In the anatomy of female mammals, a nipple or mammary papilla is a small projection of skin containing the outlets for 15-20 lactiferous (milk bearing) ducts arranged cylindrically around the tip.

The skin of the succulant nipple is rich in a supply of special nerves that are sensitive to certain stimuli.

The main purpose of nipples is to deliver to the infant milk produced in the female mammary glands during lactation. In the male, nipples are often not considered functional with regard to breastfeeding, although male lactation is possible. Though men have nipples, few know that they also have mammary glands (and can, in fact, get breast cancer, 1 to 100,000). Normally, there is so little mammary tissue that it is unnoticeable; if the male breasts develop visibly, the condition is called gynecomastia.

Nipples stay to remind us that gender is anything but clear-cut, especially in utero: we do not have gender inside the mother's womb, just our chromosomes bear it. For the first several weeks, a developing embryo follows a "female blueprint," from reproductive organs to nipples.

Some scientists believe that female sex is the ancestral one, that's why this pattern emerges. Only after about 60 days does the testosterone kick in (for those of us with a Y chromosome), changing the genetic activity of cells in the genitals and brain. But by then those mammary papillae aren't going anywhere. In fact, they are dormant till puberty, when hormonal boosts determine the development of the female nipple.

So the question is: why do male nipples come equipped with nerves and blood vessels?

Some studies even showed that about 25 % of men possesses nipples sensitive to touch ...In many male mammals nipple formation is stunted by hormones, but not in humans.

Did prehistoric men nurse their young? Rather not. Perhaps, as full-grown nipples do not bother in any way the survival or reproduction of the individual, natural selection has just not eliminated them.