Homo Mysterious: Evolutionary Puzzles of Human Nature
status.
    Beyond this, there is essentially no variability with regard to concealed ovulation in our species. If natural selection were indifferent to whether human ovulation was hidden or advertised, then we would expect substantial variability since public ovulators, concealed ovulators, and in-betweeners would all be pretty much equally fit and thus equally abundant. There is, for example, substantialvariability in human skin color, eye color, blood type, and so forth, all traits about which natural selection is evidently more or less indifferent. But there are no women whose ovulation is even remotely like a chimpanzee’s.
    The likelihood, therefore, is that human ovulation isn’t just neutral or subtle but that it is actively hidden. Yet a moment’s thought suggests that if nothing else, any woman who knows when she is fertile (whether or not she informs others) should be better equipped to become pregnant, or avoid pregnancy, or choose her offspring’s father than would someone who hasn’t a clue and doesn’t give any.
    Earlier, when considering menstruation, we considered and for the most part rejected the idea that it might have evolved as a social signal. Could the same be true, but reversed, for concealed ovulation? What of the prospect that human ovulation is concealed as a way of
suppressing
a social signal? It is—pardon the expression—conceivable.
    Thus, it could be argued that by concealing ovulation, our early hominid ancestors obscured their reproductive status, thereby limiting possible aggressive competition from other, more dominant women. Consistent with this idea, there is growing evidence that—contrary to the generalizations still popular in evolutionary biology about the exclusive maleness of same-sex competition—females generally and women in particular do in fact compete, albeit more subtly than via the chest-beating, fangs-bared style more characteristic of males. Hence, it might well have contributed to a woman’s ultimate evolutionary success if she kept her reproductive status under wraps. Almost literally.
    This seems a plausible hypothesis, except that it would be stronger if ovulation were more concealed among women living in more densely interactive social environments and comparatively unobscured when the woman in question was the only show in town. This isn’t the case. Similarly, this hypothesis would be more convincing if younger, less dominant women concealed their ovulation, while older, more socially and physically secure women flaunted theirs. But they don’t.
    It is reasonable to hypothesize that concealed ovulation is essentially an evolutionary shell game whereby women who hid their time of maximum fertility kept “their” men in a kind ofsexual thrall. Among species in which ovulation is clearly signaled, males are free to copulate at this time, then essentially abandon their inamorata, often in search of other short-term partners. Such, for example, is the notorious sex life of chimps and bonobos. But in
Homo sapiens
, in which ovulation is concealed, men wanting confidence of paternity are obliged to remain in attendance throughout the female’s cycle, engaging in regular sexual relations during the month. A study examining 68 different primate species, looking for correlations between mating system and visible signs of estrus, found that not a single monogamous species was a conspicuous ovulator. 4
    Human beings are also unusual among living things—not just primates—in the extent to which they copulate without much regard to ovulation or the details of a woman’s hormonal condition. This further suggests that concealed ovulation may have evolved as a tactic whereby our great-, great-, great-grandmothers made sure that our great-, great-, great-grandfathers kept close tabs on them, instead of (or in spite of) lusting after someone else. One of
Homo sapiens’
signature characteristics is our long period of infancy and childhood dependency, which is why even
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