Why Do Men Have Nipples?

Men don’t usually lactate – but they can! Christian explores how breast tissue develops in human embryos of both sexes, and explains why mice are more advanced, evolution-wise.

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Episode Transcript

Speaker 1 (00:00):
Deep in the back of your mind. You've always had
the feeling that there's something strange about reality. There is
supernoid death, much nanopartic, mechanical messiahs, punch evolution. On our
award winning science podcast Stuff About Your Mind, we examine
neurological quandaries, cosmic mysteries, evolutionary marvels, and our trans human future.
New episodes come out Tuesdays and Thursdays on iTunes, Google Play, Spotify,

(00:22):
and anywhere you get your podcast. Welcome to brain Stuff
from How Stuff Works. Hello there, brain Stuff, it's Christian
Sager here. I wanted to talk to you about one
way that mice could be considered more highly evolved than humans. Nipples.

(00:45):
Whereas human males develop a pair of nipples or sometimes
more that generally serve no biological function, Male mice exit
the womb with their bellies smooth and nipple free. So
why do men have nipples? Come along with me as
I explore embryonic development. Three or four weeks after conception,
all human embryos developed parallel memory ridges called milk lines.

(01:08):
They extend from the top of the chest to the
lower abdomen, and at that stage the embryo still has
what scientists refer to as in different gonads, meaning they're
still capable of developing into either testes or ovaries. It's
not until week seven or so that genes in the
embryo sex chromosomes, you know, the usual X X or

(01:28):
X Y kick in. They're what caused the formation of
sexual dimorphisms, the physical traits that distinguished biological males from
biological females. But those genes don't tinker with the already
developing milk lines. The milk lines recede naturally as the
fetus grows, leaving behind nipples. Now in mice, mammary tissue

(01:50):
also forms in both male and female embryos during early pregnancy, and,
according to Yale University research first published in a particular
ular protein stops the process in male mice. Just a
few days after the mammary tissue starts to form, it
produces a protein known as parathyroid hormone related peptide or

(02:11):
pt r P. In male embryos, this protein signals the
mammary cells to form hormone receptors that attract the male
hormones already circulating in the embryo's blood. Those hormones shut
down the mammary growth process and degenerate what tissue had
already formed, leaving the dude rodents. Nippleis by birth. Mice

(02:32):
are among an elite group of mammals with such efficient
male nipple destroying genes. Horses and platypuses are in there too.
Human males lacking any such system are left with tissue
that's even capable of producing milk under the right circumstances.
So that's how men have nipples. But why well? Nipples
and healthy breast development are closely linked with female reproductive success,

(02:57):
So closely linked that biologist figure it was more evolutionary
advantageous for all embryos to develop breast tissue, whether they'd
wind up using it or not. Check out the brainstuff
channel on YouTube, and for more on this and thousands
of other topics, visit how stuff works dot com