3. Theory of a more modern male anthropologist (Donald Symons).
Symons noted that a male chimpanzee who kills a small animal is more likely to share the meat with an oestrus female than with a non-oestrus female. This suggested to Symons that human females might have evolved a constant state of oestrus, in order to ensure a frequent meat supply from male hunters by rewarding them with sex. As an alternative theory, Symons noted that women in most hunter—gatherer societies have little say in selection of a husband. The societies are male-dominated, and male clans just suit themselves by exchanging daughters in marriage. However, by being constantly attractive, even a woman wed to an inferior male could privately seduce a superior male and secure his genes for her children. Symons' theories, while still male-orientated, at least represent a step forward in that he views women as cleverly pursuing their own goals.
4. Theory produced jointly by a male biologist and a female biologist (Richard Alexander and Katherine Noonan).
If a man could recognize signs of ovulation, he could use that knowledge to fertilize his wife by copulating with her only while she is ovulating. He could then safely neglect her the rest of the time and go off and philander, secure in the knowledge that the wife he left behind was unreceptive, if not already fertilized. Hence women evolved concealed ovulation to force men into a permanent marriage bond, by exploiting male paranoia about fatherhood. Not knowing the time of ovulation, a man must copulate often with his wife to have a chance of fertilizing her, and that leaves him less time to develop dalliances with other women. The wife benefits, but so does the husband. He gains confidence in his paternity of his children, and he need not worry that his wife will suddenly attract many competing men by turning bright red on a particular day. At last, we have a theory seemingly grounded in sexual equality.
5. Theory of a female sociobiologist (Sarah Hrdy).
Hrdy was impressed by the frequency with which many primates—including not only monkeys but also baboons, gorillas, and common chimps—kill infants not their own. The bereaved mother is thereby induced to come into oestrus again and often mates with the murderer, thus increasing his output of progeny. (Such violence has been common in human history: male conquerors kill the vanquished men and children but spare the women.) As a counter-measure, Hrdy reasoned, women evolved concealed ovulation in order to manipulate men by confusing the issue of paternity. A woman who distributed her favours widely would thereby enlist many men to help feed (or at least not to kill) her infant, since many men could suppose themselves to be the infant's father. Whether this theory is right or wrong, we must applaud Hrdy's overturning of conventional masculine sexism and transferring sexual power to women.
6. Theory of another female sociobiologist (Nancy Burley).
The average 7-pound newborn human weighs double a newborn gorilla, but the 200-pound gorilla mother dwarfs the average human mother. Because the newborn human is so much larger in relation to its mother than are newborn apes, birth is exceptionally painful and dangerous in humans. Until the advent of modern medicine, women often died in childbirth, whereas I have never heard of such a fate befalling a female gorilla or chimpanzee. Once humans had evolved enough intelligence to associate conception with copulation, oestrous women could have chosen to avoid copulating at the time of ovulation, and could have thereby spared themselves the pain and peril of childbirth, but such women would have left fewer descendants than women who could not detect their ovulation. Thus, where male anthropologists saw concealed ovulation as something evolved by women for men (Theories 1 and 2), Nancy Burley sees it as a trick that women evolved to deceive themselves.
Which of these six theories for the evolution of concealed ovulation is correct? Not only are biologists uncertain; it is only in recent years that the question has begun to receive serious attention. This dilemma exemplifies a pervasive problem in establishing causation in evolutionary biology, as well as in history, psychology, and many other fields where one cannot manipulate variables to perform controlled experiments. Such experiments would afford the most convincing way to demonstrate cause or function. If we could remodel one tribe of people so that all women advertised their day of ovulation, we could then see whether cooperation within or between couples broke down, or whether the Women used their new knowledge to avoid becoming pregnant. In the absence of such experiments, we can never be certain what human society Would really be like today without concealed ovulation. If it is hard to determine the function of things happening today under our eyes, how much harder must it be to determine functions in the vanished past! We know that human bones and tools were different hundreds of thousands of years ago, when concealed ovulation may have been evolving. Probably human sexuality, including the function of concealed ovulation, may also have been different then, in ways now hard for us to picture. Interpretation of our past runs the constant risk of degenerating into mere 'paleopoetry' stories that we spin today, stimulated by a few bits of fossil bone, and expressing like Rohrschach tests our own personal prejudices, but devoid of any claim to validity about the past.
Nevertheless, having mentioned six plausible theories, I cannot just walk away from the problem without attempting some synthesis. Here again, we come up against another pervasive problem in establishing causation. It is rare for complex phenomena such as concealed ovulation to be influenced by only a single factor. It would be as silly to seek a single cause of concealed ovulation as to claim that there was a single root cause of the First World War. Instead, there were many independent factors in the period 1900–1914 pushing towards war, others pushing towards peace. War finally broke out when the net weight of factors tipped towards war. Yet that does not excuse going to the opposite extreme of 'explaining' complex phenomena by an unweighted laundry list encompassing every conceivable factor.
As a first step towards pruning down our laundry list of six theories, let's realize that, whatever factors caused our distinctive sexual habits to evolve in the distant past, they would not be persisting today if there were not some factors still sustaining them. But the factors responsible for their initial appearance need not have been the same as the ones now operative. In particular, although the factors behind Theories 3, 5, and 6 may have been major ones long ago, they do not seem to be so now. Only a minority of modern women uses sex either to obtain food or other resources from a number of men, or to confound paternity and induce many men simultaneously to support a woman's child. Postulates of their former role are paleopoetry, albeit plausible paleopoetry. Let's just content ourselves with trying to understand why concealed ovulation and frequent private copulation might make sense now. At least, our guesses can be guided by introspection about ourselves plus observations of others.
The factors behind Theories 1, 2, and 4 seem to me still operative today, and to be facets of the same paradox, the most distinctive feature of human social organization. That paradox is that a man and woman desirous for their child (and genes) to survive must cooperate with each other for a long time to rear their child, and must also cooperate economically with many other couples living close by. It is obvious that regular sexual relations between a man and woman intensifies their connection, compared to their connections with other women and men whom they see daily but with whom they are not involved sexually. Concealed ovulation and constant receptivity advance this 'new' function of sex (new by the standards of most mammals) as a social cement, not just a device for fertilization. This function is not, as in the traditional male chauvinist version of Theories 1 and 2, a sop thrown by a cold, calculating woman to a sex-starved man, but instead an inducement for both sexes. Not only have all signs of female ovulation vanished, but the act of sex itself takes place privately, to emphasize the distinction between sexual and non-sexual partners within the same close group. As for the objection that gibbons remain monogamously involved without the reward of constant sex, that is easy to explain: each gibbon couple has minimal social—and no economic—involvement with other gibbon couples.