Why is Sex Fun?: the evolution of human sexuality by Jared Mason Diamond Read Free Book Online Page A

chance that her first clutch might be destroyed and he would require a replacement clutch. But she might also choose to pursue her own interests, seeking out some other male available immediately to receive her second clutch. If her first clutch survives and continues to occupy her former mate, her polyandrous strategy has thereby doubled her genetic output.
    Naturally, other females will have the same idea, and all of them will find themselves in competition for a dwindling supply of males. As the breeding season progresses, most males become tied up with their first clutch and unable to accept further parental responsibilities. Although the numbers of adult males and females may be equal, the ratio of sexually available females to males rises as high as seven-to-one among breeding Spotted Sandpipers and Wilson's Phalaropes. Those cruel numbers are what drive sex-role reversal even further toward an extreme. Though females already had to be slightly larger than males in order to produce large eggs, they have evolved to become still larger in order to win the fights with other females. The female reduces her own parental care contribution further and woos the male rather than vice versa.
    Thus, the distinctive features of shorebird biology— especially their precocial young, clutches of few but large eggs, ground-nesting habits, and severe losses from preda-tion-predispose them to male uniparental care and female emancipation or desertion. Granted, females of most shorebird species can't exploit those opportunities for polyandry. That's true, for instance, of most sandpipers of the high Arctic, where the very short breeding season leaves no time for a second clutch to be reared. Only among a minority of species, such as the tropical jacanas and southerly populations of Spotted Sandpipers, is polyandry frequent or routine. Though seemingly remote from human sexuality, shorebird sexuality is instructive because it illustrates the main message of this book: a species' sexuality is molded by other aspects of the species' biology. It's easier for us to acknowledge this conclusion about shorebirds, to which we don't apply moral standards, than about ourselves.
    The remaining type of exception to the predominant pattern of male desertion occurs in species in which, like us, fertilization is internal but it's hard or impossible for a single parent to rear the young unassisted. A second parent may be required to gather food for the coparent or the young, tend the young while the coparent is off gathering food, defend a territory, or teach the young. In such species the female alone would not be able to feed and defend the young without the male's help. Deserting a fertilized mate to pursue other females would bring no evolutionary gain to a male if his offspring thereby died of starvation. Thus, self-interest may force the male to remain with his fertilized spouse, and vice versa.
    That's the case with most of our familiar North American and European birds: males and females are monogamous, and they share in caring for the young. It's also approximately true for humans, as we know so well. Human single-parenthood is difficult enough, even in these days of supermarket shopping and babysitters for hire. In ancient hunter-gatherer days, a child orphaned by either its mother's or its father's death faced reduced chances of survival. The father as well as the mother desirous of passing on genes finds it a matter of self-interest to care for the child. Hence most men have provided food, protection, and housing for their spouse and kids. The result is our human social system of nominally monogamous married couples, or occasionally of harems of women committed to one affluent man. Essentially the same considerations apply to gorillas, gibbons, and the other minority mammals practicing male parental care.
    Yet that familiar arrangement of coparenthood does not end the battle of the sexes. It does not necessarily dissolve the tension between