting conclusions await testing of other hunter-gatherers. The answers are likely to vary among tribes and even among individuals. From my own experience in New Guinea, Hawkes's conclusions are likely to apply even more strongly there. New Guinea has few large animals, hunting yields are low, and bags are often empty. Much of the catch is consumed directly by the men while off in the jungle, and the meat of any big animal brought home is shared widely. New Guinea hunting is hard to defend economically, but it brings obvious payoffs in status to successful hunters.

What about the relevance of Hawkes's conclusions to our own society? Perhaps you're already livid because you foresaw that I'd raise that question, and you're expecting me to conclude that American men aren't good for much. Of course that's not what I conclude. I acknowledge that many (most? by far the most?) American men are devoted husbands, work hard to increase their income, devote that income to their wives and kids, do much child care, and don't philander.

But, alas, the Ache findings are relevant to at least some men in our society. Some American men do desert their wives and children. The proportion of divorced men who renege on their legally stipulated child support is scandalously high, so high that even our government is starting to do something about it. Single parents outnumber copar-ents in the United States, and most single parents are women.

Among those men who remain married, all of us know some who take better care of themselves than of their wives and children, and who devote inordinate time, money, and energy to philandering and to male status symbols and activities. Typical of such male preoccupations are cars, sports, and alcohol consumption. Much bacon isn't brought home. I don't claim to have measured what percentage of American men rate as show-offs rather than providers, but the percentage of show-offs appears not to be negligible.

Even among devoted working couples, time budget studies show that American working women spend on the average twice as many hours on their responsibilities (defined as job plus children plus household) as do their husbands, yet women receive on the average less pay for the same job. When American husbands are asked to estimate the number of hours that they and their wives each devote to children and household, the same time budget studies show that men tend to overestimate their own hours and to underestimate their wife's hours. It's my impression that men's household and child-care contributions are on the average even lower in some other industrialized countries, such as Australia, Japan, Korea, Germany, France, and Poland, to mention just a few with which I happen to be familiar. That's why the question what men are good for continues to be debated within our societies, as well as between anthropologists.

Most wild animals remain fertile until they die, or until close to that time. So do human males: although some men become infertile or less fertile at various ages for various reasons, men experience no universal shutdown of fertility at any particular age. There are innumerable well-attested cases of old men, including a ninety-four-year-old, fathering children.

But human females undergo a steep decline in fertility from around age forty, leading to universal complete sterility within a decade or so. While some women continue to have regular menstrual cycles up to the age of fifty-four or fifty-five, conception after the age of fifty was rare until the recent development of medical technologies using hormone therapy and artificial fertilization. For example, among the American Hutterites, a strict religious community that is well nourished and opposed to contraception, women produce babies as fast as is biologically possible for humans, with a mean interval of only two years between births, and a mean final number of eleven children. Even Hutterite women stop producing babies by age forty-nine.

To laypeople, menopause is an inevitable fact of life, albeit often a painful one anticipated with foreboding. But to evolutionary biologists, human female menopause is an aberration in the animal world and an intellectual paradox. The essence of natural selection is that it promotes genes for traits that increase the number of one's descendants bearing those genes. How could natural selection possibly result in every female member of a species carrying genes that throttle her ability to leave more descendants? All biological traits are subject to genetic variation, including the age of human female menopause. Once female menopause somehow became fixed in humans for whatever reason, why did not its age of onset gradually become pushed back until it disappeared again, because those women who experienced menopause later in life left behind more descendants?

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.

Many biologists would balk at what I have just said. They would argue that human female menopause does not pose an unsolved problem, and that there is no need to discuss it further. Their objections are of three types.

First, some biologists dismiss human female menopause as an artifact of a recent increase in human expected life span. That increase stems not just from public health measures within the last century but possibly also from the rise of agriculture ten thousand years ago, and even more likely from evolutionary changes leading to increased human survival skills within the last forty thousand years. According to this view, menopause could not have been a frequent occurrence for most of the several million years of human evo-lution, because (supposedly) almost no women or men survived past the age of forty. Of course, the female reproductive tract was programmed to shut down by age forty, because it would not have had the opportunity to operate thereafter anyway. The increase in human life span has developed much too recently in our evolutionary history for the female reproductive tract to have had time to adjust-so goes this objection.

