Nipple, Nipple

The skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli. The physiological purpose of nipples is to deliver milk to the infant, 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. Mammalian infants have a rooting instinct (moving their head so as to bring their mouth towards whatever is touching their face) for seeking the nipple, and a suck instinct for extracting milk.

Rather than requiring a sucking action, the discharge of milk is powered by maternal muscles. The calf takes the extended nipple into its mouth, and the mother ejects or expels her milk into the mouth of the calf.

Sexual arousal may also cause the nipples to become erect. The nipple and areola of males and females can be erotic receptors, sometimes intense enough to elicit orgasm in some individuals of either sex. They are not, however, sex organs because since mammals of both sexes all have nipples, nipples don't possess the defining quality of a sex organ of differentiating the sex of any animal.

M. Hussain, L. Rynn, C. Riordan and P. J. Regan, Nipple-areola reconstruction: outcome assessment; European Journal of Plastic Surgery, Vol. 26, Num. 7, December, 2003 . Pregnancy and nursing tend to increase nipple size, sometimes permanently. Pregnancy also increases the pigmentation. The erection of the nipple is partially due to the cylindrically arranged muscle cells found within it. In many women, there are small bulges on the areola, which are called 'Montgomery bodies'. The nipple is innervated by the 4th intercostal nerve in humans.

This is best said by http://www.scientificamerican.com/article.cfm?id=why-do-men-have-nipples : The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them. Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them. In a sense, male nipples are analogous to vestigial structures such as the remnants of useless pelvic bones in whales: if they did much harm, they would have disappeared.

Source: Wikipedia > Nipple