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If men can't nurse, then why do they have nipples? The answer has less to do with evolution and more to do with your personal development as a teeny tiny embryo. Short version: We're all girls -- at least at first. Hank explains!

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Sources:
http://www.scientificamerican.com/article.cfm?id=why-do-men-have-nipples
http://science.howstuffworks.com/life/human-biology/why-men-have-nipples.htm
http://www.scientificamerican.com/article.cfm?id=strange-but-true-males-can-lactate
http://www.livescience.com/32467-why-do-men-have-nipples.html
http://www.huffingtonpost.com/2012/09/04/why-do-men-have-nipples_n_1847460.html
http://ghr.nlm.nih.gov/gene/SRY
http://www.nature.com/news/1999/990805/full/news990805-1.html
http://www.salon.com/1999/06/08/nipples/
http://www.emouseatlas.org/emap/home.html
Nipples. We all got 'em. Obviously, ladies need them to breast feed babies, but why do us guys have them taking up valuable chest space? So that we can have more body piercing options? So that we can experience the special torture that is chafing while jogging? or are they just there to taunt hungry infants? 

Turns out, dudes have nipples because way back when we were tiny little embryos, we were actually girls. Sort of. You heard me, tough guy.

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You know our modern human bodies have a number of vestigial features like wisdom teeth that we don't need, or useless tailbones, but males nipples are different. They aren't left over from some evolutionary event in the past where males used to breastfeed. They're artifacts of our own personal development.

In the womb, mammals go through a series of developmental stages as they grow from an eye twinkle to a screaming infant. The process goes: zygote, embryo, fetus, red-faced baby. Most babies are usually male or female. Girls with 2 X chromosomes, and boys with an X and a Y. As embryos we start out following a female blueprint, before our hormones get serious about sex. So our nipples actually develop before our sex is determined.

Within the first several weeks, a pair of milk ridges form on every embryo, a thickening of the epidermis that runs from armpit to thigh.  Eventually, these structures pull back to form two nipples, although a surprising number of people, mostly guys, end up with an extra supernumerary nipple, somewhat along the original milk line, even if they uh--think it's probably just a mole.  It's not a mole.  

Shortly after these nipples form, in babies of the Y chromosome, a special sex determining gene switches on and declares the embryo officially male, kick-starting the development of male hormones and eventually, male anatomy.  But we get to keep the nipples.  

Interestingly enough, not all male mammals have nipples, take mice for example.  In early embryonic development, both males and females form mammary tissue, but Yale University researchers found that a few days after that tissue starts to form, a special protein called PTHRP gets produced signalling different results in males and females.  In female mice, the protein stimulates those mammary buds to grow into nipples and all the requisite milk-producing plumbing that comes with them, but in males, it not only stops further mammary development, it basically changes those cells into special receptors for the flood of male hormones that also starts circulating, so the mammary cells don't just stop developing, they actually degenerate, and when a male mouse is born, it has no trace of a nipple.

Of course, all that changes during puberty when those estrogen waves start to stimulate mammary gland and breast tissue development in bio-sex female.  Okay, but still, if they're so useless, why do I have them?  

Well, the separation of male and female traits in a species, or uncoupling, tends to happen only if there's a good reason for it in terms of reproductive success.  Evolutionarily speaking, female breasts are essential to the survival of the species, male nipples aren't, but why mess with something that's so important to one sex and merely benign to another?  So, dudes don't have nipples because those features have been selected for in males, but rather because simply they haven't been selected against.  Not all traits have adaptive explanations.  In the end, male nipples are just a genetic by-product of female breasts, so maybe someday in the distant future, my buddies and I will all look like Ken dolls, but I'm not holding my breath.  Frankly, I kinda like 'em, it's like, uh, it's like a pec cap for my pecs.  It's a little hat.  

Thanks for watching this SciShow Dose, if you have any questions, comments, or ideas for us, we're on Facebook and Twitter and down in the comments below, and don't forget to go YouTube.com/SciShow and subscribe.  

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