The Myth of Monogamy: Fidelity and Infidelity in Animals and People (35 page)

Others, notably biologist Alexander Harcourt, have criticized specific aspects of Baker and Bellis's work more directly, pointing out that mammals are unlikely to produce designated, nonfertilizing sperm for several reasons:

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given a high natural loss, males may well be unable to afford production of sperm that are guaranteed not to be potential fertilizers and whose supposed aggressive/defensive activities may never even be called for. Moreover, secretions from the accessory gland of males seem sufficient by themselves to coagulate semen and generate copulatory plugs, at least in other animals; a male who used these secretions for such a purpose and continued to produce fertilizing sperm instead of diluting his ejaculate with kamikazes would be at an evolutionary advantage.

Most impressive, however, is this finding: Males of polyandrous species are under more intense sperm competition than in monandrous species (whether monogamous or polygynous) in which females mate with only one male. And yet a review of research findings shows that polyandrous species do not produce a greater number, or even a higher proportion, of deformed (and, presumably, nonfertilizing) sperm. Nor do they produce more slow-swimmers, which might be expected if such kamikazes were specialized to stay behind and duke it out with a competitor's sperm. Harcourt concludes that sperm competition--at least in mammals--occurs via what ecologists call "scramble competition," in which contestants struggle individually toward a goal, irrespective of their fellows, as opposed to "contest competition," in which individual contestants would seek to best their fellows
mano a mano
(rather, spermo a spermo).

The male penchant for producing vast numbers of sperm may therefore exist because fertilization is a simple "raffle," rather than a direct competitive struggle. (Even a raffle would still involve sperm competition, but one in which the contestants compete by buying as many tickets as possible rather than by tearing up each other's entries.) Or maybe males make lots of sperm simply because, considering their very high mortality--even
without
sperm competition--it behooves males to make lots of little wigglers if fertilization is to occur at all. After all, the low pH of the vaginal environment is as hard on human sperm as it is on other vertebrates, phagocytes roam through every woman's reproductive tract, many sperm end up being helplessly absorbed into the uterine wall, and they do have a long way to swim, not to mention the fact that fully one-half can be expected to swim up the wrong side of the uterus, ending up perhaps at the right (wrong) fallopian tube when a fertile egg is waiting at the left, and vice versa. In short, the importance of sperm competition may simply be overblown.

But we doubt it.

Any concept comes across as all the more powerful when it sheds new potential light on old facts. For example, take pornography. It, too, could be related to sperm competition, as follows. Earlier, we mentioned the case of Grevy's zebras, in which stallions adjust their sexual performance de-

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pending on the predilections of the mares, mating more--in a sense, being more sexually aroused--when associating with females who are prone to a higher level of sexual activity. A similar pattern is found in bighorn sheep: Dominant males mate females immediately after subordinate males have done so. And it is interesting to note that male great apes, too, mate more often when their females are polyandrous. Often they are stimulated by any indications of sexual intercourse, a connection that makes sense given the presumed payoff of introducing one's own sperm to compete with those of a possible competitor.

There is little doubt that, in our own species, men are agitated and often infuriated by indications of their mates' EPCs. It is unclear whether they are sexually aroused as well, although anectodal evidence suggests that this is not uncommon. Where does pornography fit in? Part of the excitement of pornography--especially for men--may be that it conveys the basic message that "sexual activity is going on nearby," which in turn is converted (among males especially) into competitive sexual arousal. On a primitive, biological level, pornographic images may activate the same system that prehistorically enabled men to respond to the dictates of sperm competition: If "your" woman may recently have been having sex with someone else, you would be well advised to have sex with her, too, and right away!

There is recent evidence that sperm competition is pronounced in both human beings and chimpanzees, compared to the situation in gorillas, for example. (This makes sense, since unlike chimps or human beings, gorillas live in rigidly controlled polygynous harems, in which a given female mates only with the dominant silverback male; as a result, male gorillas need to concern themselves with whatever it takes to obtain and keep control of a harem--that is, with competition at the level of bodies rather than sperm.) Researchers at the University of Chicago have found that three different genes controlling sperm function have been evolving at an especially rapid rate in human beings and also in chimps, suggesting that these two species have been experiencing substantial sperm competition, which in turn has been leading to rapid evolutionary change. By contrast, the same system has been evolving more slowly in gorillas, which makes sense given that a male gorilla pretty much monopolizes sexual access to his females, whereas this is emphatically not the case for chimps or--evidently--human beings.

But, in the interest of full disclosure, these findings could mean something very different. For example, women (like other mammals) have evolved various defenses to protect their uteruses from bacterial and other infection, and these chemical defenses--such as low pH--turn out to be harmful to sperm. So rapid evolutionary change among chimp and human sperm might not indicate competition among sperm at all, but rather the

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need for sperm to evolve defenses against the uterus's efforts to further its own self-defense! The question would then become whether chimp and human females are evolving reproductive tract self-defense more rapidly than are gorilla females and, if so, why. And one answer might be that insofar as the former are more likely to copulate with multiple males, they are exposed to more risks, thereby necessitating more defenses. Everything, it seems, is connected!

he majority of women (happily for them) are not much troubled

with sexual feelings of any kind." So wrote a prominent

.X.
nineteenth-century physician in an influential medical textbook of the Victorian era. Happily for us all, he was wrong. Female sexual feelings, although somewhat different from those of men, are no less vigorous. Female sexuality is, however, less "up front" than its male counterpart. "We and our fantasies are the fruit of evolution," writes Natalie Angier in her esteemed book,
Woman.
"And we are waiting to be known."

