Why Is Sex Fun__ The Evolution of Human Sexuality - Jared M. Diamond [25]
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 the 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 tinamous.
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