Dina: “I had no idea you could milk a cat.”
Greg: “Oh yeah, you can milk anything with nipples.”
Jack: “I have nipples, Greg. Could you milk me?”
The short answer is no, you can’t milk Robert DeNiro. Barring specific medical conditions—like a tumor on the pituitary gland—men generally lack the necessary levels of prolactin to stimulate lactation and cannot produce milk. So if they aren’t able to be useful and help feed their offspring, why the heck do men even have nipples? The answer comes down to timing of sex determination during embryonic development.
Humans are mammals, which means they are warm-blooded, hairy vertebrates that breathe air and produce milk for babies. Up until genes on the Y-chromosome kick in after week 4 in development, however, male and female embryos develop identically. The primary formation of mammary glands and tissues are highly conserved across mammalian species and begin to form early in development, before the gender-specific processes take place.
The embryo’s gonad appears around week 4 of development and is considered bipotential or indifferent, meaning that gender is not playing a role in development at that point. This will continue for a few more weeks. During week 8, germ cells start to undergo sex determination. Males will then secrete factors that block the development of female ducts and structures. Once the male embryo produces testosterone, the hormone can influence other sex-specific traits around the body.
Men having nipples doesn’t really have any evolutionary advantage, but it usually doesn’t hurt anything either. As a result, the trait was never selected against. Developing those structures must also not be very energetically costly in the grand scheme of things. Most of the work with developing breast tissue and mammary gland function in females happens during puberty, while prolactin levels aren’t ramped up until pregnancy.
Despite having a limited amount of underdeveloped breast tissue, men are still capable of getting breast cancer. It is extremely rare for a man to develop breast cancer, and men account for less than 1% of all breast cancer cases, but it can happen. Risk factors include estrogen levels, obesity, alcohol consumption, and liver disease.