This is a question that pops up time to time in my lessons, especially when we are learning body parts. 

A simple answer is that men have nipples because nipples develop in the womb before embryos become distinctly male or female. So by the time a Y chromosome kicks in to distinguish a fetus as male, the nipples have already secured their place.

🤷‍♀️You might wonder-why hasn’t evolution selected against this trait? It’s not necessary for survival, so why hasn’t it been eliminated? If you think about it, we have many nonessential traits (like wisdom teeth) that are just left over from our development as a species. (Apparently such traits are called vestigial, meaning we still have them because they’re not a priority for evolution to select against.)

🙎‍♂️It’s not like male nipples are hurting anyone, so it’s no big deal for evolution to simply leave them be. They are also an erogenous zone 💖

😊With regards to lactation ➡️For transgender men, possible steps for physical transition can include surgery, taking hormones, or nothing at all.

🥛 So, depending on the physical and hormonal changes that have taken place, lactation can happen just like it does for cisgender women. But even cisgender men can lactate if a particular hormone, called prolactin, takes effect.
It’s a condition known as male galactorrhea.
It’s usually the result of:
* medication
* malnutrition
* a health condition like an overactive thyroid

Super interesting, huh?!

(Info mainly from healthline.com)