The Myth of Monogamy: Fidelity and Infidelity in Animals and People (23 page)

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helped care for his bastard children, although it is interesting that one of them threatened a human observer visiting the widowed female's nest; male blue tits have never been reported to behave in this way toward a nest not their own... so perhaps this male was displaying at least a modicum of parental inclination.

Aside from the potential downside that a cuckolded male may provide less parental care, EPCs may subject offspring to enhanced risk in another way: Without EPCs, all the nestlings would be fathered by the same male (as well as conceived by the same female, of course). As a result, they would all be full siblings, with a high probability of shared genes and, thus, shared interests in one another's success. With EPCs, some of the nestlings would have different fathers and would therefore be half-siblings, as a result of which their genetic relatedness is diminished by a factor of 2. It is increasingly clear that shared genes correspond to greater altruism and less selfishness; as a result, EPC-produced offspring, being less closely related, may well be less benevolently inclined toward one another. This, in turn, might result in less food sharing, less mutual defense, and generally diminished success. And it has already been found that when nestmates are less closely related, they tend to beg more loudly and selfishly; this, in turn, could attract additional predators.

It might also be costly for females to incur the jealousy of their mates, aside from the possibility of physical punishment or abandonment. Even being guarded by one's mate could have a negative impact, and it is not unreasonable to suppose that females who are prone to EPCs are liable to being guarded more closely. For example, male dunnocks shadow their female partners while the latter are foraging, and this results in a diminished food intake. Among dunnocks that are truly monogamous, as opposed to those groups consisting of two or more males and one female, males guard their females less closely, and so the latter get to forage unencumbered and thus more efficiently.

It is time to end this chapter. But first, let's look briefly at a strange aspect of extra-pair reproduction on the part of some female birds. It goes by the indelicate term
egg dumping.

Normally, we think of EPCs as producing a situation in which the male is cuckolded because "his" female has mated with someone else. As a result, some of the offspring produced are not his ... although they are still hers. But not necessarily. It occasionally happens that females deposit fertilized eggs in someone else's nest, leaving the foster parents to perform the child care. These cases are well known for a number of animals known as "nest parasites," species such as the European cuckoo or the North American

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cowbird, which typically do not care for their own offspring; rather, they sneak their eggs into the nests of unwitting "host" species.

This is not considered egg dumping; rather, it is a species-wide reproductive strategy, albeit a parasitic one. True egg dumpers are found in species that normally take care of their own offspring but in which a small number of individuals have opted for this tactic of semi-parasitism. Not surprisingly, female birds are more likely than mammals to foist their offspring onto another female. And among birds, a successful "dumper" can "cuckold" another female, no less than her mate. At the same time, it is also possible for the dumper to have copulated (via an EPC) with the mate of the victimized female ... in which case, we might expect that this male would be especially solicitous of the offspring, assuming that he somehow knows that the eggs were deposited by his clandestine lover! There already exists a phrase in the technical literature that describes this situation in which the attending male, but not the female, is the genetic parent:
quasi-parasitism.

We have long known that among many species of birds, there exists a kind of female underworld, a population of so-called floaters--nonbreeding individuals who are kept in their spinster status by a shortage of resources, usually suitable nest-sites. A recent discovery has been that these floaters are not quite so nonbreeding as had been thought. For example, one study of European starlings captured floaters by attracting them to artificially constructed nestboxes. Nearly 50 percent of these homeless, unmated, and ostensibly nonbreeding females were found to have either laid an egg in a nest or carried a fully developed egg in their reproductive tracts, indicating that they had mated with someone and were about to deposit their egg somewhere. Since they did not have their own nests, that "somewhere" was going to be someone else's nest! As a rule, floating females are significantly younger and smaller than normal breeders.

Generally, it isn't clear whether egg dumping is largely the work of floaters, but it quite likely is. Among the waterbirds known as coots, for example, this is well established. In any event, the likelihood is that it isn't only males that are prone to a "mixed reproductive strategy." WHereas for males such a strategy typically involves monogamy combined with extra-pair copulations, for females it includes the additional option of being a floater who dumps her eggs into someone else's nest (relying, therefore, on others to do the parenting). As a rule, however, egg dumping is unlikely to be a preferred strategy. Rather, in most cases it is probably an attempt by disadvantaged females to make the best of their bad situation, forced on them by the animal equivalent of poverty.

On the other hand, in at least one documented case, egg dumpers are the cream of the crop: Among goldeneye ducks, the oldest and strongest females not only maintain their own nests, they also lay additional eggs in the nests

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of other females. It appears that younger, weaker females are incapable of producing so many eggs; hence, they are less likely to dump and more likely to be dumped upon. ,

Although it is relatively uncommon, egg dumping nonetheless occurs in perching birds, such as starlings and house and savanna sparrows, as well as in ducks. One study found that among cliff swallows (which nest monog-amously, but in colonies consisting of dozens, sometimes hundreds, of individuals), nearly one-quarter of all nests contained eggs laid by one or more females who were not incubating or feeding the young. The researcher concluded that egg dumping is a major downside of colonial life, at least among birds. Among mammals, females have complete confidence of their maternity (a certainty that males can never attain). But among birds, even females cannot know for certain that their nestlings are really theirs!

