feeding, while marmoset fathers spend much time carrying their twin offspring.
All these excuses for male nonlactation still leave open the possibility that some other mammal species could exist in which male lactation might be advantageous to the male and his offspring. The Dyak fruit bat may turn out to be such a species. But even if there are mammal species for which male lactation would be advantageous, its realization runs up against problems posed by the phenomenon termed evolutionary commitment.
The idea behind evolutionary commitment can be understood by analogy to devices manufactured by humans. A manufacturer of trucks can easily modify one basic truck model for different but related purposes, such as transporting furniture, horses, or frozen food. Those different purposes can be fulfilled by making a few minor variations on the same basic design of the truck's cargo compartment, with little or no change in the motor, brakes, axles, and other major components. Similarly, an airplane manufacturer can with minor modifications use the same model of airplane to carry ordinary passengers, skydivers, or freight. But it is not feasible to convert a truck into an airplane or vice versa, because a truck is committed to truckhood in too many respects: heavy body, diesel motor, braking system, axles, and so on. To build an airplane, one would not start with a truck and modify it; one would instead start all over again.
Animals, in contrast, are not designed from scratch to provide an optimal solution for a desired lifestyle. Instead, they evolve from existing animal populations. Evolutionary changes in lifestyle come about incrementally through the accumulation of small changes in an evolutionary design adapted to a different but related lifestyle. An animal with many adaptations to one specialized lifestyle may not be able to evolve the many adaptations required for a different lifestyle, or may do so only after a very long time. For instance, a female mammal that gives birth to live young cannot evolve into a birdlike egg layer merely by extruding her embryo to the outside within a day of fertilization; she would have to have evolved birdlike mechanisms for synthesizing yolk, eggshell, and other avian commitments to egg laying.
Recall that, of the two main classes of warm-blooded vertebrates, birds and mammals, male parental care is the rule among birds and the exception among mammals. That difference results from birds' and mammals' long evolutionary histories of developing different solutions to tho problem of what to do with an egg that has just been fertilized internally. Each of those solutions has required a whole set of adaptations, which differ between birds and mammals and to which all modern birds and mammals are now heavily committed.
The bird's solution is to have the female rapidly extrude the fertilized embryo, packaged with yolk inside a hard shell, in an extremely undeveloped and utterly helpless state that is impossible for anyone except an embryologist to recognize as a bird. From the moment of fertilization to the moment of extrusion, the embryo's development inside the mother lasts only a day or a few days. That brief internal development is followed by a much longer period of development outside the mother's body: up to 80 days of incubation before the egg hatches, and up to 240 days of feeding and caring for the hatched chick until it can fly.
Once the egg has been laid, there is nothing further in the chick's development that uniquely requires its mother's help. The father can sit on the egg and keep it warm just as well as the mother can. After hatching out, chicks of most bird species eat the same food as their parents, and the father can collect and bring that food to the nest as well as the mother can.
In most bird species the care of the nest, egg, and chick requires both parents. In those bird species in which the efforts of one parent suffice, that parent is more often the mother than the father, for the reasons discussed in chapter 2: the female's greater obligate internal investment in the fertilized embryo, the greater opportunities foreclosed for the male by parental care, and the male's low confidence in paternity as a result of internal fertilization. But in all bird species the female's obligate internal investment is much less than that in any mammal species, because the developing young bird is “born” (laid) in such an early stage of development compared to even the least developed newborn mammal. The ratio of development time outside the mother-a time of duties that in theory can be shared by the mother and the father-to development time inside the mother is much higher for birds than for mammals. No mother bird's “pregnancy”-egg formation time— approaches the nine months of human pregnancy or even the twelve days of the briefest mammalian pregnancy.
Hence female birds are not as easily bluffed as female mammals into caring for the offspring while the father deserts to philander. That has consequences for the evolutionary programming not only of birds' instinctive behaviors but also of their anatomy and physiology. In pigeons, which feed their young by secreting “milk” from their crops, both the father and the mother have evolved to secrete milk. Biparental care is the rule in birds, and while in those bird species that practice uniparental care the mother is usually the sole caretaker, in some bird species it is the father, a development unprecedented among mammals. Care by the father alone characterizes not only those bird species characterized by sex-role-reversal polyandry but also some other birds, including ostriches, emus, and tina-mous.
The bird solution to the problems posed by internal fertilization and subsequent embryonic development involves specialized anatomy and physiology. Female but not male birds possess an oviduct of which one portion secretes albumin (the egg white protein), another portion makes the inner and outer shell membranes, and still another makes the eggshell itself. All of those hormonally regulated structures and their metabolic machinery represent evolutionary commitment. Birds must have been evolving along this pathway for a long time, because egg laying was already widespread in ancestral reptiles, from which birds may have inherited much of their egg- making machinery. Creatures that are recognizably birds and no longer reptiles, such as the famous Archaeopteryx, appear in the fossil record by 150 million years ago. While the reproductive biology of Archaeopteryx is unknown, a dinosaur fossil from about 80 million years ago has been found entombed on a nest and eggs, suggesting that birds inherited nesting behavior as well as egg laying from their reptilian ancestors.
Modern bird species vary greatly in their ecology and lifestyle, from aerial fliers to terrestrial runners and marine divers, from tiny hummingbirds to giant extinct elephant birds, and from penguins nesting in the Antarctic winter to toucans breeding in tropical rainforests. Despite that variation in lifestyle, all existing birds have remained committed to internal fertilization, egg laying, incubation, and other distinctive features of avian reproductive biology, with only minor variations among species. (The principal exceptions are the brush turkeys of Australia and the Pacific islands: they incubate their eggs with external heat sources, such as fermentative, volcanic, or solar heat, rather than with body heat.) If one were designing a bird from scratch, perhaps one could come up with a better but entirely different reproductive strategy, such as that of bats, which fly like birds but reproduce by pregnancy, live birth, and lactation. Whatever the virtues of that bat solution, it would require too many major changes for birds, which remain committed to their own solution.
Mammals have their own long history of evolutionary commitment to their solution to the same problem of what to do with an internally fertilized egg. The mammalian solution begins with pregnancy, an obligate period of embryonic development within the mother that lasts much longer than in any mother bird. Pregnancy's duration ranges from a minimum of twelve days in bandicoots to twenty-two months in elephants. That big initial commitment by a female mammal makes it impossible for her to bluff her way out of further commitment and has led to the evolution of female lactation. Like birds, mammals have evidently been committed to their distinctive solution for a long time. Lactation does not leave fossil traces, but it is shared among the three living groups of mammals (monotremes, marsupials, and placentals), which had already differentiated from each other by 135 million years ago. Hence lactation presumably arose in some mammal-like reptilian ancestor (so-called therapsid reptiles) even earlier.
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
