Welcome to Brainstuff production of I Heart Radio. Hey brain Stuff, Lauren foke obamb here, I want 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 develop parallel mammary ridges called milk lines. They extend from the top of the chest to the lower abdomen. At that stage, the embryo still has what scientists referred to as indifferent gonads, meaning they're still capable of developing either into testes or ovaries. It's not until week seven or so that genes and the embryos sex chromosomes kick in, you know, usually xx
or x Y, but there's a bunch of variations. They're what caused the formation of sexual dimorphisms, the physical traits that distinguished the typical biological male from the typical biological female. Again, there are variations, but pertinent to our discussion today, those
genes don't tinker with the already developing milk lines. The milk lines recede naturally as the fetus grows, leaving behind nipples and milk producing glands called lobules and ducks, and some fatty tissue in between in both males and females. Now in mice, memmory tissue also forms in both male and female embryos during early pregnancy, but according to Yale University research first published in a particular 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 RP. 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 has already formed, leaving the hey rodents nipple lists by the time of birth. Mice are among an elite group of mammals with such efficient male nipple des
drawing 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 biologists figure it was more evolutionarily advantageous for all embryos to develop breast tissue, whether they'd
wind up using it or not. Today's episode was written by me and produced by Tyler Clang. Brain Stuff is a production of iHeart Radio's How Stuff Works from are in this a bunch of other curious topics. Visit our home planet, how stuff Works dot com, and for more podcasts for my heart Radio, visit the iHeart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.