Man Milk & Dad Lactation
One day, according to my Dad, when I was a wee bitty baby, I latched onto his nose and started suckling. Apparently it took about 4 seconds before I realized no milk was forthcoming and let my displeasure be known.
“Let me sing you the song of my people” -Me at 6 weeks old & my Dad
So why don’t men lactate?
After all, they have the equipment, kind of. Males in many mammalian species have nipples and mammary tissue. The presence and development of these structures in males depends on the timing of sexual differentiation during embryonic development. In humans, out of 100,000 men, 1-2 will develop breast cancer annually (Speirs and Shaaban 2009).
And the equipment works, kind of. Testosterone deficiency in men can, in rare circumstances, lead to the medical condition of galactorrhea- when a milk substance is spontaneously produced from the mammary gland. There are also a few reports of men producing milk and nursing their infants, but Big Foot is better documented.
Baby Bigfoot Bjorn
After a decade studying rodents and right after the launch of the highly influential book “Sex, Evolution and Behavior” with Margo Wilson, Martin Daly wrote the egregiously under-cited paper “Why don't male mammals lactate?” (1979). He compellingly argues that developmental barriers during pre-natal life, at puberty, and in adulthood prevent mammalian males from the ability to copiously synthesize milk. Most notably, functional development of the mammary occurs during pregnancy from hormonal signals from the ovaries, fetus, and for eutherian mammals, the placenta. Males don’t have ovaries and don’t get pregnant, substantial impediments.
Sidebar: In order for natural selection to favor a trait, first it must randomly occur due to mutations and/or recombination of genetic code during DNA replication. This means that for male lactation to even have the opportunity to be favored, vast swaths of DNA must spontaneously change- so that the expression of genes underlying manly man hormones are down-regulated and the expression of “lady” hormones are up-regulated in major ways. The probability that such dramatic spontaneous reorganization of the genome in a viable concepti would occur is really, really times a gazillion, really unlikely.
But wait, even if the genetic cards stacked up just right- would selection favor male lactation? The requisite hormonal changes might reduce sperm count, libido, and access to ladies, reducing the number of offspring a male will sire in his lifetime. Which means male lactation has to seriously improve offspring quality to trade-off against the reduced quantity. Since lady mammals with their wonderfully functional mammary glands are there nourishing young, how much added value can “man milk” provide for selection to favor its evolution?
art by T. MOORMAN 2009 Scientific American 300: 44 - 51
However, Kunz and Hosken (2009) posit that the barriers to male lactation may not be as insurmountable as proposed by Daly. Mammalian males already convert testosterone to estrogen via aromatization. If this occurs in the mammary gland can locally regulate mammary development avoiding hormonal cascades throughout the body. Moreover many plants produce phytoestrogens, such as soy, so a particular diet may facilitate mammary gland development and male lactation. Could male lactation actually be possible outside of manipulated domesticated animals and human medical conditions?
Challenging the paradox of useless man mammaries is Dyacopterus spadecius, the Dyak fruit bat. Lactation biologists studying this species in Malaysia report that wild-living males produce milk (Francis et al. 1994). Adult male dyak fruit bats surveyed produced some milk upon hand-stripping of the nipple, and dissection of the mammary gland in a subset of these males revealed that they had lacteriforus ducts with milk that flowed to the nipple similar to the lactating female Dyak mammary gland.
However, before getting too enamored with the possibilities of manbat milk, we should be cautious. Infant suckling produces tell-tale textures on the nipple (the academic term is ‘cornified’), which were absent in male Dyak fruit bats. This suggests that male Dyak fruit bats were not habitually nursing young (Francis et al. 1994). And even more problematically, males produced 2% the volume of milk as a lactating female (Racey et al. 2009, Francis et al. 1994), suggesting that copious milk production may be beyond the capacity of their mammary gland. The composition of the milk has not yet been described, whether or not its similar to female milk is therefore not known. Taken all together, the adaptive function of male lactation in the Dyak fruit bat remains uncertain. As per usual, more research is needed.
But just as moms are so much more than breastmilk, dads are so much more than no milk. Among mammalian species that show bi-parental care, fathers play an integral role in infant survival and development (Huck and Fernandez-Duque 2013). Direct paternal care, such as carrying, food-sharing, and cuddling, are linked to important hormonal changes in the male. Naturally, this Biology of Fatherhood applies to humans as well, but since this blog is about milk, check out the Evolving Father!
Aw yeah.
Happy Father’s Day.
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Literature Cited:
Daly, M. (1979). Why don't male mammals lactate?. Journal of theoretical biology, 78(3), 325-345.
Francis, C. M., Anthony, E. L., Brunton, J. A., & Kunz, T. H. (1994). Lactation in male fruit bats.
Huck, M., & Fernandez-Duque, E. (2013). When dads help: male behavioral care during primate infant development. In Building Babies (pp. 361-385). Springer New York.
Kunz, T. H., & Hosken, D. J. (2009). Male lactation: why, why not and is it care?. Trends in ecology & evolution, 24(2), 80-85.
Racey, D. N., Peaker, M., & Racey, P. A. (2009). Galactorrhoea is not lactation. Trends in ecology & evolution, 24(7), 354.
Speirs, V., & Shaaban, A. M. (2009). The rising incidence of male breast cancer. Breast cancer research and treatment, 115(2), 429-430.
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