The Myth of Monogamy: Fidelity and Infidelity in Animals and People (16 page)

If females engage in EPCs with males who offer especially good genes, then an interesting--and controversial--possibility arises, suggested by the observation that in some species females resist EPCs, sometimes quite vigorously. The possibility is this: Females could gain an advantage for their offspring (good genes) if they make sure that their EPC partner really is of high quality by resisting males' EPC attempts, only submitting to one who shows himself to be unusually determined, competent, and--almost literally--irresistible. As a result, her male offspring might also likely be determined, competent, and comparably irresistible when it comes to obtaining EPCs themselves. A chip off the old block.

On the other hand, female resistance to EPCs, when it occurs, may be genuine: Sometimes no really does mean NO! On balance, in fact, female EPC resistance is probably more frequent than acquiescence or solicitation. (Given the obvious payoff to them, it is not surprising that males seek EPCs and that they typically do so more actively than females. The reason for examining female solicitation of and acquiescence in EPCs is that the phenomenon is so counterintuitive--and yet so frequent.)

Is there a simple, one-size-fits-all, easily discernible characteristic that might provide convenient information about whether an individual is carrying "good genes"? Maybe there is. The characteristic is symmetry, specifically the degree of left-right correspondence between the two sides of an individual's body, whether arms, legs, eyes, ears, wings, flippers, and so forth. All vertebrates are bilaterally symmetrical (jellyfish, sea urchins, and starfish, by contrast, are radially symmetrical). In the case of bilaterally symmetrical creatures, left and right are not controlled by different genes, so asymmetry--difference between left and right--is widely assumed to reflect

76
THE MYTH OF MONOGAMY

some sort of developmental perturbations, whether caused by poor nutritional status, toxins, mutations, or pathogens.

It turns out that males with low asymmetry have high mating success and vice versa; males who are symmetrical are widely seen as attractive, and vice versa for those who are lopsided. This has been found for a variety of animals, from insects to primates. Two research papers dealing with barn swallows and published in the same year by noted Danish researcher Anders Moller tell an impressively logical tale. One was titled "Female Swallow Preference for Symmetrical Male Sexual Ornaments." In plain English: Female swallows prefer males whose forked tails are equal in length. The second study was titled "Parasites Differentially Increase the Degree of Fluctuating Asymmetry in Secondary Sexual Characters." In plain English: Males infested with parasites tend to be lopsided rather than symmetrical. Put the two together: Female swallows prefer males who are symmetrical, in all likelihood because such males are not parasite-laden.

As it happens, some of the most persuasive evidence for the role of asymmetry comes from studies of human beings. The procedure is surprisingly simple: Measure a number of body parts that are bilateral (such as feet, hands, ankles, wrists, elbows, ear length, and ear width), obtain a composite index of degree of symmetry (or asymmetry), and see if the resulting measure correlates with perceptions of physical attractiveness. It does: More symmetry equals better looking. Not only that, but symmetrical men generally have a relatively high number of
sexual
partners, so the judgment by women isn't merely theoretical! Women even report more orgasms when having sex with symmetrical men.

This leads to the prediction--especially relevant for our purposes--that symmetrical men will have a comparatively large number of EPC partners and vice versa. The prediction holds. A study of more than 200 college students asked them a number of questions, guaranteeing--for obvious reasons--to keep their responses anonymous. Specifically, they were asked about any sexual liaisons they had had (1) with someone who was already involved in a romantic relationship with someone else and (2) while they were themselves involved in a romantic relationship with someone else. In addition to being measured for physical symmetry, respondents were queried as to their age, socioeconomic status, likely future salary, and emotional attachment style. They were also photographed for independent assessment of their physical attractiveness.

Among the interesting findings: Symmetrical men reported more EPC partners--both when they were paired with someone else and when they were the "third person"--than did asymmetrical men, a result that persisted when any effects of social status, likely salary, age, and even physical attrac-

undermining the myth: females (choosing male genes)
77

tiveness were eliminated. So, when it comes to already-paired females engaging in some sex on the side, barn swallows are not alone: Women, too, prefer to dally with members of the opposite sex who are symmetrical. Such a preference may well work the other way, too, although to date it has received less research attention: It is a good bet that men also prefer symmetrical women. The suggestion has even been made that part of the widespread male fascination with female breasts is that such protuberant, bilateral organs provide a good opportunity to assess symmetry! (By contrast, a penis would seem to offer much less opportunity for a connoisseur's assessment, since--whether dangling limply or standing proudly erect--it is nonetheless a lowly singleton, a mere midline member. Tough luck. But whose?)

You might have noticed at least two logical problems with all this. First, the information on numbers of EPCs was obtained by so-called self-reports, that is, what people say they did, as distinct from what they actually have done. This may be a serious, if unavoidable, problem. On the other hand, difficulty arises only if symmetrical (or asymmetrical) people are consistently prone to exaggerate (or, alternatively, understate) their frequency of sexual dalliances ... situations that seem unlikely. A second potential problem is one of interpretation: Even if his physical symmetry genuinely correlates with a man's extra-pair copulations, it isn't clear, for example, whether women are attuned to the actual physical symmetry of potential sexual partners or whether symmetry correlates with something else (self-confidence, unknown pheromones, cosmic emanations, whatever).

