Mating systems

The concept of differential parental investment holds the key to understanding various mating systems: monogamous, polygynous, and more rarely, polyandrous. What has been described to this point as the basis for sexual selection is a polygynous system, in which one male mates with a number of females. Characteristically, in polygynous systems of birds and mammals, most of the females of reproductive age become inseminated and attempt to reproduce. Their reproductive success is determined more by the resources available to them—a suitable breeding site relatively safe from predators and with access to sufficient food to feed the young—than it is by the availability of males. The reproductive success of males, on the other hand, is more likely to be limited by competitive encounters with other males or with other dangers, for example, predators, to which the male is exposed during conspicuous and persistent courtship behavior. As a result, the variability (again, the variance) of the reproductive success of males tends to be greater than that of females. As with elephant seals, one or a few males may achieve most of the matings in a strongly polygynous system in which there has been vigorous sexual selection, and some males may be cut out of the picture entirely.

The Canada goose makes long-term pair-bonds. In summer the geese travel to the far north to breed, returning south in the winter to find open water and a supply of food. Their honking vee formation is a nostalgic reminder of the changing seasons, and at least in our culture, belief in their monogamous mating system seems to stir human emotions with a similar power. In fact, most birds are monogamous, at least in the limited sense of forming pairs within a single breeding season, whereas most mammals are to some extent polygynous. Only about four percent or fewer of mammalian species approach monogamy. What circumstances tilt the mating system toward monogamy? Simply put, the less disparity there is in parental investment, the more likely the mating system will tend toward monogamy. It has been argued that the difference between the monogamy of most birds and the polygyny of most mammals follows from the fact that female birds deposit their developing embryos outside of their bodies in large, shell-covered, yolk-containing eggs, whereas female mammals gestate the embryos internally, nourish them before birth through a special structure called the placenta, and following birth feed them milk, also produced by the mother.15 The mammalian form of maternal investment is enormous, and for many species the paternal role is correspondingly reduced. Following insemination of the female, to the extent that young will develop successfully to weaning without any further paternal input, the reproductive interests of the male are best fulfilled by seeking other females to inseminate. For birds, on the other hand, particularly those whose young hatch from the egg in a relatively naked and helpless state, sustained parental investment by the male may be an absolute necessity in order just to feed the gaping mouths. As long as the bonds remain stable enough to rear young successfully, strict monogamy may nevertheless be compromised by sneak fertilizations. These are always to the advantage of males, and can be equally advantageous for females if a genetically superior male should present himself. Strict monogamy, defined in terms of sexual encounters, is probably more a human ideal than it is a common biological reality.

The nature and degree of polygyny vary greatly in their expressions, even among related species.16 Frequently they appear to be tuned to other aspects of life history. Among the hoofed mammals that graze and browse for sustenance, there are many different examples. The dik-dik is a diminutive African antelope that lives in brushy country and hides from predators. Dik-diks are found in seemingly monogamous male-female pairs. At the other extreme are the largest grazers like the eland and buffalo that congregate in herds where individual males fight to control the copulations of as many females as possible. In between is the impala: modest in size but considerably larger than the dik-dik. Impalas move in herds and flee from predators. When breeding, adult males establish territories and wait for a group of females and immature males to pass through. The male's life then becomes a frantic rush to copulate with as many females as possible while chasing young males. The male has to scramble for his reproduction rather than having to fight for it, and anyone who witnesses this performance is likely to feel pity for his plight as well as wonder how he accomplishes anything.

The basic social unit of the hamadryas baboon of Ethiopia is a single male and a harem of several females. These units may congregate in larger assemblages, forming bands and troops. The males jealously guard their females, biting them on the neck if they stray, and keeping other males from mingling with them. Other species of baboons from farther south in Africa have a somewhat different social structure consisting of troops of two or three dozen individuals of both sexes, adults and juveniles. Females remain in the troops of their birth, whereas there is some exchange of males between troops. The males have a dominance hierarchy that is reasonably stable over time but subject to rearrangement as individuals come, go, mature, and age. The frequency of copulations that males achieve corresponds with their social rank; higher ranking males are more successful in mating females. The system is not as exclusive as in the closely related hamadryas baboon, because a given female is not monopolized-by a single male.

The primates as a group show great variation in social and mating systems. The orangutan (an ape) and a number of nocturnal prosimians (tree shrews, lemurs, lorises, galagos, and relatives) are solitary. Both the Old World and New World families of monkeys contain examples of species that congregate in single-male, multi-female bands or multi-male, multi-female bands, along the lines described for baboons.

