Why do Men Have Nipples?

All humans begin life in the womb as females. If no Y chromosome is present in the foetus, then the embryo will continue to develop as and be born as a female. The presence of nipples on males seems strange and given that males do not produce any milk. Are these Remnants of evolution? There are a couple of good reasons men have nipples. One is related to sexual stimulation, and the other is simple efficiency.

Humans are mammals. They are warm-blooded, hairy vertebrates that breathe air and produce milk for babies. Until genes on the Y-chromosome kick in after week 4 in development, both 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 men to develop breast cancer, and men account for less than 1% of all breast cancer cases, but still it can happen. Risk factors include oestrogen levels, obesity, alcohol consumption, and liver disease.