In any event, it is also interesting to note that in the study just described, the number of a woman's out-of-pair partners correlated with her "emotional attachment style." Each subject (male and female) was given an "attachment index," based on two different styles: "avoidant attachment" or "anxious attachment." Avoidant attachment included agreement or disagreement with such statements as "I am nervous whenever anyone gets too close to me," while a typical sample item for the anxious-attachment scale would be "I often worry that my partner doesn't really love me." The results? Women with a higher level of anxious attachment had more out-of-pair lovers, whereas those with a higher level of avoidant attachment had fewer. A woman's degree of physical symmetry did not predict her number of out-of-pair partners.

These combined findings are consistent with the basic biology of male-female differences: Men's out-of-pair sex correlated with a physical trait that presumably says something about their desirability, whereas women's out-of-pair sex correlated with a mental trait that presumably says something about their willingness to have such a relationship. The

78
the myth of monogamy

implication is that men are generally willing, that women are generally able, and that women are most sexual with men who are symmetrical.

This much is clear: Females are inclined to have EPCs with males who have good genes. And as we have seen, "good genes" can include many things: being sufficiently different from the female in question (but not too different), being genetically complementary in other ways, or carrying health-related genes. But this isn't all. If certain characteristics (symmetry, bright plumage) indicate good genes and if, as a result, females are at an evolutionary advantage if they prefer these characteristics, then the stage is set for yet another wrinkle in the EPC saga: Females can benefit by preferring those males whose only virtue is that they are preferred by other females! Such a preference might well begin with traits that are "genuine," such as symmetry or bright plumage, but as the pioneering evolutionary geneticist R. A. Fisher pointed out decades ago, it could quickly develop a life of its own.

Indeed, there have been studies showing that in some cases female choice is driven by nothing other than female choice itself; that is, females sometimes choose mates not because they produce healthier or longer-lived offspring, but simply because those offspring--especially the sons--are themselves likely to be chosen by the next generation of females. This idea, now known as the "sexy son hypothesis," suggests that females may choose males simply because other females (a generation later) are likely to have the same preference. As a result, a female is well advised to be seduced by sexy males, even if these males are not exceptionally healthy or even likely to produce exceptionally healthy offspring, so long as the female's sons will be "sexy" ... that is, attractive to the next generation of females. A kind of bandwagon effect.

For example, in a species of sandfly, females evince clear preference as to mates. In one experiment, females were denied the opportunity to exercise choice and were forced to mate with either preferred males or males who would otherwise be shunned. There was no impact of paternity on the overall health or viability of their offspring. But the offspring of preferred males were themselves preferred, just as shunned males produced sons who were shunned in turn.

When it comes to EPCs, females of many species are especially likely to mate with males who are more attractive than their partner. You can almost hear the females--whether already mated or not--spotting the animal equivalent of a movie star and sighing to themselves: "I want to have
his
kids." If so, the reason appears to be that, at an unconscious level, they can

undermining the myth: females (choosing male genes)
79

hear the echoes of other females saying the same thing about
their
future offspring, thereby promising a larger number of grandchildren for the besotted, starstruck, would-be mother... who is now a candidate for one or more extra-pair copulations with the lucky hunk.

The converse also holds: Make a male less attractive, and his mate is more likely to look elsewhere for male genes. There is, for example, a small, strikingly colored socially monogamous Euroasian bird known as a bluethroat. Males have--not surprisingly--bright-blue throats; female throats are white. When researchers from the University of Oslo, in Norway, used dye to diminish the blueness of their mates' throats, female bluethroats were more likely to engage in EPCs. (It is also interesting that the de-blued males apparently perceived somehow that they were less attractive than before, perhaps because of changes in their mates' behavior, since they increased their mate-guarding activities, although to no avail.)

Female choice can also be influenced by what is popular or stylish at the moment. The phenomenon has been called "mate copying." Here's how it works: A female guppy is given a choice between two different males. This female then observes the male she had rejected being chosen by another female (actually, an artificial model of a female, manipulated by the experimenters). Then the choice test is repeated, whereupon the female is likely to change her mind and prefer the male she had initially rejected but whom she had subsequently observed to be "popular." Not only that, but younger females are likely to copy the preferences shown by older females.

There is abundant evidence that sexy males get more EPCs, independent, perhaps, of whether they are really healthier or carrying genes that are "better" in any other sense. Male swallows whose forked tails are artificially lengthened obtain a mate 10 days earlier than normal males; they are eight times more likely to mate again and produce a second brood; and they are twice as likely to have one or more EPCs with an already-mated female. In one especially impressive study, three different kinds of male barn swallows were created: those whose tail forks were shortened, those whose tail forks were lengthened, and a control group whose tails were cut but then glued back together with no change. The results: Extra-pair offspring made up about 60 percent of the nestlings associated with tail-shortened males, as compared to 40 percent of controls and about 12 percent of males whose tails had been artificially elongated. At the same time, the number of biological young reared in their nests increased directly with tail length.

Among house sparrows, long forked tails don't make a male sexy; large black throat patches do. Males engaging in EPCs are particularly likely to have impressive throat patches, and females are more likely to be involved in an EPC with a male whose black throat patch is larger than that of their

80
the myth of monogamy

"husband." A comparable finding applies to another type of bird, the zebra finch, among whom male attractiveness depends on the color of the beak (in the world of zebra finches, a red beak is "hot"). Researchers have even found that, in the case of zebra finches, the color of leg bands, installed by the experimenters, influences male attractiveness and, thus, females' penchant for EPCs. Again, red is desirable: Females mated to males sporting red bands are unlikely to mate outside the pair, while those mated to green-banded males are more likely to have "affairs," with the resulting young fathered by more appealing males.