Our closest primate relatives, the chimpanzees, usually live in multi-male, multi-female groups, but some all-male groups also exist. When individuals transfer between groups, they are usually females, an unusual pattern for mammals but one also shared with gorillas and a few species of monkeys. The human social structure based on male kinship employing exchanges of women may therefore have its antecedents in the social organizations of the great apes. At one time it was thought that chimpanzees were indiscriminate philanderers, and it was hypothesized that male-male competition occurred primarily at the level of sperm production.17 (A chimpanzee produces more than twice as many sperm per ejaculation than a man, and twelve times more than a gorilla.) More recent field observations, however, suggest that copulatory successes of male chimpanzees are not uniform.18 Frequently at the time of estrus a female effectively pairs off for a few days with a single male. Higher ranking males are more successful in monopolizing females in this manner.

The theory of parental investment is supported by cases in which the roles of males and females are to some extent reversed.19 In a family of fish that includes the little sea horse, the female transfers her eggs to a pouch on the male, who then gestates and nourishes the developing young. Sexual selection has been at work on these fish, for the females are not only the more brightly colored but also the active wooers in courtship.

Phalaropes, a small group of shorebirds, show a similar reversal in the usual pattern of sexual selection. The females are larger, brighter, arrive on the breeding grounds first, and guard the males from other females. The males, for their part, take sole responsibility for brooding the eggs and newly hatched chicks. Why have male phalaropes become the limiting reproductive resource, with the greater parental investment? A plausible explanation for this phenomenon draws on the breeding ecology of shorebirds. These creatures nest on the ground in open country, usually in the far north. The breeding season is short, and the nests are subject to predation. Since the young hatch able to feed themselves (like ducks and geese and unlike familiar songbirds), both parents are not required once the eggs are laid. If the female can defect and find another mate, she does so. In fact polyandrous mating (one female, several males) is practiced by other species of shorebirds that experience similar environmental problems and share the same precocial pattern of development. Polyandry is relatively uncommon among vertebrates but it is an exception that supports the rule of differential parental investment. We shall describe in a later chapter the special and equivalently rare case of polyandry in human societies.

Life-history strategies

The mating systems monogamy, polygyny, and polyandry represent but one measure of reproductive diversity. Some animals (and plants) concentrate their reproductive energies in a single but frequently massive effort. The butterfly characteristically lays her eggs and dies; the migratory salmon returns to the site of its birth only to terminate its life after an arduous upstream swim as part of a crescendo of reproductive exertion. Other species, however, reproduce repeatedly. The queen honeybee does little but lay eggs for several years, and birds and mammals generally reproduce seasonally, often more, sometimes less frequently. In animals, the capacity for repeated reproduction appears to be a prerequisite for the evolution of social organizations.

Life histories can also be characterized on a related dimension. At one extreme are species that produce many offspring, develop rapidly, exploit habitats quickly, and frequently have means for dispersal to distant sites. They frequently are found in unstable environments such as puddles or forest clearings, and there is a premium on at least some of the progeny finding suitable new places to live and reproduce. At the other end of the spectrum are longer-lived species that occupy more stable habitats, generally have larger bodies, develop more slowly, and reproduce repeatedly but have relatively few offspring. The former depend for reproductive success on a large number of offspring and often on the ability to colonize new areas. The latter depend more on their ability to exploit the habitat in which they live. Viewed another way, they also represent two extremes of parental investment, the first in fecundity, the second in nurture.

The two extremes were originally called, respectively, r-selected and K-selected species, a lifeless terminology that refers to parameters in the equations for growth of populations. The original concept was that some species have evolved with less emphasis on fecundity and more on competitive efficiency in the extraction of energy from their environment. As a consequence, in these latter species population sizes would tend to be more stable over time, but the densities might approach more nearly the carrying capacity of the environment. At the risk of oversimplification, this theoretical basis for r- and K-selection has been criticized on the grounds that r (the intrinsic rate of increase of the population) might reasonably be the object of selection, but K (the carrying capacity of the environment) is not a parameter of the same quality.20 Regardless of how one formulates causal evolutionary hypotheses, however, animals do display this variety of life histories, and it is useful to think of this diversity as different designs for fitness, or more specifically, as different strategies of parental investment. We see once again that there are multiple solutions to the evolutionary challenge of reproduction.

