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The phenomenon of male lactation in humans has become more common in recent years due to the use of medications that stimulate a man's mammary glands. Though boys and men have nipples, many are unaware that they also have mammary glands. Ordinarily there is so little mammary tissue that it is unnoticeable; if the male breasts develop visibly, the condition is called gynecomastia. Under the appropriate hormonal stimulus—that nature provides to human females when they become pregnant and give birth—the mammary glands of human males can also produce milk. The volume of milk produced is low relative to that of a lactating female. Male lactation has, in some cases, commenced without hormonal treatments as well. Newborn baby boys (and girls) can occasionally produce milk because of the intense hormones involved in their mother's pregnancy and the hours of childbirth; this is called witch's milk.
Male lactation is most commonly caused by hormonal treatments given to men suffering from prostate cancer. Female hormones are used to slow the production of cancerous prostate tissue, but the same hormones also stimulate the mammary glands. Male-to-female transsexuals may also produce milk due to the hormones they take to reshape their bodies. Extreme stress combined with demanding physical activity and a shortage of food has also been known to cause male lactation. The phenomenon was first studied in survivors of the liberated Nazi concentration camps after World War II. Some American POWs returning from the Korean and Vietnam Wars also experienced male lactation.
It is also possible for males (and females) to induce lactation through constant massage and simulated 'sucking' of the nipple over a long period of time (months).
The phenomenon of male lactation occurs in one non-human species, the Dayak fruit bat (Dyacopterus spadiceus), and the lactating males may assist in the nursing of their infants.
Spontaneous male lactation and even nursing have occasionally been observed in humans.
In Why Is Sex Fun?, Jared Diamond reports of male and female cancer patients being treated with estrogen who proceeded to lactate when injected with prolactin, and suggests that mechanical stimulation of male breasts, by releasing prolactin, could result in lactation. He also mentions teenage boys lactating after self stimulation of their nipples.
The complete, up-to-date and editable article about Male Lactation can be found at Wikipedia: Male Lactation