The Myth of Monogamy: Fidelity and Infidelity in Animals and People (33 page)

Historically, it is at least possible that the combination of monogamy plus adultery was as important--or more so--than polygyny. In general, adultery is more common than polygyny, at least in hunter-gatherer societies, simply because it is very difficult for a man to acquire more than one wife--and to keep her. This might also help explain our human penchant for a high frequency of sexual intercourse, as follows.

Among those species that copulate very often--lions, bonobo chimpanzees, acorn woodpeckers--it seems that the threat of sperm competition is the driving force. But these animals do not even make a pretense of monogamy. A better model for us humans might well be those socially monogamous species--notably certain birds, such as the white ibis--in which males as well as females make a significant investment in care of the young and which are driven by ecological considerations into living in high-density colonies. Socially monogamous nonhuman primates generally live such isolated lives that the threat of EPCs is greatly reduced ... even if the predilection still persists. But as noted, despite the fact that human beings are biologically inclined to polygyny, most people end up being socially monogamous and also--like our fathered friends--living in dense "colonies" known as towns, villages, or cities.

Under these circumstances, with a premium placed on biparental care but also a substantial risk of cuckoldry, it makes sense that
Homo sapiens
is a pretty sexy creature, inclined to engage in lots of sexual intercourse. Admittedly, it is unromantic to see much of human lovemaking as driven by the threat of adultery and, hence, the need to prove one's worth, not to mention the even more "mechanical" payoff that comes from prevailing in sperm competition. Many will reply that we make love a lot (by most animal standards) because we love each other a lot, or because it feels good, or simply because we like to do so. But
why
is sexual intercourse so closely

WHAT ARE HUMAN BEINGS, "NATURALLY"?
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connected to love in human beings?
Why
does it feel so good?
Why
do we like to do it, even when we aren't interested in reproducing ... sometimes especially when this is the case?

Although there are exceptions (some of which we have reviewed), among most species of monogamous mammals, sexual behavior is neither especially frequent nor especially fervent. For the great majority of socially monogamous mammals, sex takes a back seat to resting together, mutual grooming, and simply "hanging out." In most species, when the pair-bond is well established, relatively little energy is expended on sex or obvious social interactions of any sort. Yet sexual behavior is prominent among
Homo sapiens,
often identified as an important component of love and, thus, monogamy. Maybe our unusual preoccupation with
sexual
love has developed because, unlike the great majority of monogamous mammals, which live rather isolated, hermit-like lives, for which the risk of EPCs is very low, we are highly social and, thus, sorely tempted to stray. Seeking to maintain a degree of monogamy despite living so close to one another, perhaps human beings have added lots of sex both as a way of reconfirming and, if need be, reestablishing the pair bond, while also meeting the demands of sperm competition by providing men with sufficient confidence of genetic relatedness for them to invest in their mate's offspring.

nd so we come to sperm competition, a difficult topic in many

ways. It is difficult to study, difficult to arrive at firm conclusions

-i- A.
about, and emotionally difficult because it challenges some of the deepest and most anxiety-ridden undercurrents of our emotional lives. Especially the lives of men. (As we shall see, women may or may not set up conditions for sperm competition; if they do, then men--knowingly or not--have to participate.)

The big question is this: Is sperm competition a significant factor for human beings? Recall that sperm competition takes place when the sperm from more than one male compete to fertilize the eggs of a female. For sperm competition to be important among human beings, during the course of human evolution--possibly continuing into modern times as well-- women must have frequently had successive episodes of sexual intercourse with more than one man during a brief time span; that is, while they were fertile. Strictly monogamous women could not promote sperm competition. Women who are polyandrous, promiscuous, prostitutes, rape victims, or socially monogamous but also prone to EPCs could.

The most ardent advocates of the importance of human sperm competition are two British biologists, Robin Baker and Mark Bellis. Their work is controversial, accused by critics of being inadequately supported by the data

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THE MYTH OF MONOGAMY

and sometimes verging on shallow sensationalism, while lauded by supporters as being bold, innovative, and path-breaking. The jury is still out, while the evidence is just starting to dribble in.

It appears that the average mating interval for moderately young, healthy heterosexual couples is three days. Whether coincidental or not, such a frequency of IPCs--intra-pair copulations--maintains an almost continuous supply of sperm within the female's reproductive tract. EPCs, by definition, tend to lead to sperm competition (except, of course, when the female has not been having sexual intercourse with her spouse and she has only one EPC partner). Not surprisingly, it is very difficult to say how frequent human EPCs are and, more specifically, how frequently a woman will mix sperm from more than one man. To carry around a mix of sperm, a woman would have to have sex with two different men within about a five-day interval.

IPCs are pretty much evenly divided throughout a woman's reproductive cycle, if anything somewhat more frequent during the postovulation phase, when fertility is substantially reduced. By contrast, Baker and Bel-lis report that EPCs are actually more frequent when women are most fertile! According to the two researchers, "at some time in their lives the majority of males in western societies place their sperm in competition with sperm from another male and the majority of females contain live sperm from two or more different males." They estimate that in Great Britain 4 to 12 percent of children are conceived by "sperm that has prevailed in competition with sperm from another male." This is consistent with standard estimates of "paternal discrepancy" among human beings generally: about 10 percent, which, if accurate, is enough to bespeak genuine sperm competition.

