Evolution's Rainbow. Joan Roughgarden
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to Paris. A bachelor on a love-boat cruise is entertained by women offering duty-free Cuban cigars and football lore memorized from the 49ers playbook. Let’s extend this idea.
If one sex, say A, is providing most of the parental care, then few are receptive to mating at any particular time because most are occupied with raising offspring. Conversely, the other sex, B, is not very involved with raising offspring and has many individuals ready and willing to mate. This asymmetry in the supply and demand of mates leads to a dynamic tension between the sexes. The Bs compete for access to and control of the As. Provided their freedom of choice is not thwarted by the Bs’ control, the As choose which B they wish to mate with.
Biologists call the ratio of receptive females to willing males the “operational sex ratio.” The operational sex ratio isn’t fifty-fifty because the sex with the higher parental investment is occupied with raising the offspring and is relatively unavailable for mating compared with the other sex.6
Returning to the seahorses and pipefish, we can ask which sex is relatively unavailable for mating because of their efforts in raising offspring. Swedish investigators found two nearby North Sea pipefish species that are indeed sex-role reversed. The females from both these species produce enough eggs for about two males during the time it takes for one male to raise his young. In the wild, the number of females with ripe eggs consistently exceeds the number of receptive males. Females in these species are polyandrous, with a harem of males. In addition, these females are larger than the males and develop bright colors at courtship time, presumably for the males to choose among, reversing Darwin’s classic peacock story. Furthermore, the females, not the males, compete with one another, forming dominance hierarchies for access to the males who will tend their eggs. Nine other pipefish species in which the females alone have sexual coloration and/or grow larger than the males are thought to be sex-role reversed as well.
On the other hand, seahorses and certain other pipefish species are not sex-role reversed; they follow the model of Darwin’s peacocks. Male seahorses can raise their young and get ready for the next embryos faster than female seahorses can produce egg batches. The result is a net surplus of males wanting eggs compared to females offering eggs. Males aggressively tail-wrestle and snout-snap one another for access to females, whereas females don’t have any specific aggressive behaviors among themselves. Male seahorses tend to be larger, more colorful, and more distinctly patterned than females.7
Thus sex-role reversal definitely occurs in nature. Many feel that the concept of an operational sex ratio effectively extends Darwin’s theory of sexual selection to cover sex-role reversed species—after all, the logic is the same for the mating strategies in both sex-role-typical species and sex-role-reversed species, with the identities of the excess sex and rate-limiting sex simply flipped. But no theory has been proposed to explain why sex-role reversal occurs in the first place.
Sex-role reversal is found in birds, especially aquatic and sea birds. When sex-role reversal occurs, the double standard can reverse too. Wattled jacanas from the Chagres River in Panama are large, squat black birds with white wing tips, a red face, and a long, yellow probing bill used to feed among shallow freshwater plants like hyacinths. The raucous, beefy females spend their days jousting with one another at the borders of their territories. Within these territories, harems of smaller males tend the eggs and chicks.
DNA fingerprinting has shown that males raise eggs laid by the female who controls their harem, even when the eggs were fathered by males outside the harem. The females clearly went outside their harem to obtain matings and yet burdened the males within their harem with the job of raising the young. The investigators, themselves male, were outraged, asserting that male jacanas were being “cuckolded” in spite of contributing so much parental care. One investigator stated, “It’s about as bad as it can be for these guys.”8
The converse probably wouldn’t have provoked such outrage. A female in a harem controlled by a male might raise a chick fathered by that male and placed there by a female from a neighboring harem. We could imagine many reasons for such an adoption. The female might find it advantageous to raise the chick in return for the controlling male’s provisioning and protection of the young she has mothered herself. Similarly, a male jacana might find it advantageous to raise a chick mothered by the controlling female in return for the controlling female’s provisioning and protection of the young he has fathered himself. Thus, sex-role reversal implies that the double standard also reverses. This idea takes some getting used to.
Other birds showing sex-role reversal include two shore birds, Wilson’s phalarope and the spotted sandpiper.9 Apparently, no mammals exhibit sex-role reversal, presumably because of the very high parental investment by mammalian females. In addition to the egg, a mammalian female supplies milk to the embryo and carries the young to term, either in a placenta or a pouch. For a mammalian male, this act is hard to follow. To exceed this already high parental investment by a female, a male would require a social system allowing him to care for his offspring well beyond the age of weaning, as may be approached in humans.
The evolution of the mammalian placenta and pouch is usually presented as a physiological advance, an adaptation for nurturing embryonic development in a climate that has cooled globally since the time of dinosaurs. Alternatively, the evolutionary force behind the placenta and pouch may have been for females to assume control of their offspring. A side effect is that males then acquire an incentive to control females.
5
Two-Gender Families
Let’s move on now to species with two genders that don’t change sex, do not have intersexual body parts, and aren’t sex-role reversed. Are such animals “normal”? Have we come at last to the familiar gender roles performed by ordinary bodies, as depicted on nature shows? Or are nature shows perhaps not telling the whole story? What goes on in two-gender animal families, and how are such families organized?
Many of us were raised to admire the nuclear family as a norm and were taught that single-parent families, families of same-sex couples, or communes were second-best alternatives or, even worse, wrong. Yet the meaning of a human family is in flux. In the United States, public attention has focused on the problem of how to define “family” as a result of a recent Supreme Court case about the rights of grandparents to visit grandchildren despite parental objection. The thirty-million-member American Association of Retired People (AARP) states that grandparents are the primary caretakers of 1.4 million children because many nuclear families have dissolved.1
The American Center for Law and Justice, which represents the Christian right, claims that “the traditional family, consisting of married parents and their children” is the building block of society. A leader of the Nation of Islam similarly declares, “Whenever . . . 50% of those who marry get divorced within the first three years, these are signs of the decline of a civilization.”2
Meanwhile, the Lambda Legal Defense and Education Fund, a gay rights organization, argues that neither side, grandparents or parents, sufficiently protects children being reared in nontraditional families, affirming the primary importance of “the quality and security of the relationship between individual children and adults rather than blood ties or labels.”3 Indeed, in June 2002, the California Supreme Court ruled unanimously that a man who cared for a six-year-old boy could be considered the boy’s parent, even though the man was not the boy’s biological father and was never married to the boy’s mother. The man was given custody of the child over the mother’s objections, showing that “parenthood could be achieved through love and responsible conduct.”4
With so much controversy about the meaning of family and parenthood, asking how animals raise their young may be helpful. What is an animal family? Does any family organization emerge as a particularly efficient way to raise young? And does biology support the belief that the nuclear family should have a privileged status in our society?
SEX AND POWER
Oh, I wish the simplest of animal families were a blissfully pair-bonded male and female. Alas, males and females negotiate over power in even the most elementary of animal families. Feminist writings call attention to a power differential between the sexes: “The image of the cage