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, feeding castrated animals with phytoestrogens or animals with pituitary tumors. Hence it was hypothesized that while most mammals could easily develop the ability to lactate this does not provide the males, or the species with any evolutionary advantage. While the males could in theory improve the chance to pass on their genes by improving the feeding their offspring by male lactation, most of them have developed other strategies such as mating with additional partners. Presently only 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. In rare cases male cats have been known to lactate in response to stimulation by a kitten, especially if the male has been neutered. 
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.
See also 
- Voyage de Humboldt et Bonpland ; 1-3. Voyage aux régions équinoxiales du Nouveau Continent : fait en 1799, 1800, 1801, 1803 et 1804. Tome 1 / par Al. de Humboldt et A. Bonpland ; rédigé par Al. de Humboldt; J. Smith (Paris), 1814-1825, p. 376, (Online at gallica)
- Descent of Man, Chapter I
- Descent of Man, Chapter VI
- Kunz, T; Hosken, D (2009). "Male lactation: why, why not and is it care?". Trends in Ecology & Evolution 24 (2): 80–85. doi:10.1016/j.tree.2008.09.009. PMID 19100649.
- Gómez MA, Garcés-Abadías B, Muñoz A, Vásquez F, Serrano J, Bernabé A (1999). "Structural and Ultrastructural Study of GH, PRL and SMT Cells in Male Goat by Immunocytochemical Methods". Cells Tissues Organs 165 (1): 22–29. doi:10.1159/000016670. PMID 10460970.
- "'Kitten Nurses on Male Cat - AllExperts.com". Retrieved February 26th, 2012.
- Nikhil Swaminathan (2007-09-06). "Strange but True: Males Can Lactate". Scientific American. Retrieved 2008-05-27.
- Angier, Natalie; New York Times, February 24, 1994. Cr. J. Covey.
- Francis, Charles M., et al.; "Lactation in Male Fruit Bats," Nature, 367:691, 1994.
- Fackelmann, K.A.; Science News, 145:148, 1994.
- Anomalies and Curiosities of Medicine G.M. Gould and W.L. Pyle