Mixed fatherhood is most dramatic in certain cases of nonidentical twins, for example, when one infant turns out to be Caucasian and one Asian, and so forth. The most famous "twins," Castor and Pollux, were said to have had two fathers, one being Zeus and the other, their mother's mortal husband. In a survey of ijearly 4,000 sexually experienced women (having had at least 500 copulations), 1 in 200 claimed to have had sexual intercourse with two different men within 30 minutes of each other on at least one occasion; within 24 hours, the number jumped to nearly 30 percent. If these data are reliable, they, too, suggest plenty of opportunity for sperm competition.

Most insects and birds have a "last male advantage," which means that the last male to mate with a female is likely to fertilize the lion's share of her eggs. In the case of mammals, however, the situation is much less clear. It is completely unknown whether, among primates in general or human beings

WHAT ARE HUMAN BEINGS, "NATURALLY" ?
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in particular, the first male, the last, or anyone in between has any advantage. The probability of genetic fatherhood among human beings may simply be determined by having the most sperm in play, the reproductive equivalent of the old military adage of being "firstest with the mostest." Regular and frequent IPCs can be seen, therefore, as a means of "topping off" a woman's reproductive tract, replacing sperm likely to have died or become disabled since the last coitus and thereby maintaining a more or less constant and optimal level of sperm population. This would be adaptive under any circumstances, simply to maximize the chances of fertilization. But it would be especially appropriate in an environment of sperm competition; that is, when the woman might copulate with other men as well.

Baker and Bellis point out that both men and women have numerous ways--nearly all of them unconscious--of influencing the outcome of sperm competition. Women first.

Women are not merely passive vessels within which men and their sperm carry out a series of competitive events. They can, and do, exert substantial choice, evaluating men by numerous criteria--emotional, physical, intellectual, financial--in a search for appropriate qualities as a parent, protector, friend, colleague, lover. And like other living things, there is no reason why women could not find themselves socially mated to one male but inclined to engage in EPCs with another. It is noteworthy that many female primates give loud calls during copulation. Among free-living baboons, these calls are most frequent when their sexual swelling is most intense: Females called in 97 percent of copulations. Such calls are longest when ejaculation takes place (which, for baboons, is only about one-third of the time). This suggests that females call to encourage other males and, thus, to promote sperm competition among them.

Human women are no less likely to be concerned about the overall quality of the males who fertilize their eggs, although there is no evidence for anything quite as immediate and explicit as the baboon system of mobilizing competition. The internal reproductive tract of women produces its own barriers as well, including antisperm antibodies that can interfere with fertilization by immobilizing or even destroying sperm and impairing their ability to penetrate the egg, while other antibodies act against the egg's membrane, preventing early egg cleavage and development. A key point is that these antibodies do not necessarily reduce absolute fertility; rather, they diminish the fertility of particular male-female pairs. A woman caught in this biochemical fertility trap may enhance her reproductive success--whether she consciously realizes it or not--by seeking a different reproductive partner while perhaps still retaining her social, marital partner.

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Aside from the array of behavioral strategies of which women are capable, it seems likely that they also manipulate sperm directly... although, again, not consciously. Especially prominent among these sperm manipulations is "flowback." Up to one-third of the seminal fluid deposited within the vagina leaks out within a few minutes of intercourse. Semen is also discharged when urinating, at which time it is expelled with substantial force, as compared to dribbling out after coitus when a woman stands up--or even if she remains lying down. About 12 percent of the time, this flowback results in the expulsion of essentially all sperm deposited inside a woman's reproductive tract. So, at this level alone, women are capable of exercising substantial control over sperm (recall the Atalanta solution, discussed earlier for other species).

Baker and Bellis surveyed women who were either married or otherwise involved in a serious one-to-one heterosexual relationship and evidently obtained the cooperation of many, who were remarkably open about some of the most private aspects of their lives. They report that women are especially likely to engage in EPCs when they are fertile, suggesting an unconscious strategy of choosing more desirable males, via EPCs, as potential fathers for their offspring... even if such a strategy is intentionally thwarted by use of birth control. Baker and Bellis also somehow convinced a number of women to capture flowback after both EPCs and IPCs; the results show a
lower
level of sperm retention associated with sexual intercourse with their main partner. In other words, not only are women more likely to engage in EPCs when they are more fertile, but they also retain more semen after such encounters. (According to Baker and Bellis, women achieve higher sperm retention during EPCs by reducing their frequency of noncopulatory orgasms via masturbation; in other words, they claim that contractions during female orgasm actually push out semen and that by masturbating less and thus having fewer orgasms, women end up retaining more EPC sperm.)

It has long been known that insects and birds have sperm storage organs, whereas mammals, it was claimed, lack anything comparable. Nonetheless, the argument has been made that sperm are stored by the millions in so-called cervical crypts, tiny cavities lining a woman's cervix. From here, they could be released over a period of hours, even days, after intercourse, with peak release over a period of 2 to 24 hours. This is important for our purposes because the ability of women to store sperm from successive copulations sets the stage for sperm competition among successive males.