Long life, slow development, and extensive parental care all require attention to garnering food resources and a greater need for the individual to ensure its own safety over extended periods of time. So we might expect to find in such species evolutionary changes that have increased the effectiveness of sensory detection and efficiency of exploitation of energy resources, including behavioral corollaries such as territoriality and sociality. With this form of life history, prolonged survival becomes necessary for successful reproduction, and with fewer offspring and a long period of development, mechanisms to forestall premature demise are also necessary. Behavior comes to exercise greater control over the rates of both births and deaths. In later chapters we shall elaborate on the idea that much of what is referred to as human nature is understandable as a consequence of the evolution of a long-lived, slowly developing, resource-requiring, mildly polygynous social primate that also happens to be highly intelligent.

What about the mating behavior of humans?

The origins of the relationships between the sexes in human societies have received much attention in recent years. Evolutionary hypotheses about male domination have been advanced, based on observations of contemporary hunter-gatherer cultures and on supposed divisions of responsibility in hunter-gatherer societies during the early evolution of humankind. To some extent these hypotheses recognize biological realities such as lactation, but they nevertheless remain narrowly based. As there is very little direct evidence as to how such societies were organized tens of thousands of years ago, it becomes possible to assert that they were truly egalitarian and that the domination of women is a cultural phenomenon that came later.21 This is both fashionable and facile, and as others have sensibly argued, the very common asymmetry between the sexes of animals means that arguments about the causes of sexual selection should not be based on the characteristics of any one species.

The results of sexual selection in humans are evident to those who wish to see. The mild sexual dimorphism of body size and strength is accompanied by competition between young males for women, a greater variance in reproductive success among men than women, extensive efforts of men to control the reproductive destinies of women, a greater tendency of men to seek multiple partners, rape as a coercion of women by men and not vice versa, and prostitution as a female profession. Human males are capable of nurturing their offspring to an extent unparalleled among mammals, but that has not neutralized the force of sexual selection. Women still make the greater parental investment, and the evolved reproductive strategies of men and women are not congruent.

Among the primates, about eighteen percent of the species are seemingly monogamous. Where do humans fit in this picture? Some regularized pairing of men and women akin to marriage is so widely distributed across cultures as to be a characteristic of the human species. Features of Homo sapiens that contribute to this practice include slow development of the young with correspondingly great parental investment, long lifetimes, and elaborate social structures that are made possible by the cognitive capacity of the human brain. But what do we see in human mating practices that fits into the larger comparative and evolutionary picture that we have been painting?

By one estimate, about eighty percent of human societies are at least mildly polygynous and twenty percent monogamous.22 These numbers alone, however, are misleading. Monogamous unions are in fact the most prevalent, even in societies where polygyny is acceptable. Polygyny is made practical when one man is able to accumulate sufficient resources to support more than one wife. Among the !Kung San people of Africa, whose traditional way of life is probably the clearest reflection still available of a preagricul-

tural, hunter-gatherer culture, about ninety-five percent of the pairings are monogamous.23 And in Western Europe and North America, where monogamy is decreed by law, divorce and remarriage effectively relax the formal imposition of monogamy.

The idea that polygyny is associated with accumulations of wealth and power is documented by an interesting analysis performed by Laura Betzig.24 She compiled data on 104 autonomous societies from different places in the world and from different times in history with an eye to comparing information on polygyny and the means by which conflicts were resolved. She focused on a subset of twelve societies that exhibited the most complex political structure, with four hierarchical levels of organization. All were characterized by a despotic political organization, with vast power vested in a chief or king who settles disputes, frequently to his personal advantage. In all twelve, punishment of the aristocracy was less harsh than that of commoners. Interestingly, the concentration of power and despotic behavior correlates with extensive polygynous control of women, sometimes in such vast numbers as to preclude the potentate's sexual activity with all of them. In Incan society, access to wives correlated throughout the political structure. Depending on where a man stood in the formal hierarchy of principal persons, governors, administrators, and petty chiefs the system might provide "for their service and multiplying people in the kingdom" women in the numbers of 50, 30, 20, 15, 12, 8, 7, 5 or 3. Clearly a substantial number of men at the bottom of the hierarchy must have been without wives. This is a condition that ordinarily can be expected to generate considerable unrest and requires a despotic political structure to keep it from erupting.

