The Myth of Monogamy: Fidelity and Infidelity in Animals and People (4 page)

Two abbreviations will also be useful, and hence repeated throughout: EPC stands for
extra-pair copulation,
which simply means a copulation in which at least one participant is already socially mated to someone else. In human terms, it is equivalent to an extramarital affair or adultery (if the pair is married) or "cheating" (if the pair is simply "going together" or "dating" so seriously that the EPC violates the other's expectation, knowledge, or consent). For our purposes, animals can engage in EPCs no less than people can. Similarly, IPC stands for
intra-pair
(or
in-pair) copulation,
which means that two individuals who are socially mated also mate sexually.

Such abbreviations are intended to facilitate communication, mostly by clarity and brevity. But they have another, unintended effect, one that may be fortuitously helpful: Their aura of scientific objectivity conveys a degree of detachment that should enable us to consider difficult, emotionally charged material with at least a degree of detachment. This is probably a good thing, since not only are we as a species new to the practice of monogamy (social as well as sexual), but we are also novices when it comes to understanding this most fascinating as well as vexatious way of life.

CHAPTER TWO

Undermining the Myth: Males

Astory is told in New Zealand about the early nineteenth-century visit of an Episcopal bishop to an isolated Maori village. Everyone was about to retire for the night after an evening of high-spirited feasting and dancing, when the village headman--wanting to show hospitality to his honored guest--called out, "A woman for the bishop." Seeing a scowl of disapproval on the prelate's face, the headman roared even louder,
"Two
women for the bishop!"

This story, of course, is one of cross-cultural misunderstanding. But underlying it is something quite different: cross-cultural, species-wide similarity, notably, the widespread male fondness for (1) sex generally and (2) sexual variety when possible. We presume that the Episcopal missionary turned down the headman's offer, but we also smile at the latter's immediate assumption that what troubled the bishop was not the prospect of spending the night with a new woman, but that he had only been offered one! To be sure, human beings can even elect celibacy (and, what is more remarkable, some can remain true to it), but virtually everyone agrees that such denial is denial indeed; it does not come naturally, and it requires saying "no" to something within.

Most people, male and female, like sex. But the Maori headman revealed an acute sensitivity to another pan-cultural human trait: the widespread eagerness of men in particular for sexual variety. This is not to claim that men necessarily seek a nonstop sexual carnival or a lifetime of wildly erotic, simultaneous encounters with multiple partners. But compared to women, men in particular--and, as we shall see, males in general--have a lower threshold for

16 the myth of monogamy

sexual excitation and a greater fondness for sexual variety, or, to look at it more negatively, a penchant for equating monogamy with monotony.

It isn't rocket science to understand how the biology of male-female differences leads to differences in sexual preference, nor is it terribly difficult to see why, from the male perspective, monogamy is so difficult. In fact, this recognition has been around for several decades now, and it has become part of the received wisdom of evolutionary biology and one of the major energizing principles of sociobiology, sometimes known as evolutionary psychology. (The other side of the coin--why the
female
perspective also generates departures from monogamy--is a different story, one that is only now being unraveled; we'll discuss it in the next two chapters).

Fundamentally, the "standard" sociobiological explanation for male-female differences is a matter of sperm and eggs. Nearly all living things are divided into male and female, and this distinction, in turn, is based on the kind of sex cells they produce, whether tiny and generated in vast numbers (sperm) or large and relatively scarce (eggs). This, in fact, is how we
define
maleness and femaleness: not by the presence or absence of beards, breasts, penises, or vaginas, and not even by who gives birth. After all, male seahorses carry their offspring inside their body, eventually releasing them to the outside after a series of violent contractions that are remarkably like those of a woman in labor. But even for seahorses, there is no question that the individual giving birth is a "he," not a "she." This is because he is the one who contributed sperm; she provided the eggs.

Most birds lack external genitals altogether, and yet biologists have no difficulty distinguishing males from females, even in those cases such as sparrows or gulls in which males and females are often indistinguishable based on physical appearance. If it lays eggs, it is female; if it makes sperm, it is male.

This is not simply a matter of theoretical dictionary definitions. It turns out that whether one is an egg-maker or a sperm-maker has important consequences. To understand these consequences, the next step is to look at the energetic expenditures that eggs and sperm entail. A female bird, for example, will lay a clutch of eggs that might well tip the scales at 20 percent of her total body weight; her male partner will ejaculate a fraction of a teaspoon of sperm. Sperm are cheap and readily replaced; eggs are expensive and hard to come by. Not surprisingly, therefore, we find that males are generally profligate with their sperm, whereas females tend to be careful and choosy about how they dispose of their eggs.

The situation for mammals is, if anything, even more asymmetric. Even though the mammalian egg is very small--almost microscopic--each sperm

undermining the myth: males 17

is smaller yet. A single human ejaculation, for example, contains about 250 million sperm, whereas, by contrast, it takes about a month for a single egg to be ovulated. (During that month, a healthy man will produce literally
billions
of sperm.) More importantly, however, each egg represents an immensely greater investment on the part of a woman--or female mammal generally--than does a sperm. If fertilized, that egg will develop within the body of its mother, nourished from her bloodstream. Following birth, the infant mammal (human or otherwise) will receive proportionately even more nourishment, in the form of milk, via the mother's breasts. By contrast, the father has only invested a few moments of his time and a squirt or two of semen, expending the energy equivalent of eating a few potato chips!

