The Myth of Monogamy: Fidelity and Infidelity in Animals and People (21 page)

At the same time, sometimes a little sexual jealousy can be a useful thing. Female eider ducks, for example, enjoy a higher feeding rate when they are attended by a guarding male. In these circumstances, the female is subjected to less harassment by other males. R. E. Ashcroft, the biologist who first described this phenomenon more than 25 years ago, believed that he had identified an additional function of the pair-bond--diminished harassment--in this species. With growing awareness of the importance of EPCs, it is now possible to see mate-guarding and even the pair-bond itself as a response to the risk of EPCs, with such possible benefits as improved foraging efficiency likely to be at most a secondary by-product of the primary reason for such behavior: efforts by males to thwart EPCs.

Similarly, the mating choices of females may be driven by the payoff that comes from keeping other importunate males at bay. We have already considered the hypothesis that female primates may be inclined to mate with more than one male because, by doing so, prospective mothers may recruit a cadre of expectant-father candidates who might therefore inhibit their otherwise infanticidal tendencies. Moreover, they might even choose males especially likely to provide protection for their offspring. Such cagey sexuality--assuming, of course, that it occurs at all--need not be limited to primates. For example, female mallard ducks paired with attractive males produce heavier chicks than when paired with unattractive males. This might conceivably be due to attractive males producing healthier offspring, but the actual reason appears to be even more straightforward: Females paired with desirable males lay larger eggs. No one knows the mechanism involved, but it may be as simple as that females mated with more socially dominant males aren't harassed as much by other males and are therefore able to feed more efficiently. (More than a few women are familiar with the tactic of

undermining the myth: females (other considerations)
101

choosing to spend the evening with the most physically imposing man at a bar or nightclub, thereby purchasing a kind of social immunity for themselves ... at least insofar as being bothered by
other
males is concerned!)

Maybe one reason attractive males tend to invest less in their offspring is that their females tend to invest more. There is evidence from a number of different species that females often lay more eggs, each of which is larger and heavier, and that they also feed their chicks more actively when mated to an attractive male. Under these conditions--with females eagerly picking up the slack, or at least the pace--perhaps it isn't so surprising that males invest less! "Peahens Lay More Eggs," proclaimed the title of one noteworthy research article, "for Peacocks with Larger Trains."

Among human beings, divorce is strongly correlated with EPCs, and vice versa. Most people assume, in fact, that EPCs, once discovered, lead to divorce. Indeed, adultery is among the most oft-cited grounds for marital dissolution.

But causation may often go the other way: Couples heading for a breakup are probably more likely to have EPCs. Carry this one step further--as some researchers have been doing in their studies of animals--and we get this: One reason for engaging in extra-pair copulations may well be to explore the possibility of divorcing one's current mate and reaffiliating with the EPC partner.

Sometimes, this might simply be another case of females in particular taking out a kind of insurance, analogous to the patterns of "fertilization insurance" described in Chapter 3. In this case, EPCs may be a suitable strategy for a female who might find herself divorced, or widowed, in the future.

Even for the linguistically squeamish, it should be no more incongruous to speak of animal "divorce" than to describe animal "courtship," "mating," "dominance hierarchies," "territories," "reproduction," "parental care," or, for that matter, to employ other words also used to describe the activities of human beings, such as "eating," "sleeping," "migrating," "digesting," "copulating," "defecating," "respiring," and "perspiring."

First, let's note that divorce is not unknown among animals; just as mateships are formed, they can be broken, and not only by the death of one of the partners. Some animals (most eagles, geese, beavers, and possibly foxes) are believed to mate "for life," but we are finding that even in these species, individuals occasionally desert their mate and establish a relationship with someone else. Take, for example, gibbons, those long-armed apes of Indonesia and Southeast Asia whose aerial acrobatics make them favorites at zoos and who have been admired for another reason, too: They were long considered paragons of lifetime monogamy. It is now clear, however, that permanent gibbon monogamy is a myth. Thus, one study has

102
THE MYTH OF MONOGAMY

referred to their "dynamic pair-bonds," which is a nice way of saying that the "pair-bond" among gibbons turns out to be less a bond than a band ... a rubber band. Gibbon mating systems are so "dynamic" as to make us conclude that gibbons are swingers after all, and not just in matters of brachia-tion. Thus, of 11 heterosexual pairs observed in a long-term study in Sumatra, 5 were severed when one individual left its mate, typically to join a neighboring adult of the opposite sex.

There are many possible reasons for pairs--animal no less than human--to break up. These include incompatibility between them, either partner sensing that it has a better option, recognition that one or the other has made a mistake, a third-party intruder who forces a pair member out, and so forth. It used to be thought that animal divorce resulted simply from incompatibility (behavioral, genetic, physiological, even anatomical) between the partners, in which case a breakup would be beneficial to both. Thus, a renowned study of cliff-nesting gulls known as kittiwakes found that the best predictor of divorce in this species was a failure to breed successfully the previous year.

An alternative view, recently advanced, is the so-called better options hypothesis, which suggests that divorce results from a unilateral decision made by one member of a mated pair seeking to improve his or her situation. This leads to the possibility that EPCs and divorce may be closely related, if the former are a means whereby a female determines whether to initiate the latter.

