Male lactation in zoology means production of milk from mammary glands in the presence of physiological stimuli connected with nursing infants. It is well documented in the Dayak fruit bat.
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The term male lactation is not used in human medicine. It has been used in popular literature, such as Louise Erdrich's The Antelope Wife, to describe the phenomenon of male galactorrhea which is a well documented condition in humans.
Newborn babies of both sexes can occasionally produce milk; this is called neonatal milk (also known as "Witch's milk") and not considered male lactation.
Male lactation was of some interest to Alexander von Humboldt, who reports in Voyage aux régions équinoxiales du Nouveau Continent of a citizen of the village Arenas (close to Cumana) who allegedly nurtured his son for three months when his wife was ill, as well as Charles Darwin, who commented on it in The Descent of Man, and Selection in Relation to Sex (1871):
It is well known that in the males of all mammals, including man, rudimentary mammae exist. These in several instances have become well developed, and have yielded a copious supply of milk. Their essential identity in the two sexes is likewise shown by their occasional sympathetic enlargement in both during an attack of the measles.
Darwin later considered the nearly perfect function of male nipples in contrast to greatly reduced structures such as the vesicula prostatica, speculating that both sexes may have nursed young in early mammalian ancestors, and subsequently mammals evolved to inactivate them in males at an early age.
Evolution and biology
Male mammals of many species have been observed to lactate under unusual or pathogenic conditions such as extreme stress, castration and exposure to phytoestrogens, or pituitary tumors. Therefore, it is hypothesized that, while most male mammals could easily develop the ability to lactate, there is no selective advantage to male lactation. While male mammals could, in theory, improve offspring's survival rate through the additional nourishment provided by lactation, most have developed other strategies to increase the number of surviving offspring, such as mating with additional partners. Presently, very few species are known where male lactation occurs and it is not well understood what evolutionary factors control the development of this trait.
Nonhuman animal male lactation
The phenomenon of male lactation occurs in some other species, notably the Dayak fruit bat (Dyacopterus spadiceus), and the lactating males may assist in the nursing of their infants. In addition, male goats are known to lactate on occasion.
Human male lactation
The phenomenon of successful human male breastfeeding has been credibly observed in several cases. However, the cases are not sufficiently documented to allow distinguishing of possible pathologic galactorrhea.