Today, we bring you a tale of mice and men. And their nipples.
What are nipples on men for? To understand that, you've got to first know that are the protrusions on mammary glands and serve as a way to deliver breast milk produced in a mother's body to her young.
Human males develop a pair of nipples while they are still in the womb, but male mice are born nipple-free. As our BrainStuff host Cristen Conger explains in the video above, Yale University researchers found that a specific protein halts nipple development in male mice just a few days after mammary tissue beings to form in utero. The protein is parathyroid hormone-related peptide (PTHrP), and it tells the mammary cells to form hormone receptors that attract the male hormones already circulating in the embryo's blood.
These hormones stop the growth of mammary tissue, even going so far as to degenerate what tissue is already formed. The result? Male mice are born sans nipples. Male horses and platypuses also are born without nipples, making this "male mammal no nipple club" pretty darn exclusive.
Male humans certainly aren't part of the club. About three to four weeks after conception, all human embryos have mammary ridges that run parallel on each side of the body, from the top of the chest to the lower abdomen. Right around week seven, sex hormones — XX or XY — determine whether the growing embryo will be male or female, a distinction known as sexual dimorphism.
Despite these sex differentiation genes, the already-developing milk lines (think of the line of nipple along the underside of a dog's body, and that's the milk line) remain largely unaffected. Although the milk lines recede as the embryo grows, they do not distinguish one sex from another and, in the end, leave behind nipples on male babies, too.
And here's where we get to the "what are they for" of the matter. Nipples and healthy breast development are so closely linked to the female reproductive process that evolutionary biologists believe it was advantageous for all embryos to develop breast tissue — whether they needed it or not. Instead of a "why," this is more of a "why not?"