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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. 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. 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
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 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 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 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, 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