Males' nipples are regarded as a sign that till a certain age, inside the mother's womb, we are not assigned to a specific sex. So the embryo prepares to produce either a girl or a boy; when the genetic information says boy, the nipple bud stops its development.

But in certain conditions, men can lactate. "Up until a certain age, boys and girls, as fetuses, are indistinguishable, really, so we retain-women retain some remnants of the vas deferens, which is the canal that sperm follows. If you have no Y chromosome, then certain hormones are released that say, 'Okay, we'll set up this child's breast tissue to develop at puberty so that she will be able to produce milk.' Men didn't [secrete those hormones], so we don't usually have breast tissue", said Jack Newman, a Toronto-based doctor and breast-feeding expert. "Actually a significant number of boys around the age of puberty do develop breasts, so the tissue is there, but it regresses."

Proofs of lactating men come from the Ancient Jew Scripts to modern literature.

Science, too, found evidence; scarce, but genuine.

Even recently, in 2002, media signaled the case of a 38-year-old man in Sri Lanka who nursed his two daughters after his wife died during the birth of her second child.

Some say that in the current society, were females are increasingly equal to males, fathers producing milk to care offspring would just back all the career mothers, dealing with the job's demands and family. Some researchers have said that men could induce lactation by nipple stimulation, but it is basically a hormonal process. "The father just put the baby to the breast and he would produce milk; I think that's pretty unlikely. It could be that you have this man with this pituitary tumor and he produces milk once the baby starts suckling", said Newman.

The hormone involved in lactation is prolactin, whose high levels trigger it and is produced by the brain's pituitary gland. Thorazine, an antipsychotic drug employed in the mid-20th century, affected the pituitary gland, inducing high levels of prolactin and a heart drug named digoxin also induces lactation.

A lesion to the pituitary gland can also induce milk production. "It would be the same reason: increased prolactin levels-in the one case drug-induced, in the other due to a tumor or some other sort of neurological problem", said Newman.

Starvation can decrease the activity of the hormonal glands and of the hormone-absorbing liver, triggering lactation, as happened with survivors of Nazi concentration camps and Japanese POW camps during the World War II. "The glands recover much faster than the liver when normal nutrition is resumed, so hormone levels soar unchecked", said physiologist Jared Diamond, who advocated that nipple stimulation, too, plays an important role.

Cases of lactating males have been detected in many mammalian species; but by now only in one species - the Dayak fruit bat of Southeast Asia - this was observed to occur spontaneously.