Examples of male lactation (due to hormonal therapies, stressful conditions, etc.), both in humans and other animals, have been well documented, so it is clear that men have potential capacity for it, which gets activated under certain hormonal conditions. It's just that lactation, unlike orgasm, isn't intrinsically fun and that it is not as easy for men to lactate than for women to have orgasm, so most men don't try to develop their capacity for lactation. An example of poor design that shows that God doesn't exist. :->
<blockquote>Let's start with the facts. Of the 23 pairs of human chromosomes, 22, and the genes that they carry, are the same in men as in women. Only the twenty-third, the "sex chromosome," differs between men and women: women have two matched copies termed X chromosomes, while men have one X chromosome plus a smaller Y chromosome.
The genes on chromosome 23, acting in concert with genes on other chromosomes, ultimately determine all differences between our sexes. Those differences, of course, include not only the possession of ovaries as opposed to testes but also the postadolescent differences in beards, pitch of voice, and breast development. Blocking a single gene--say, one that normally codes for the cell receptor that binds testosterone--can make someone who genetically is otherwise a normal male develop breasts and a vagina.
The actual effects of testosterone and its chemical derivatives, called androgens, vary with age, organ, and species. Animals differ greatly in how the sexes develop. Adult male gorillas, for example, are much larger than females (weighing roughly twice as much), have a differently shaped head, and a silver-haired back. Human males also differ from females, though much less obviously, in being slightly heavier (by 20 percent on average), more muscular, and bearded. But males and females of some gibbon species look so similar that you couldn't distinguish them unless they permitted you to examine their genitals.
Both sexes of all mammals have mammary glands. While the glands are generally less well developed and nonfunctional in males, the degree of underdevelopment varies among species. At one extreme, in mice and rats, the mammary tissue never forms ducts or a nipple and remains invisible from the outside. At the opposite extreme, in dogs and primates (including humans), the gland does form ducts and a nipple in both males and females and scarcely differs between the sexes before puberty.
During adolescence the visible differences between mammalian sexes increase under the influence of a mix of hormones from the gonads, adrenal glands, and pituitary gland. Among the hormonally caused changes is a growth spurt in the mammary glands in females. Hormones released in pregnant females produce a further mammary growth spurt and start milk production, which is then stimulated by nursing. In humans, milk production is especially under the control of the hormone prolactin. (In cows the responsible hormone is somatotropin, alias growth hormone, the substance behind the current debate over the hormonal stimulation of milk cows.)
It should be emphasized that male and female differences in hormones aren't absolute but a matter of degree: one sex may have higher concentrations of a certain hormone and more receptors for it. In particular, becoming pregnant is not the only way to acquire the hormones necessary for breast growth and milk production. Direct injection of estrogen or progesterone (hormones normally released during pregnancy) has triggered breast growth and milk production in virgin cows--and also in male goats, male guinea pigs, and a steer. Granted, the hormonally treated steer produced much less milk than a virgin cow, and you shouldn't count on seeing steer's milk in the supermarket anytime soon. But that's not surprising, since the steer had previously limited his options: he hadn't developed an udder to accommodate all the mammary gland tissue that the hormonally treated cows could accommodate.
There are numerous conditions under which injected or topically applied hormones have produced inappropriate breast development and milk secretion in humans, both in men and in nonpregnant or non-nursing women. In one study, male and female cancer patients who were being treated with estrogen proceeded to secrete milk when injected with prolactin. Lactation has likewise been observed in people taking tranquilizers that influence the hypothalamus (which controls the pituitary gland, the source of prolactin), in people recovering from surgery that somehow stimulated the nerves related to the suckling reflex, and in women on prolonged courses of estrogen and progesterone birth-control pills.
All these cases involved medical intervention, but it is not always necessary. Mere repeated mechanical stimulation of the nipples suffices in some cases, since mechanical stimulation is a natural way of releasing hormones. For instance, sexually mature but virgin female marsupials can regularly be stimulated to lactate just by placing another mother's young on their teats. "Milking" of virgin female goats similarly triggers them to lactate. That principle might be transferable to men, since manual stimulation of the nipples causes a prolactin surge in men as well as in women.
For this phenomenon, my favorite human example is a letter to Dear Abby from an unmarried woman about to adopt a newborn. The writer longed to nurse the infant and asked Abby whether taking hormones would help her to do so. Abby's reply was "Preposterous, you'll only make yourself sprout hair!" Several indignant readers then wrote in to describe cases in which women succeeded in nursing an infant by repeatedly placing it at the breast. Experience suggests that most adoptive mothers begin producing some milk within three or four weeks. The reported examples included grandmothers up to the age of 71, as well as Ruth's mother-in-law, Naomi, in the Bible.
Breast development occurs commonly, and spontaneous lactation occasionally, in men under conditions of starvation. Thousands of cases were recorded among prisoners of war released from concentration camps after World War II; one observer noted 500 cases among survivors of one Japanese POW camp alone. The most likely explanation is that starvation inhibits not only the glands that produce hormones, but also the liver, which destroys those hormones. The glands recover much faster than the liver when normal nutrition is resumed, so hormone levels soar unchecked.
(Jared Diamond, "Father's Milk: Male Mammals' Potential for Lactation," Discover, <a href="http://www.findarticles.com/p/articles/mi_m1511/is_n2_v16/ai_16051177/print">February 1995</a>)</blockquote> -- Yoshie
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