Thus, divorce and EPCs could be intimately connected as part of a "mate-sampling" process by which females use EPCs to assess the quality of potential mates prior to divorcing their current partner. A research paper titled "Why Does the Typically Monogamous Oystercatcher ... Engage in Extra-Pair Copulations?" answered that question as follows: to identify a better mate. There is, however, an alternative possibility. Maybe EPCs occur particularly when divorce is
not
an option! Assuming that most, if not all, living things are inclined to "move up" if the option exists, an equally consistent strategy would be that if you cannot, and if you are stuck with a partner who is somehow inadequate--or, rather, less desirable than someone else who might be available--then try EPCs.

A growing body of evidence suggests, however, that females engaging in EPCs may be trying to pave the way for an eventual switch of partners, strengthening a prospective pair-bond with a future mate. Spotted sandpipers, for example, are likely to pair with individuals with whom they have previously engaged in EPCs. Such behavior has been described as a "mate-acquisition tactic," and it is employed particularly by females. Similarly, in another bird species, the ocean-going razorbills, EPCs are evidently used for mate appraisal.

undermining the myth: females (other considerations)
103

As we have seen, females of many species are now known to intrude into the territory of extra-pair males, where they solicit copulations. They may even be increasing their chances of gaining more EPCs by choosing to settle down someplace where there are many male neighbors. It has also been suggested that the tendency of many species to breed in dense social groups ("colonies") may be due to the insistence of females upon mating with males whose nest-sites are near the territories of other males specifically because this gives these females the opportunity to mate with other males if they choose! (If so, males would have relatively little leverage; a male who refused to participate and insisted on maintaining his solitary splendor so as to be unthreatened by the prospect that his mate might copulate with a neighboring male might end up solitary indeed.)

When eastern bluebirds have already succeeded in breeding together successfully and are "old married couples," mated for at least the second time, they are significantly less likely to engage in EPCs than are "newlyweds" breeding together for the first time. Maybe female bluebirds are less likely to chance an EPC if they are already "happily married." In bird species as diverse as lesser scaup (a duck), barn swallows, and indigo buntings, females paired with younger males are more likely to stray sexually; those paired with older males are less likely to do so.

Generally, divorce rate is positively associated with EPC rate; in short, divorce and adultery are closely connected in animals, just as in human beings. It seems likely that females suffer possible mate desertion if their partner discovers that they have engaged in EPCs (more on this shortly). But despite many efforts to test such a connection, virtually no research has thus far confirmed it. A comparative study, looking at many different species, found a positive association between divorce and the frequency of EPCs in many different species; this is certainly suggestive, but it could be due to something else. Thus, if divorce is more likely when there is some inadequacy in either individual or incompatibility between them, then this inadequacy or incompatibility could give rise to EPCs and also-- independently--to divorce, rather than divorce arising
because
the male detected his female's infidelity and therefore terminated the relationship.

In our own species, it seems likely that the connection between divorce and sexual infidelity works in several ways: To begin with, adultery is likely to lead to divorce; indeed, it has long been the most frequently cited reason for marital dissolution. But at the same time, adultery doesn't occur in a vacuum. An otherwise happily married individual doesn't suddenly find him- or herself being adulterous--in our more antiseptic terminology, engaging in one or more EPCs--without any antecedents, as one might suddenly be struck by lightning, pulverized by an errant meteor, or victimized by a flowerpot accidentally dropped by someone from a window 10 stories

104
the myth of monogamy
^!

overhead. More likely, even if the adultery has not been carefully planned, it is predisposed by a degree of vulnerability within the married pair. When either husband or wife perceives that something is wrong (even if just a little bit wrong, and even if the existing discontent is below the level of conscious awareness), there may well be an increased openness to "prospecting" for an alternative partner. In this sense, impending divorce--or, at least, problems within the marriage--can also cause adultery.

Let's turn now to some reasons for
not
engaging in EPCs. Some likely ones are the most obvious. Maybe a female's in-pair male is simply more desirable than any extra-pair prospects. Maybe her male's mate-guarding is just too effective to permit any extracurricular copulations. Or maybe female choice is prevented by males in other ways. Although there is no doubt that female choice is important, often overwhelmingly so, there are situations in which females simply don't have much choice, typically when males are substantially larger and more aggressive, as in elk, for example, or elephant seals. In such cases, when males may be several times the females in size, as well as sporting potentially lethal weaponry (antlers, tusks, etc.), females are likely to go along with the male who gathers them together into his harem.

The physically impressive and behaviorally intimidating bull elk almost certain owes his attributes to male-male competition: Large, ferocious males succeed in besting other males who are a bit smaller and less ferocious. In the process, they can also work their will upon the females ... who, after all, generally have little reason to complain, since it is in their interests to mate with males whose traits have brought them such success. (Because their offspring, in turn, will be likely to have these traits and enjoy similar success.)

In other cases, female choice and male-male competition coexist uneasily. For example, the following experiment was conducted with captive brown trout. Female trout were exposed to two physically separated males who differed in the size of their adipose fin (long thought to be the key trait used by female trout to choose their mates). After the female revealed her choice--by preparing her nest closer to one of them, nearly always the one with the larger adipose fin--the two males were released. They proceeded to fight each other for the opportunity to spawn with the female in question. The result was that fewer than half the females ended up spawning with the male they had chosen. So, although mate choice is a real phenomenon for female brown trout, such a choice--at least in the laboratory--can be overwhelmed by male-male competition. It is interesting to note that successful males had higher levels of androgens, suggesting that while females were choosing based on a physical trait (size of the adipose fin), male success in

undermining the myth: females (other considerations)
105

competing with one another can be influenced by other, physiological factors, such as hormone levels.