Why Do Men Have Nipples?

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.

  • Males Originally Follow A Female Blueprint

    Males Originally Follow A Female Blueprint
    Photo: Andrzej Skawina/Antoni Marsinek MD / Wikimedia Commons / CC BY-SA 3.0

    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.

  • Males Develop Milk Lines Before Chromosomes Kick In

    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.

  • Testosterone In The Womb Makes Nipples Smaller

    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.

  • Some Newborn Boys Lactate

    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.

  • Men Can Sometimes Breastfeed

    For a while, scientists and historians theorized that early human males nursed their young just as often as human females, and that's why human males have nipples. While this idea has been widely rejected, there are communities where males do nurse. An African pygmy tribe, the Aka, has documented cases of males nursing babies. Men have also been known to begin nursing their children after their wife has died. 

    However, these cases are rare, because male nipples are typically not equipped to create milk and support a baby. But cuddling or spending time with a baby has been shown to increase the amount of prolactin males produce while cutting down on their testosterone. This allows males to develop their nipples into effective breasts.

  • Men Are More Likely to Have Extra Nipples

    Humans can be born with extra nipples, and it tends to happen more often to males than females. By some counts, the rate of increased nipple growth is one in every forty newborn babies. These bonus nipples often show up on the left side of male bodies, and tend to be small and underdeveloped. They are usually harmless, and can be easily removed.