However, this view ignores the fact that the human male reproductive tract, and every other biological function of both women and men, continue to function in most people for many decades after age forty. One would therefore have to assume that every other biological function was able to adjust quickly to our new long life span, leaving unexplained why female reproduction was uniquely incapable of doing so. The claim that formerly few women survived until the age of menopause is based on paleode-mography, that is, on attempts to estimate age at time of death in ancient skeletons. Those estimates rest on un-proven, implausible assumptions, such as that the recovered skeletons represent an unbiased sample of an entire ancient population, or that ancient adult skeletons really can be aged accurately. While paleodemographers' ability to distinguish the ancient skeleton of a ten-year-old from that of a twenty-five-year-old is not in question, the ability they claim to distinguish an ancient forty-year-old from a fifty-five-year-old has never been demonstrated. One can hardly reason by comparison with skeletons of modern people, whose different lifestyles, diets, and diseases surely make their bones age at different rates from the bones of ancients.

A second objection acknowledges human female menopause as a possibly ancient phenomenon but denies that it is unique to humans. Many or most wild animals exhibit a decrease in fertility with age. Some elderly individuals of a wide variety of wild mammal and bird species are found to be infertile. Many elderly female individuals of rhesus macaques and certain strains of laboratory mice, living in laboratory cages or zoos where their lives are considerably extended over expected spans in the wild by gourmet diets, superb medical care, and complete protection from enemies, do become infertile. Hence some biologists object that human female menopause is merely part of a widespread phenomenon of animal menopause. Whatever that phenomenon's explanation, its existence in many species would mean that there is not necessarily anything peculiar about menopause in the human species requiring explanation.

However, one swallow does not make a summer, nor does one sterile female constitute menopause. That is, detection of an occasional sterile elderly individual in the wild, or of regular sterility in caged animals with artificially extended life spans, does nothing to establish the existence of menopause as a biologically significant phenomenon in the wild. That would require demonstrating that a substantial fraction of adult females in a wild animal population become sterile and spend a significant portion of their life spans after the end of their fertility.

The human species does fulfill that definition, but only one or possibly two wild animal species are definitely known to do so. One is an Australian marsupial mouse in which males (not females) exhibit something like menopause: all males in the population become sterile within a short time in August and die over the next couple of weeks, leaving a population that consists solely of pregnant females. In that case, however, the postmenopausal phase is a negligible fraction of the total male life span. Marsupial mice do not exemplify true menopause but are more appropriately considered an example of big-bang reproduction, alias semelparity-a single lifetime reproductive effort rapidly followed by sterility and death, as in salmon and century plants. The better example of animal menopause is provided by pilot whales, among which one-quarter of all adult females killed by whalers proved to be postmenopausal, as judged by the condition of their ovaries. Female pilot whales enter menopause at the ago of thirty or forty years, have a mean survival of at least fourteen years after menopause, and may live for over sixty years.

Menopause as a biologically significant phenomenon is thus not unique to humans, being shared at least with one species of whale. It would be worth looking for evidence of menopause in killer whales and a few other species as possible candidates. But still-fertile elderly females are often encountered among well-studied wild populations of other long-lived mammals, including chimpanzees, gorillas, baboons, and elephants. Hence those species and most others are unlikely to be characterized by regular menopause. For example, a fifty-five-year-old elephant is considered elderly, since 95 percent of elephants die before that age. But the fertility of fifty-five-year-old female elephants is still half that of younger females in their prime.

Thus, female menopause is sufficiently unusual in the animal world that its evolution in humans requires explanation. We certainly did not inherit it from pilot whales, from whose ancestors our own ancestors parted company over fifty million years ago. In fact, we must have evolved it since our ancestors separated from those of chimps and gorillas seven million years ago, because we undergo menopause and chimps and gorillas appear not to (or at least not regularly).

The third and last objection acknowledges human menopause as an ancient phenomenon that is unusual among animals. Instead, these critics say that we need not seek an explanation for menopause, because the puzzle has already been solved. The solution (they say) lies in the physiological mechanism of menopause: a woman's egg supply is fixed at her birth and not added to later in her life. One or more eggs are lost by ovulation at each menstrual cycle, and far more eggs simply die (termed atresia). By the time a woman is fifty years old, most of her original egg supply has been depleted. Those eggs that remain are half a century old, increasingly unresponsive to pituitary hormones, and too few in number to produce enough estra-diol to trigger the release of pituitary hormones.

