" " The simplest explanation for men having nipples is that all human embryos start out with them. Asia Images/Getty Images

Male mice could be considered more highly evolved, so to speak, than human males in one anatomical regard: nipples. Whereas men develop a pair of nipples — and sometimes more — that serves little purpose other than offering topographical variety on the chest, male mice exit the womb with no such markings, their furry bellies smooth and nipple-free. Alongside stallions and male platypuses, mice are among a small cluster of mammalian species that don't go ahead and sprout nipples as though in symbolic solidarity with their female kin [source: McCarthy].

During early pregnancy, both male and female mice embryos form mammary tissue, the foundational tissue that becomes the full-fledged nipples, nerves and glands that later facilitate female milk production. In 1999, Yale University researchers published a study identifying the protein responsible for lobbing off those nipple precursors in male mice. A few days after mice mammary tissue starts to form, it produces a protein known as PTHrP. In male mice, PTHrP signals the mammary cells to form male hormone receptors, and those hormones effectively destroy the tissue, leaving the he-rodents nipple-less [source: Lawrence].



Male humans meanwhile undergo a similar embryonic process, minus the mammary tissue takedown. After three or four weeks, all embryos develop parallel mammary ridges called milk lines before the 23rd chromosomes, XX or XY, have a chance to tinker with sexual dimorphisms, or physical traits that distinguish biological males from females. As fetuses grow, those milk lines that extend from the top of the chest to the lower abdomen recede and typically leave behind a pair of nipples, along with milk-producing glands called lobules, ducts and the fatty tissue in between [source: Adams]. Without a protein like PTHrP stepping into action to trigger hormonal roadblocks as it does in mice, human embryos, male and female alike, are given the same set of internal plumbing in the breast region. Then, during puberty, estrogen in girls spurs breast tissue and mammary gland development.

Therefore, the simplest explanation for men having nipples is that all human embryos start out with them, and evolution didn't get around to selecting against their existence on the male bust. Since nipples and healthy breast development are so closely linked with female reproductive success, evolutionary biologists suppose that the adaptive pressure to wean out male nipples wasn't strong enough [source: Simons].

And though the purpose of male nipples rarely extends beyond the decorative, to lump them with the appendix, wisdom teeth and other vestigial anatomy would be a technical mistake. How could, after all, men's nipples be relegated to the evolutionary rubbish bin when they can -- oh, yes, they can — produce milk?