We can expect that prospective victims^-both male and female--would be on guard to prevent such dumping .. . except, as noted, if the male had recently copulated with the dumper. Then he and she could conceivably be in cahoots. (In a novella by Joseph Conrad, a man's determination to adopt an orphaned child leads his wife to suspect--wrongly, it turns out--that her husband had fathered the child via an extramarital affair.) In Dr. Seuss's children's book
Horton Hatches the Egg,
Mayzie bird--"lazy bird"-- leaves an egg in the care of our hero, a big-hearted elephant, who steadfastly guards it through thick and thin. Mayzie eventually returns and demands her egg, just before it hatches; righteousness is ultimately rewarded, however, since the creature that emerges bears a striking resemblance to generous, gentle Horton!

Maybe, while Horton the elephant was incubating Mayzie's egg, some of Horton's genes somehow leaked through the shell (a one-of-a-kind event). Or maybe Horton and Mayzie had earlier partaken of an EPC or two on the side (equally improbable, since mammals and birds do not normally interbreed, not even a little bit). In any event, Dr. Seuss's poetic justice satisfies our notion of what is fair: Mayzie the egg dumper gets trumped.

CHAPTER FIVE

Why Does Monogamy Occur At All?

The eminent eighteenth-century English essayist and critic Dr. Samuel Johnson once wrote this about a dog walking on its hind legs: "It is not done well; but you are surprised to find it done at all." We might say the same about monogamy: Considering how many strikes there are against it, how wobbly most living things are when they seek to balance on monogamous limbs, it is remarkable that it is done at all. Since both males and females have reasons to deviate from it, why does monogamy ever occur?

To be sure, monogamy is rare. But it does happen sometimes, and so logic suggests that it must--at least on occasion--have something going for it. There are several possibilities, some of them unique to human beings and others that seem to apply to animals generally. Let's look at the general patterns first.

Better the devil you know than the one you don't. Maybe some animals (and, occasionally, people) form monogamous unions because they are, in a sense, conservative. After all, courtship and mating are risky, requiring that both partners venture out of their personal shell and become vulnerable to rejection, injury, bad choices, or just plain wasting of time and energy. Having done so once--and succeeded in obtaining a mate--it is possible that certain individuals might just elect to stop such anxious and risky prospecting and settle down to a life of cozy, comfortable domesticity. "If it ain't broke, don't fix it." Leave well enough alone.

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On a more positive note, having found a reliable and mutually gratifying relationship, why rock the boat? More positively yet, it has been documented among animals that the longer pairs are together, the more likely they are to be successful at rearing offspring. This may be because experience and familiarity with each other make for better and more efficient parenting. Familiarity doesn't have to breed contempt; it can also breed competence (particularly competence at breeding!). On the other hand, causation may also work the other way: mates that remain together in succeeding years are generally more likely to be those that are reproductively successful. Thus, competence breeds contentment. And, in turn, commitment. Divorce, as we have seen, is common in nature, often correlating with whether the pair had been reproductively unsuccessful in the past. Among kittiwake gulls, for example, couples are more likely to split up after a year in which they failed to breed.

It is also likely that many living things--human beings included--have a sense of their own self-worth as measured by their potential success in what might be called the "mating market." One of the best-documented tendencies in animal (and human) courtship and mate selection is "assortative mating," which refers to the phenomenon whereby mates tend to be similar. To be sure, opposites attract--at least opposite sexes--but otherwise, it is remarkable to what extent individuals choose members of the opposite sex who are
similar
to themselves. Whether in matters of physical size, cultural background, intelligence, political inclinations, or overall degree of personal attractiveness, people gravitate toward mates who are like themselves. (How often have you met an unattractive woman married to a very handsome man, or vice versa?) A similar pattern is found in animals, too.

WTiat does this have to do with monogamy? Just this: WTien a mated pair consists of individuals who are not only mated but also matched, there is probably a greater chance that their monogamy will persist. The greatest stability would presumably arise from a situation in which each partner actually is--or perceives itself to be--just a bit less desirable than the other! In this case, each would likely think that he or she has gotten a good deal (that is, a somewhat "better" mate than entitled) and would be unlikely to risk rocking the boat by reaching higher yet. The greater the disparity, the greater the chance that the more highly valued individual will attempt either to terminate the relationship or, failing that, to go outside the monogamous union.

Biologists have long known that females are generally the choosy sex and that males tend to be much less fussy. There are interesting exceptions, however. For example, when males perceive themselves to be especially desirable, they tend to become proportionally more demanding of high-class partners only. Not surprisingly, the same applies to females to an even

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greater degree, since they have something--their large eggs or, in the case of mammals, the promise of nourishing offspring during pregnancy and lactation--that males want.

Sometimes, however, the options for either sex are restricted simply by the force of circumstance. The result is a higher probability of monogamy, simply because there is little alternative. If there aren't many potential partners from whom to choose, or if it is literally difficult to get near anyone else, the likely result is a higher degree of fidelity--not because of choice or because the partners are especially virtuous, but simply out of necessity.

Some studies dealing with fish reveal a pattern of exceptions to female choosiness (or rather, examples of fine-tuning). In one case, researchers designed an experimental setup in which female fish had to swim against a current of water in order to get to different males. In this situation, males that had been unacceptable suddenly became highly attractive,
if
the normally preferred alternatives were unattainable.

In an earlier publication, the same researchers had reported that female fish use male coloration in mate choice, thereby avoiding parasitized males (who are unable to produce bright coloration). These findings led Silvia Lopez, then a doctoral student at Oxford University, to ask the following question: What about the effect, on
females,
of being parasitized? Thus, what happens when the females themselves are less healthy and, thus, less desirable? Do they become less choosy? For her research, Lopez chose gup-pies, the common, brightly colored aquarium fish.