What's the purpose of male nipples? This question has probably been asked by every man and woman who has ever lived. Are they vestigial? Did men nurse babies at some point? The very existence of male nipples raises a ton of mystifying questions. But in reality, science is well aware of the numerous reasons for why male nipples exist, and the answers are refreshingly straightforward.
For one thing, humans have a lot of body parts that just don't seem to make a lot of sense. The appendix, pinkie toes, wisdom teeth - they all seem somewhat random. Male nipples aren't all that different, though they are probably the most noticeable bodily outliers. And yes, they are there for more reasons than to just to add some visual interest to the male chest.
Why men have nipples is a fascinating conundrum. The short answer to the question of "why do guys have nipples?" is that all fetuses begin as female, and develop certain female characteristics. The long answer involves testosterone, natural selection, and body chemistry.
In the first four weeks of development in the womb, human embryos follow a sort of genetic and developmental blueprint - essentially, a female one. During those four weeks, a fetus will begin to grow like a female, regardless of what sex and gender it turns out to be.
At week four, the Y chromosome begins to kick in, and the fetus becomes male. Before then, the baby has already started to develop some female characteristics, including nipples.
As a fetus grows in the womb, one of the first things it develops are milk lines that run from the upper torso to the lower abdomen. These milk lines actually develop before either the XX chromosomes or the XY chromosomes kick in.
Once chromosomes are in play, the milk lines begin to recede. The baby is then then left with a noticeable mark where the milk lines once extended to, as well as milk-producing glands called lobules. Females eventually develop breasts. Males just get a pair of nipples.
The presence of testosterone in the womb spurs the fetus's body to shrink those milk lines and breast tissue. The Y chromosome begins to change the genetic activity in a male's cells, particularly in his genitals and brain.
These changes impede any further development in the nipples and mammary papillae (the breasts), and even begin to shrink them somewhat. However, a fetus's body doesn't produce enough testosterone to make nipples go away or shrink to an unnoticeable size. So, all babies are born with nipples, regardless of sex.
It seems like a reasonable assumption that men's nipples are completely non-functioning. But that's not the always the case. Mothers have a hormone called prolactin, which helps them with breast milk development. This hormone can pass from the mom into the baby via the placenta. When this happens, babies' bodies might try to lactate as well. Some newborn boys will begin to leak milk from their nipples, in a phenomenon known as "witch's milk."
This can also happen later in life as well, but in that case male lactation is usually linked with hormone imbalance or illness.