Alternatively, think about the consequences of making a mistake: If a female mammal makes a bad choice and is inseminated by an inferior male, say, one whose offspring will fail to survive or (nearly the same thing, in evolutionary terms) to reproduce, she pays a substantial toll in risk as well as in lost time and energy. Such a female may spend several weeks or many months pregnant, not to mention lactating once her offspring is born, only to have nothing to show for it on her evolutionary ledger. By contrast, a male mammal who makes himself available for one or many sexual dalliances has invested comparatively little. If in the process he succeeds in fertilizing one or more females, he is that much ahead; if he fails, then, unlike most females in a similar situation, he has not lost very much. As a result, evolutionary pressures tend to favor males who are sexually available, readily stimulated, and interested in multiple sexual relationships--who are, in the words of noted evolutionary theorist George C. Williams, "aggressive sexual advertisers." At the same time, females have generally been endowed by natural selection with a tendency to be more sexually discerning, or, as Williams has put it, "coy, comparison shoppers."

An important conceptual breakthrough came when Robert L. Trivers pointed out that the key (or, at least, one key) to male-female differences in behavior derives from differences in what he called "parental investment." Parental investment is simply anything costly--time, energy, risk--that a parent spends or endures on behalf of its offspring and that increases the chances of the offspring's being successful, at the cost of the parent's being unable to invest in other offspring at some other time. Feeding one's offspring is parental investment. So is defending, educating, cleaning, or scratching when and where it itches. And so, also, is producing the big, fat, energy-rich mother lode of nutrients called an egg. A sperm, by contrast, is a pitiful excuse for parental investment, consisting merely of some DNA with a tail at the other end.

Trivers showed that when there is a big difference between the parental investment offered by members of the two sexes, the sex investing more

i

18 the myth of monogamy

(nearly always female) will become a valued "resource," sought after by individuals of the sex investing less (nearly always male). Several important consequences flow from this. For one thing, males tend to compete with each other for access to females. This is because females have something of great value: their eggs or, in the case of mammals, their promise of a placenta and, eventually, lactation. Not only that, but successful males may get to inseminate numerous females, whereas unsuccessful males have nothing to show for their efforts. As a result, natural selection will generally favor males that succeed in their competition with other males and that therefore are relatively big and aggressive, outfitted with dangerous weapons (fangs, tusks, antlers, horns) and a propensity for bluff, bluster, and violence as well as sexual adventuring.

Moreover, because females generally provide such abundant parental investment, males are in many cases superfluous in terms of the success of any offspring produced. As a result, they are "liberated" to pursue as many additional reproductive opportunities as they can find. Trivers pointed out that even in cases of supposedly strict monogamy, when the direct involvement of both father and mother are required for offspring to be reared successfully, the evolutionary optimum for males will often be to adopt a "mixed reproductive strategy." In such cases, males mate with a chosen female and assist her in rearing offspring, but also make themselves available for additional reproductive liaisons with other females ... whom, in most cases, they will not help. Because of the small investment entailed in sperm-making, males will typically be more fit, in the evolutionary sense, if at some level they are willing--even eager--to make their gametes as widely available as their lifestyles allow. It is important to realize, at the same time, that such individuals are not simply cads or scoundrels. Usually, males who seek EPCs (extra-pair copulations) are resident territorial proprietors--happily married, respectable burghers who are simply susceptible to "a little something on the side."

Once males developed this tendency to play fast and loose, it likely became self-perpetuating, so that departures from monogamy may actually be responsible, in part, for the further evolution of maleness itself; namely, for the production of especially tiny sperm. Under the pressures of sperm competition, males would probably have been pushed by natural selection to make sperm in ever-greater numbers, and--since the amount of energy that can be expended in such pursuits is ultimately limited--each one would necessarily have to be very small.

There is another way of making sense of this phenomenon of male sexual avidity, although it is not entirely distinct from looking at the low parental investment that characterize males generally. The idea is to focus, instead, on reproductive potential. In the long run, males and females have

undermining the myth: males 19

the same reproductive potential, since whenever sexual reproduction occurs, one male and one female are equally responsible. But the two sexes differ in how reproductive success is distributed among their members. Because of their high parental investment, most females are likely to be at least somewhat successful; usually there are no dramatic differences between the most successful females and the least successful. Even "low-quality" females are generally able to get inseminated, if only because males are typically ready and willing to fertilize any females who might otherwise go unmated. And because of their high parental investment, even "high-quality" females are limited in how many children they can produce. By contrast, it is possible among males for a small number of well-endowed individuals to be hugely successful, while others are total failures.

Consider this: During her lengthy pregnancy, a cow elk is fully occupied with only one calf. By contrast, a bull can inseminate additional females every day. Admittedly, the fact that most healthy male mammals--including mice and men--release a few hundred million sperm in just one ejaculation does not mean that they are capable of fathering a few hundred million children. Take the human situation, however: During the nine months that a woman is pregnant--not even counting the added time lactating--a man has much greater reproductive potential... that is,
if
he inseminates additional women. Another way of looking at it: What limits the reproductive success of any given male would appear to be his access to females, rather than inherent limitations of his reproductive anatomy.

And so, once again we have the same basic pattern: Males, which make a relatively small parental investment and have a large potential reproductive success, tend to be sexually eager. This does not in itself require departing from monogamy, but in fact such sexual eagerness is likely to be especially pronounced when it comes to new potential partners, who, once inseminated, will produce offspring yielding the kind of evolutionary payoff that selects for precisely such behavior. By contrast, a rigidly monogamous male--with no eye for the ladies other than his own--has fewer opportunities for reproductive success. The result? From the male perspective, strict monogamy is not likely to be the best of all possible worlds.