Does polygyny increase male fitness? It certainly does for those males who hold resources and power. In all of the societies in Betzig *s study, polygyny is associated with "a high degree of differential reproduction." This indicates why, in evolutionary terms, resources and power are important ends in themselves, a matter to which we shall return in later chapters. The promiscuous tendencies of men would seem to be obvious, and in Western cultures they have also been documented by attitudinal surveys.25 Eight or nine times as many married men as women—in total, nearly half of the men—would like to engage in extramarital sex. The same individuals also held the view that men have a greater sexual desire than women.

The sexual interest of male mammals can frequently be aroused by novel females. Are human males an exception? This phenomenon is called the Coolidge effect,26 supposedly in honor of a Presidential visit to a poultry farm. According to the story, Mrs. Coolidge, out of earshot of her husband, inquired whether a rooster could copulate more than once a day. On being told "dozens of times a day" she requested that this information be conveyed to the President. When this exchange was reported to Mr. Coolidge, he asked, "Same hen?" On hearing "no, a different one each time" he is alleged to have said, "Tell that to Mrs. Coolidge."

Monogamy is a mating practice that increases the parental investment of the male, which is to the advantage of the female. Although any offspring will have the female's genes, we have seen that the male cannot have the same certainty as the female about the presence of his own. The female's genetic interests are fulfilled if the male sustains his parental investment in her children, but the male's interests are furthered only if he is their father. The male may therefore gain in fitness by casual liaisons with other females, but he loses heavily (in terms of Darwinian fitness) if he invests in another man's offspring at the expense of what might have been his own. On the other hand, female infidelity has a genetic cost only if she loses the support of her mate, and under some circumstances infidelity could have a genetic benefit. Monogamy increases the male's confidence in paternity, but it does not make it certain. It is therefore no accident that through history human males have expended much effort in trying to control the reproductive activities of females.

There are numerous manifestations of this asymmetry, both cultural and emotional.27 Female chastity is a virtual commodity in many societies. Unmarried women are chaperoned, veiled, hobbled, or otherwise protected or mutilated so that their value as future wives will not be compromised, and a transgression on their part can irrevocably soil the family honor. Violence induced by male sexual jealousy is a familiar theme in literature and life, and infidelity is the principal cause for the killing of wives by husbands. Until very recent times, most cultures have had a double standard regarding infidelity in that the crime is defined in terms of the marital status of the woman. The cuckolded husband is the aggrieved individual and is frequently excused of violence if he takes the law into his own hands. Margaret Mead's myths to the contrary notwithstanding, male sexual jealousy is a significant source of violence everywhere in the world.28

Martin Daly and Margo Wilson,29 in a fascinating analysis of homicide statistics, have provided a great deal of insight into matters usually left to sociologists. Contemporary urban violence in the United States frequently seems to us now to be the result of drug wars, which to a large extent are also conflicts over resources. They are also largely conflicts between young men with otherwise limited economic prospects. In another generation, as indeed seems to have been the case for centuries in Europe and America (for which historical studies exist), a substantial fraction of all homicides involved young men fighting over utter trivia like a spilled glass of beer, a minor gambling debt, or a perceived insult to pride or honor. This is but one manifestation of what appears to be a need to demonstrate one's manhood, and it seems particularly prone to flare into violence where the participants have strained economic circumstances with little likelihood of improvement. But why should men and not women behave in this statistically predictable and dangerous way? What is it that drives the psyche of the young male to take such seemingly foolish risks? As the phenomenon is widespread and recurrent and the stakes frequently so high, the explanation of ultimate cause must involve something quite important to males. The most likely answer is that through time, access to resources and elevated social status have been very important in determining the reproductive success of males. Respect of peers is a major determinant of social status, and considering the ever-present hidden agenda that evolutionary history has provided, it is not at all ironic that the proximate goal of the participants in these altercations is to demonstrate that they "have balls."

Rape is a special form of violence that men inflict on women. It is not unusual for women in our present society to see rape as misogyny, an expression of hatred of women. A victim of rape who recently chose to make her story public was incredulous that as her attacker left he told her how pretty she was. The columnist who reported this story, also a woman, was equally shocked at the rapist's remark. Their bewilderment followed from their beliefs about the causes of rape.

Much sociological and psychiatric effort has gone into the study of rape, and the effort has illuminated some aspects of proximate cause. A smaller number of analyses have tried to put rape into an evolutionary perspective. Women react very negatively to rape for sound evolutionary reasons; the act not only subjects them to physical harm, it deprives them of their natural role of choosing with whom and when they will mate. Quite simply, rape is not in the interests of either women or their genes. As a further complication, rape can also compromise a woman's relationship with her husband; witness the tendency of men to lay blame on the victims of rape.

