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.
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.
Not All Male Mammals Have Nipples
Humans are mammals, and they have nipples - but that's not the case for all of their hairy, warm-blooded brethren. Horses, mice, platypuses, and some other animals are born nipple-free. These animals begin to grow nipples while in the womb, but partway through development the nipples shrink or disappear entirely.
Human Males Lack A Key Nipple-Erasing Protein
In 1999, Yale University published a research study on male mice, one type of animal that does not grow nipples. According to the study, a few days after mammary tissue has started to form in mice embryos, the mouse body produces a protein called PTHrP. This chemical acts as a sort of roadblock to female hormones, and signals the male hormones to go into overdrive, especially in the mammary cells. The male hormones erase the mammary tissue and milk line remnants, leaving male mice nipple-free.
Male humans, on the other hand, do not at have PTHrP in their bodies during development. Their nipples remain.