For starters, there are two great mysteries of female sexuality: concealed ovulation and orgasm. The question, in both cases, is: Why? Wliy, for instance, are human beings so secretive about when they ovulate? We send all sorts of signals indicating our internal state: blushing when embarrassed, crying when sad, even--in some cases--conveying information we might just as soon keep private, such as the state of our gastrointestinal system when we belch or fart. By contrast, the exact (or even approximate) time of ovulation is a closely guarded female secret. There must be a reason.

Similarly, why do women have orgasms? Men's orgasm--a highly pleasurable sensation associated with ejaculation--makes obvious sense, but women have no comparable need for such a pronounced reaction to sexual stimulation. Here again, there must be a reason.

In the case of concealed ovulation, let's first note that human beings, although unusual in this respect, are not the only species to be so secretive. Others include black-and-white colobus monkeys of the African rain forest, pigmy marmosets and lion tamarins of South and Central America, blue monkeys, orangutans, woolly and spider monkeys, vervets and langurs. Nonetheless, an explanation is needed, given that our closest relatives, the chimpanzees, make a dramatic public show of their reproductive state. Human beings, for all their vaunted self-knowledge, are remarkably ignorant about their own ovulation, whereas if we were chimps, we'd have no doubt: Even a casual zoo visitor notices the vastly enlarged and gaudy pink rear end of a fertile, female chimp, whereas a woman must use an accurate thermometer or careful analysis of cervical mucus to obtain comparable information about her own body.

WHAT ARE HUMAN BEINGS, "NATURALLY"?
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Why such secrecy? There are several possibilities, all of them involving EPCs. Thus, evidence is now accumulating that human females are more likely to engage in extra-pair copulations at the midpoint of their menstrual cycle, precisely when they are most fertile. This does not necessarily bespeak a conscious desire to reproduce as a result of these EPCs. It may simply be that women feel somewhat more sexually inclined when they are at mid-cycle. But this does not preclude our asking why such feelings are especially likely to involve an EPC rather than an IPC.

One might expect women to be, if anything, somewhat reluctant to have affairs when they are most fertile, insofar as they are aware of possibly getting pregnant. Counterbalancing this, however, might be an even greater reluctance to have sex--especially in a new and exciting relationship--when they are most wfertile; that is, when menstruating. So, avoidance of "messy sex" might lead to a tendency for women to have extramarital sexual encounters when they are
more
likely to conceive ... even if they don't plan it that way and might, in fact, prefer it otherwise.

Robin Baker and Mark Bellis argue that women have a fully evolved unconscious preference for EPCs precisely when their fertility is highest, for much the same reasons that other species often engage in similar activities: to be inseminated by the best males and to encourage sperm competition among sexual partners. They point to the suggestive finding that women having a primary sexual partner are likely to walk farther during their monthly time of peak fertility, whereas females without a primary sexual partner show a tendency for reduced midcycle walking. This is consistent with the notion that paired women are more restless at midcycle, as a result of which they are more likely to encounter one or more new partners, whereas unpaired women are inclined to avoid midcycle contacts. Of course, the fact that something "is consistent" with a particular hypothesis is very different from saying that it "proves" anything.

In any event, there is increasingly strong evidence that women feel sexiest when they are at midcycle; that is, when they are fertile. For example, levels of estradiol (a female hormone) have been found to correlate with the kind of clothing women wear at a nightclub: Women at peak fertility wear tighter clothing, and less of it. They expose more skin than women who are not fertile.

Concealed ovulation is generally most common in primates among whom females have multiple sexual partners. A review of the scientific literature concluded that concealed ovulation evolved at most once among monogamous species, but between 8 and 11 times in cases of non-monogamy. WTiy? Probably because it permits females to obtain additional matings. After all, if a socially monogamous female clearly advertised her fertility, "her" male would guard her with particular care during that brief

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time . .. and since the rest of the time she would be infertile, her reproductive options would be limited to her social mate. (Another possibility cannot be excluded, however: that concealed ovulation led to monogamy, instead of vice versa. Maybe males were more likely to affiliate with a specific female if her time of maximum fertility was
not
identified, as a way of ensuring at lease some chance of fertilizing her, assuming they were willing to devote themselves full time to mate-guarding.)

Either way, the likelihood is that once ovulation is hidden, male guarding is less intense, if only because it has to be spread over a female's entire cycle instead of being concentrated during a few hours or days. The result appears to be that by concealing their ovulation, females grant themselves enhanced opportunity to mate with more than one male. As already described, they may also be granting themselves--or, rather, their offspring--enhanced opportunity of obtaining assistance from one or more would-be fathers. Or, at least, reduced risk of infanticide.