But there is a fatal counterobjection to this objection. While the objection is not wrong, it is incomplete. Yes, depletion and aging of the egg supply are the immediate causes of human menopause, but why did natural selection program women such that their eggs become depleted or unresponsive in their forties? There is no compelling reason why we could not have evolved twice as large a starting quota of eggs, or eggs that remain responsive after half a century. The eggs of elephants, baleen whales, and possibly albatrosses remain viable for at least sixty years, and the eggs of tortoises are viable for much longer, so human eggs could presumably have evolved the same capability.

The basic reason why the third objection is incomplete is because it confuses proximate mechanisms with ultimate causal explanations. (A proximate mechanism is an immediate direct cause, while an ultimate explanation is the last in the long chain of factors leading up to that immediate cause. For example, the proximate cause of a marriage breakup may be a husband's discovery of his wife's extramarital affairs, but the ultimate explanation may be the husband's chronic insensitivity and the couple's basic incompatibility that drove the wife to affairs.) Physiologists and molecular biologists regularly fall into the trap of overlooking this distinction, which is fundamental to biology, history, and human behavior. Physiology and molecular biology can do no more than identify proximate mechanisms; only evolutionary biology can provide ultimate causal explanations. As one simple example, the proximate reason why so-called poison-dart frogs are poi-sonous is that they secrete a lethal chemical named batra-chotoxin. But that molecular biological mechanism for the frogs' poisonousness could be considered an unimportant detail because many other poisonous chemicals would have worked equally well. The ultimate causal explanation is that poison-dart frogs evolved poisonous chemicals because they are small, otherwise defenseless animals that would be easy prey for predators if they were not protected by poison.

We have already seen repeatedly in this book that the big questions about human sexuality are the evolutionary questions about ultimate causal explanation, not the search for proximate physiological mechanisms. Yes, sex is fun for us because women have concealed ovulations and are constantly receptive, but why did they evolve that unusual reproductive physiology? Yes, men have the physiological capacity to produce milk, but why did they not evolve to exploit that capacity? For menopause as well, the easy part of the puzzle is the mundane fact that a woman's egg supply gets depleted or impaired by around the time she is fifty years old. The challenge is to understand why we evolved that seemingly self-defeating detail of reproductive physiology.

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.

Physiologists and many other researchers on aging tend to search for a single all-encompassing explanation of aging. Popular explanations hypothesized in recent decades have invoked the immune system, free radicals, hormones, and cell division. In reality, though, all of us over forty know that everything about our bodies gradually deteriorates, and not just our immune systems and our defenses against free radicals. Although I have had a less stressful life and better medical care than most of the world's nearly six billion people, I can still tick off the aging processes that have already taken their toll on me by age fifty-nine: impaired hearing at high pitch, failure of my eyes to focus at short distances, less acute senses of smell and taste, loss of one kidney, tooth wear, less flexible fingers, and so on. My recovery from injuries is already slower than it used to be: I had to give up running because of recurrent calf injuries, I recently completed a slow recovery from a left elbow injury, and now I have just injured the tendon of a finger. Ahead of me, if the experience of other men is any guide, lies the familiar litany of complaints, including heart disorders, clogged arteries, bladder trouble, joint problems, prostate enlargement, memory loss, colon cancer, and so on. All that deterioration is what we mean by aging.

The basic reasons behind this grim litany are easily understood by analogy to human-built structures. Animal bodies, like machines, tend to deteriorate gradually or become acutely damaged with age and use. To combat those tendencies, we consciously maintain and repair our machines. Natural selection ensures that our body unconsciously maintains and repairs itself.

Both bodies and machines are maintained in two ways. First, we repair a part of a machine when it is acutely damaged. For example, we fix a car's punctured tire or bashed-in fender, and we replace its brakes or tires if they become damaged beyond repair. Our body similarly repairs acute damage. The most visible example is wound repair when we cut our skin, but molecular repair of damaged DNA and many other repair processes go on invisibly inside us. Just as a ruined tire can be replaced, our body has some capac-ity to regenerate parts of damaged organs such as by mak-ing new kidney, liver, and intestinal tissue. That capacity for regeneration is much better developed in many other animals. If only we were like starfish, crabs, sea cucumbers, and lizards, which can regenerate their arms, legs, intestines, and tail, respectively!