WHY THE RHYTHM METHOD DOES NOT WORK
What happened before language allowed proxy mate guarding?
Here, anatomy provides an intriguing clue: Perhaps the most startling difference between the physiology of a woman and that of a
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female chimpanzee is that it is impossible for anybody, including the woman herself, to determine precisely when in the menstrual cycle she is fertile. Whatever doctors, old wives ' tales, and the Roman Catholic Church may say, human ovulation is invisible and unpredictable. Chimpanzees become pink; cows smell irresistible to bulls; tigresses seek out tigers; female mice solicit male mice—
throughout the mammal order, the day of ovulation is announced with fanfare. But not in man. A tiny change in the woman 's temperature, undetectable before thermometers, and that is all. Women 's genes seem to have gone to inordinate lengths to conceal the moment of ovulation:
With concealed ovulation came continual sexual interest.
Although women are more likely to initiate sex, masturbate, have an affair with a lover, or be accompanied by their husband on the day of ovulation than on other days,'° it is nonetheless true that human beings of both sexes are interested in sex at all times of the menstrual cycle; both men and women have intercourse whenever they feel like it, without reference to hormonal events. Compared with many animals, we are astonishingly hooked on copulation.
Desmond Morris called mankind " the sexiest primate alive " " (but that was before anybody studied bonobos): Other animals that copulate frequently—lions, bonobos, acorn woodpeckers, goshawks, white ibises—do so for reasons of sperm competition.
Males of the first three species live in groups that share access to females, so every male must copulate as frequently as he can or risk another male 's sperm reaching the egg first. Goshawks and white ibises do so to swamp any sperm that might have been received by the female while the male was away at work. Since it is clear that humanity is not a promiscuous species—even the most carefully organized free-love commune soon falls apart under the pressure of jealousy and possessiveness—the case of the ibis is the most perti-nent for man: a monogamous colonial animal driven by the threat of adultery into the habit of frequent copulation. At least the male ibis need only keep his sex-six-times-a-day routine up for a few days each season before egg laying. Men must keep up sex twice a week for years."
MONOGAMY AND THE NATURE OF WOMEN
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But concealed ovulation in women cannot have evolved for the convenience of the man. In the late 1970s there was a flurry of speculative theorizing about the evolutionary cause of concealed ovulation. Many of the ideas apply only to human beings. An example is Nancy Burley 's suggestion that ancestral women with unconcealed ovulation learned to be celibate when fertile because of the uniquely painful and dangerous business of human childbirth; but such women left behind no descendants, so the rare exceptions who could not detect their own ovulation mothered the human race. Yet concealed ovulation is a habit we share with some monkeys and at least one ape (the orangutan). It is also a habit we share with nearly all birds. Only our absurdly parochial anthropocentrism has allowed us to think that silent ovulation is special.
Nonetheless, it is worth going through the attempted explanations of what Robert Smith once called human "reproductive inscrutability " because they shed an interesting light on the theory of sperm competition. They come in two kinds: those suggesting concealed ovulation as a way of ensuring that fathers did not desert their young, and those suggesting the exact opposite: The first kind of argument went as follows: Because he does not know when his wife is fertile, a husband must stay around and have sex with her often to be sure of fathering her children. This keeps him from mischief and ensures he is still around to help rear the babies."
The second kind of argument went this way: If females wish to be discriminating in their choice of partner, it makes little sense to advertise their ovulation. Conspicuous ovulation will have the effect of attracting several males, who will either fight over the right to fertilize her, or share her. If a female wishes (is designed) to be promiscuous in order to share paternity, as chimps do, or if she wishes to set up a competition so that the best male wins her, as buffalo and elephant seals do, then it pays to advertise the moment of ovulation. But if she wishes to choose one mate herself for whatever reason, then she should keep it secret."
This idea has several variants. Sarah Hrdy proposed that silent ovulation helps prevent infanticide because neither the hus-
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band nor the lover knows if he has been cuckolded: Donald Symons thinks women use perpetual sexual availability to seduce philanderers in exchange for gifts. L. Benshoof and Randy Thornhill suggested that concealed ovulation allows a woman to mate with a superior man by stealth without deserting or alerting her husband.
If, as seems possible, ovulation is less concealed from her (or her unconscious) than it is from him, then it would help her make each extramarital liaison more rewarding since she is more likely to
" know" when to have sex with her lover, whereas her husband does not know when she is fertile. In other words, silent ovulation is a weapon in the adultery game."
This intriguingly sets up the possibility of an arms race between wives and mistresses: Genes for concealed ovulation make both adultery and fidelity easier. It is a peculiar thought, and there is at present no way of knowing if it is right, but it throws into stark contrast the fact that there can be no genetic feminine soli-darity. Women will often be competing with women: SPARROW FIGHTS
It is this competition between females that provides the final clue to the reason adultery, rather than polygamy, has probably been the most common way for men to have many mates. Red-winged blackbirds, which nest in marshes in Canada, are polygamous; the males with the best territories each attract several females to nest in them. But the males with the biggest harems are also the most successful adulterers, fathering the most babies in their neighbors ' territories, too. Which raises the question of why the males ' lovers do not simply become extra wives.
There is a small owl called Tengmalm ' s owl that lives in Finnish forests. In years when mice are abundant, some of the male owls have two mates, one in each of two territories, while other males go without a mate at alclass="underline" The females that are married to polygamous males rear noticeably fewer young than the females married to monogamous males, so why do they put up with it?
MONOGAMY AND THE NATURE OF WOMEN
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Why not leave for one of the nearby bachelors? A Finnish biologist believes that the polygamists are deceiving their victims: The females judge potential suitors by how many mice they can catch to feed them during courtship. In a good year for mice a male can catch so many mice that Ire can simultaneously give two females the impression that he is a fine male; he can provide each with more mice than he could catch for one in a normal year. 36
Nordic forests seem to be full of deceitful adulterers, for a similar habit by a deceptively innocent-looking little bird led to a long-running dispute in the scientific literature of the 1980s.