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Speaker 1: Deep in the back of your mind. You've always had

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Speaker 1: the feeling that there's something strange about reality. There is

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Speaker 1: supernoid death, much nanopartic, mechanical messiahs, punch evolution. On our

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Speaker 1: award winning science podcast Stuff About Your Mind, we examine

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Speaker 1: neurological quandaries, cosmic mysteries, evolutionary marvels, and our trans human future.

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Speaker 1: New episodes come out Tuesdays and Thursdays on iTunes, Google Play, Spotify,

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Speaker 1: and anywhere you get your podcast. Welcome to brain Stuff

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Speaker 1: from How Stuff Works. Hello there, brain Stuff, it's Christian

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Speaker 1: Sager here. I wanted to talk to you about one

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Speaker 1: way that mice could be considered more highly evolved than humans. Nipples.

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Speaker 1: Whereas human males develop a pair of nipples or sometimes

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Speaker 1: more that generally serve no biological function, Male mice exit

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Speaker 1: the womb with their bellies smooth and nipple free. So

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Speaker 1: why do men have nipples? Come along with me as

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Speaker 1: I explore embryonic development. Three or four weeks after conception,

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Speaker 1: all human embryos developed parallel memory ridges called milk lines.

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Speaker 1: They extend from the top of the chest to the

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Speaker 1: lower abdomen, and at that stage the embryo still has

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Speaker 1: what scientists refer to as in different gonads, meaning they're

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Speaker 1: still capable of developing into either testes or ovaries. It's

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Speaker 1: not until week seven or so that genes in the

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Speaker 1: embryo sex chromosomes, you know, the usual X X or

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Speaker 1: X Y kick in. They're what caused the formation of

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Speaker 1: sexual dimorphisms, the physical traits that distinguished biological males from

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Speaker 1: biological females. But those genes don't tinker with the already

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Speaker 1: developing milk lines. The milk lines recede naturally as the

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Speaker 1: fetus grows, leaving behind nipples. Now in mice, mammary tissue

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Speaker 1: also forms in both male and female embryos during early pregnancy, and,

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Speaker 1: according to Yale University research first published in a particular

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Speaker 1: ular protein stops the process in male mice. Just a

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Speaker 1: few days after the mammary tissue starts to form, it

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Speaker 1: produces a protein known as parathyroid hormone related peptide or

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Speaker 1: pt r P. In male embryos, this protein signals the

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Speaker 1: mammary cells to form hormone receptors that attract the male

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Speaker 1: hormones already circulating in the embryo's blood. Those hormones shut

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Speaker 1: down the mammary growth process and degenerate what tissue had

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Speaker 1: already formed, leaving the dude rodents. Nippleis by birth. Mice

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Speaker 1: are among an elite group of mammals with such efficient

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Speaker 1: male nipple destroying genes. Horses and platypuses are in there too.

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Speaker 1: Human males lacking any such system are left with tissue

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Speaker 1: that's even capable of producing milk under the right circumstances.

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

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Speaker 1: and healthy breast development are closely linked with female reproductive success,

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Speaker 1: So closely linked that biologist figure it was more evolutionary

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Speaker 1: advantageous for all embryos to develop breast tissue, whether they'd

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Speaker 1: wind up using it or not. Check out the brainstuff

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Speaker 1: channel on YouTube, and for more on this and thousands

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Speaker 1: of other topics, visit how stuff works dot com