WEBVTT - Why Do Men Have Nipples?

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<v Speaker 1>Deep in the back of your mind. You've always had

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<v Speaker 1>the feeling that there's something strange about reality. There is

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<v Speaker 1>supernoid death, much nanopartic, mechanical messiahs, punch evolution. On our

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<v Speaker 1>award winning science podcast Stuff About Your Mind, we examine

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<v Speaker 1>neurological quandaries, cosmic mysteries, evolutionary marvels, and our trans human future.

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<v Speaker 1>New episodes come out Tuesdays and Thursdays on iTunes, Google Play, Spotify,

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<v Speaker 1>and anywhere you get your podcast. Welcome to brain Stuff

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<v Speaker 1>from How Stuff Works. Hello there, brain Stuff, it's Christian

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<v Speaker 1>Sager here. I wanted to talk to you about one

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<v Speaker 1>way that mice could be considered more highly evolved than humans. Nipples.

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<v Speaker 1>Whereas human males develop a pair of nipples or sometimes

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<v Speaker 1>more that generally serve no biological function, Male mice exit

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<v Speaker 1>the womb with their bellies smooth and nipple free. So

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<v Speaker 1>why do men have nipples? Come along with me as

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<v Speaker 1>I explore embryonic development. Three or four weeks after conception,

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<v Speaker 1>all human embryos developed parallel memory ridges called milk lines.

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<v Speaker 1>They extend from the top of the chest to the

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<v Speaker 1>lower abdomen, and at that stage the embryo still has

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<v Speaker 1>what scientists refer to as in different gonads, meaning they're

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<v Speaker 1>still capable of developing into either testes or ovaries. It's

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<v Speaker 1>not until week seven or so that genes in the

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<v Speaker 1>embryo sex chromosomes, you know, the usual X X or

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<v Speaker 1>X Y kick in. They're what caused the formation of

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<v Speaker 1>sexual dimorphisms, the physical traits that distinguished biological males from

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<v Speaker 1>biological females. But those genes don't tinker with the already

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<v Speaker 1>developing milk lines. The milk lines recede naturally as the

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<v Speaker 1>fetus grows, leaving behind nipples. Now in mice, mammary tissue

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<v Speaker 1>also forms in both male and female embryos during early pregnancy, and,

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<v Speaker 1>according to Yale University research first published in a particular

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<v Speaker 1>ular protein stops the process in male mice. Just a

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<v Speaker 1>few days after the mammary tissue starts to form, it

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<v Speaker 1>produces a protein known as parathyroid hormone related peptide or

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<v Speaker 1>pt r P. In male embryos, this protein signals the

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<v Speaker 1>mammary cells to form hormone receptors that attract the male

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<v Speaker 1>hormones already circulating in the embryo's blood. Those hormones shut

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<v Speaker 1>down the mammary growth process and degenerate what tissue had

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<v Speaker 1>already formed, leaving the dude rodents. Nippleis by birth. Mice

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<v Speaker 1>are among an elite group of mammals with such efficient

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<v Speaker 1>male nipple destroying genes. Horses and platypuses are in there too.

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<v Speaker 1>Human males lacking any such system are left with tissue

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<v Speaker 1>that's even capable of producing milk under the right circumstances.

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<v Speaker 1>So that's how men have nipples. But why well? Nipples

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<v Speaker 1>and healthy breast development are closely linked with female reproductive success,

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<v Speaker 1>So closely linked that biologist figure it was more evolutionary

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<v Speaker 1>advantageous for all embryos to develop breast tissue, whether they'd

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<v Speaker 1>wind up using it or not. Check out the brainstuff

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<v Speaker 1>channel on YouTube, and for more on this and thousands

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<v Speaker 1>of other topics, visit how stuff works dot com