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Why do men have nipples if they can't breastfeed?

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📓 The short answer

Men have nipples because all embryos start developing the same way, and nipples form before sex-specific genes activate. They likely remain simply because there’s no evolutionary pressure to remove a harmless trait.

📚 The long answer

Great thinkers like Darwin, Aristotle, and curious children have pondered the question: Why do men have nipples if they can't breastfeed?

It all starts with how human embryos develop in the womb. For the first six weeks of gestation, male and female embryos develop identically, with no sex-specific traits. Around six weeks, sex-specific genes found in the X or Y chromosome flip on and put embryos on the path to being male or female.

Where do nipples come in? Well at around 5.5 weeks — just before the embryo's sex genes activate — a pair of milk or mammary ridges form on the skin extending from the armpit to the thigh. These milk ridges will eventually recede to leave (*usually) two nipples in both male and female embryos.

In short, nipples start forming in human embryos before sex-specific genes are activated. After that point, females will develop female traits, while males will develop male traits. Both sexes keep breast tissue and nipples thanks to this early milk ridge formation.

Now we know how nipples develop in male humans. But why hasn't evolution removed them if they seemingly serve no purpose?

Theory #1: There's no harm in males keeping their nipples.

Some argue that male nipples remain not because they have an evolutionary advantage, but simply because they have no evolutionary disadvantage. When a trait is useful in one sex, as nipples are for nursing in females, and harmless in the other sex, there's no benefit in removing them.

Nipples aren't even the only part of male anatomy that develops by default in the embryo before sex differences kick in. The prostate contains a tiny, nonfunctional structure called the prostatic utricle. It is a remnant of embryonic Müllerian ducts, which develop into the uterus and fallopian tubes in females.

Theory #2: Nipples contribute to the desire to reproduce in males.

It's also possible that nipples do serve an evolutionary advantage in male humans. Nipples contain a dense network of nerves, making them a highly stimulatory organ. Some researchers think their role in sexual pleasure helps boost the desire to reproduce.

Theory #3: Nipples allow for the possibility of male lactation.

While exceedingly rare, some male mammals are capable of lactation. The dayak fruit bat, a very rare bat species found in southeast Asia, can produce milk in both male and female bats. The exact evolutionary advantage is unclear, but dad bat may take the pressure off of mom bat from feeding the baby bat all the time.

Interestingly, human males are technically capable of lactation if there is a medication or medical condition that triggers an increase of the hormone prolactin. In these rare cases, males can produce milk. So, while far fetched, it's possible nipples haven't disappeared in males because it leaves the door open for male lactation.

🧠 Bonus brain points

Can you be born with extra nipples?

Yes, around one in 20 people have what's called supernumerary nipples. These extra nipples are leftovers from mammary crest development, rarely functional, and often appear to look like nothing more than a small mole.

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

🌐 Wikipedia article of the week

​Public Universal Friend​

"The Public Universal Friend (born Jemima Wilkinson; November 29, 1752 – July 1, 1819) was an American preacher born in Cumberland, Rhode Island, to Quaker parents. After suffering a severe illness in 1776, the Friend claimed to have died and been reanimated as a genderless evangelist named the Public Universal Friend, and afterward shunned both birth name and all pronouns. In androgynous clothes, the Friend preached throughout the northeastern United States, attracting many followers who became the Society of Universal Friends."

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