Why is Sex Fun?: the evolution of human sexuality

Like birds, mammals are committed to much specialized reproductive anatomy and physiology of their own. Some of those specializations differ greatly between the three mammalian groups, such as placental development resulting in a relatively mature newborn in placental mammals, earlier birth and relatively longer postnatal development in marsupials, and egg-laying in monotremes. These specializations have probably been in place for at least 135 million years.
Compared to those differences between the three mammalian groups, or compared to the differences between all mammals and birds, variation within each of the three groups of mammals is minor. No mammal has re-evolved external fertilization or discarded lactation. No marsupial or placental mammal has re-evolved egg laying. Species differences in lactation are mere quantitative differences: more of this, less of that. For instance, the milk of Arctic seals is concentrated in nutrients, high in fat, and almost devoid of sugar, while human milk is more dilute in nutrients, sugary, and low in fat. Weaning from milk to solid food extends over a period of up to four years in traditional human hunter-gatherer societies. At the other extreme, guinea pigs and jackrabbits are capable of nibbling solid food within a few days of birth and dispensing with milk soon thereafter. Guinea pigs and jackrabbits may be evolving in the direction of bird species with precocial young, such as chickens and shorebirds, whose hatchlings already have open eyes, can run, and can find their own food but cannot yet fly or fully regulate their own body temperature. Perhaps, if life on Earth survives the current onslaught by humans, the evolutionary descendants of guinea pigs and jackrabbits will discard their inherited evolutionary commitment to lactation-in a few more tens of millions of years.
Thus, other reproductive strategies might work for a mammal, and it would seem to require few mutations to transform a newborn guinea pig or jackrabbit into a newborn mammal that requires no milk at all. But that has not happened: mammals have remained evolutionarily committed to their characteristic reproductive strategy. Similarly, even though we have seen that male lactation is physiologically possible, and although it also would seem to require few mutations, female mammals have nevertheless had an enormous evolutionary head start on males in perfecting their shared physiological potential for lactation. Females, but not males, have been undergoing natural selection for milk production for tens of millions of years. In all the species I cited to demonstrate that male lactation is physiologically possible-humans, cows, goats, dogs, guinea pigs, and Dyak fruit bats-lactating males still produce much less milk than do females.
Still, the tantalizing recent discoveries about Dyak fruit bats make one wonder whether out there today, undiscovered, might be some mammal species whose males and females share the burden of lactation-or one that might evolve such sharing in the future. The life history of the Dyak fruit bat remains virtually unknown, so we cannot say what conditions favored in it the beginnings of normal male lactation, nor how much milk (if any) the male bats actually supply to their offspring. Nevertheless, we can easily predict on theoretical grounds the conditions that would favor the evolution of normal male lactation. Those conditions include: a litter of infants that constitute a big burden to nourish; monogamous male-female pairs; high confidence of males in their paternity; and hormonal preparation of fathers, while their mate is still pregnant, for eventual lactation.
The mammal species that some of these conditions already best describe is-the human species. Medical technology is making others of these conditions increasingly applicable to us. With modern fertility drugs and high-tech methods of fertilization, births of twins and triplets are becoming more frequent. Nursing human twins is such an energy drain that the daily energy budget of a mother of twins approaches that of a soldier in boot camp. Despite all our jokes about infidelity, genetic testing shows the great majority of American and European babies tested to have been actually sired by the mother's husband. Genetic testing of fetuses is becoming increasingly common and can already permit a man to be virtually 100 percent sure that he really sired the fetus within his pregnant wife.
Among animals, external fertilization favors, and internal fertilization mitigates against, the evolution of male parental investment. That fact has discouraged male parental investment by other mammal species but now uniquely favors it in humans, because in-vitro external fertilization techniques have become a reality for humans within the past two decades. Of course, the vast majority of the world's babies are still conceived internally by natural methods. But the increasing number of older women and men who wish to conceive but have difficulty doing so, and the reported modern decline in human fertility (if it is real), combine to ensure that more and more human babies will be products of external fertilization, like most fish and frogs.
All these features make the human species a leading candidate for male lactation. While that candidacy may take millions of years to perfect through natural selection, we have it in our power to short-circuit that evolutionary process by technology. Some combination of manual nipple stimulation and hormone injections may soon develop the latent potential of the expectant father-his confidence in paternity buttressed by DNA testing-to make milk, without the need to await genetic changes. The potential advantages of male lactation are numerous. It would promote a type of emotional bonding of father to child now available only to women. Many men, in fact, are jealous of the special bond arising from breast-feeding, whose traditional restriction to mothers makes men feel excluded. Today, many or most mothers in first-world societies have already become unavailable for breast-feeding, whether because of jobs, illness, or lactational failure. Yet not only parents but also babies derive many benefits from breast-feeding. Breast-fed babies acquire stronger immune defenses and are less susceptible to numerous diseases, including diarrhea, ear infections, early-age-onset diabetes, influenza, necrotizing enterocolitis, and SIDS (Sudden Infant Death Syndrome). Male lactation could provide those benefits to babies if the mother is unavailable for any reason.
It must be acknowledged, however, that the obstacles to male lactation are not only physiological ones, which can evidently be overcome, but also psychological ones. Men have traditionally regarded breast-feeding as a woman's job, and the first men to breast-feed their infants will undoubtedly be ridiculed by many other men. Nevertheless, human reproduction already involves increasing use of other procedures that would have seemed ridiculous until a few decades ago: procedures such as external fertilization without intercourse, fertilization of women over the age of fifty, gestation of one woman's fetus inside another woman's womb, and survival of prematurely delivered one-kilogram fetuses by high-tech incubator methods. We now know that our evolutionary commitment to female lactation is physiologically labile; it may prove psychologically labile as well. Perhaps our greatest distinction as a species is our capacity, unique among animals, to make counter-evolutionary choices. Most of us choose to renounce murder, rape, and genocide, despite their advantages as a means for transmitting our genes, and despite their widespread occurrence among other animal species and earlier human societies. Will male lactation become another such counter-evolutionary choice?