Can Males Lactate?

One of the main characteristics of mammals is their ability to feed their offspring with milk produced by their mammary glands. Kunz and Hosken (2008) were curious why this trait is seen only in mammalian females. Both males and females are physiologically capable of lactating. In primates, the male and female nipples develop more or less identically until puberty. At this point, androgens suppress the production of mammary glands and ducts which lead to the surface of the nipples. However, if this androgen suppression did not occur at the critical moments of development, males could in fact produce mammary glands. Males also already contain a mechanism to create estrogen because both testosterone and estrogen are very similar steroids and are build in very similar processes. Furthermore, lactation is not caused by pregnancy. Prolactin causes lactation to occur in females. This can be triggered by suckling stimulation or exogenous hormone administration in non pregnant mammals. Prolactin is not only secreted by females though. Males have surges of prolactin comparable to those females experience which cause lactation.

In fact, male humans have displayed the ability to lactate before. After malnourished prisoners of war from World War II were freed and given a proper diet, their bodies began to recover. While in the prisons, the soldiers suffered liver, testicular, and pituitary atrophy. The pituitaries and testes recovered first and began producing androgen and estrogen. However, the liver did not recover as quickly and could not metabolize the hormones. Due to the imbalance of hormones, the males began to lactate. Similar effects have been seen in sheep whose diets contain plants that produce estrogen like hormones.

It is believed that the only reason lactation does not occur in males is because it is a maladaptive trait when the certainty of paternity is not one hundred percent. Lactation is a very costly endeavor for females. Most male mammals show reduced care of offspring and it is therefore not worth the cost of producing milk for their offspring. However, in two species of Old World fruit bats, repeated incidences of male lactation have been seen. The males do not produce nearly as much milk as the females do and it is difficult to tell if the offspring feed from the males. While the lactation could be the result of pesticides, it is unlikely due to the high number of lactating males in the population. Further investigations are required to see if these species have adapted lactation in males, yet it remains a curious question: If males can lactate, why don’t they?

References:

Kunz, T.H. & Hosken, D.J. (2008) Male lactation: Why, why not and is it care? Trends in Ecology and Evolution, 24(2): 80-85.