An evolutionary perspective suggests that, as in animals (e.g., ducks), rape has something to do with improving the rapist's Darwinian fitness.30 In our mildly polygynous species we might predict that rape is most frequently performed by men whose educational level is low and whose economic (and thus reproductive) prospects are poor. This is in fact the case, not just in the racially troubled cities of the United States, but in the ethnic and cultural homogeneity of Denmark, and in cultures where the marriage prospects of men are constrained by the need to purchase wives, the custom of "bride-price." Economics is only one ingredient, for the typical rapist is not only a young man but one that has low self esteem and little sense of purpose in life. Whatever the proximate cause of his psychological profile, his prospects for increasing his fitness through orthodox means are poor, and he usually knows it.

The victims of rape are characteristically young women early in their reproductive years. If rape were an expression of hate of women, this would not necessarily be so. In fact, the age distribution of rape victims is skewed to younger years than the distribution of female victims of murder.

This overview of rape should not be taken to mean that men high on the socioeconomic ladder never try to coerce women into granting sexual favors. The way many men behave may be determined by their power over women and by their perceptions of the risks they are running. Groups of young men in fraternities and athletic teams can behave in insensitive, exploitative, and even criminal ways toward young women. And through history women have routinely suffered rape from conquering armies. None of this, however, detracts from the force of an evolutionary perspective.

The incidence of infanticide among mammals was mentioned previously as an example of how its explanation was muddled by the confusion of individual and group selection. Infanticide is not unknown in human societies either. Consider the advice of Moses to his people following their victory over the Midianites, in which they slew all the males. . . . Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him. But all the women children, that hath not known a man by lying with him, keep alive for yourselves.

Numbers, 31

What could be a more explicit set of instructions both for eliminating reproductive competition, present and future, as well as for assuring paternity among the appropriated females?

Infanticide does not require the conditions of war. It has been encountered by anthropologists in a variety of cultures, where it is almost always an expedient to address one of several situations.31 One is the birth of a deformed infant. A second is the wrong father. A third, and numerically the most important, occurs when the mother, usually quite young, does not have the social support and resources to care for the offspring. Whatever moral judgment one cares to make from the comfort and security of a modern Western culture, these are outcomes that are evolutionarily in the interests of the mother's ultimate fitness.

Sometimes there is a selective killing (or neglect) of daughters, and there is some evidence that this is most prevalent in highly stratified societies. A likely explanation in evolutionary terms is that in such societies the reproductive potential of sons of the upper classes can be enormous, much greater than that of daughters, and amplified if the society is polygynous.

Infanticide also occurs in modern Western cultures, although the mechanism may frequently be neglect rather than a single decisive act. The frequency is low—less than three dozen per million children per age class per year—but the circumstances are interesting. Infanticide at the hands of the mother usually results from the same situations as in "primitive" cultures, and young mothers are more likely to be involved. Children who are at risk from parents are at much higher risk during the first year of life and are at higher risk from stepparents than from natural parents. The fables of Cinderella and of Hansel and Gretel are rooted in fact.

The relationships between men and women are like an intricately cut diamond whose appearance changes when viewed from dif ferent directions. In this chapter we have been peering at our reflections from just a few of its many facets, and we have seen that there is more to human nature than can be understood without biology. The social sciences can describe, but in their present state they are unable to explain, some of the deepest questions that are posed by the behavior of their subjects.

Let me now head off one reaction that is probably inevitable in today's social climate. This is not a political essay. In invoking evolutionary biology and the concept of ultimate causation I trust I will not be saddled with the view that because something is "biological" it is necessary or appropriate or right for human society or that I am defending any social or economic status quo. Quite the contrary, there are a number of aspects of human behavior—regardless of what their origins may be—that may be maladaptive or culturally inappropriate in the technologically complicated world in which we now live. Unarguably, homicide, rape, and a host of other forms of violence and exploitation are deplorable, yet despite both moral and legal sanctions they remain disturbingly ubiquitous. I have argued above, and I shall argue repeatedly again, that in order to address biological and social problems we must accept the inherent complexity of what is meant by the word "cause." In short, where there is a problem, there is much to be said for trying to understand it before attempting to solve it.

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