Why do men have nipples? Because losing them offered no advantage
Almost every mammal has nipples, including males, who cannot use them to feed young. Biologists broadly agree the reason is anticlimactic: nothing was gained by getting rid of them, so evolution never did. Some call the male nipple a spandrel, a by-product rather than an adaptation with a purpose.
A nipple is a small projection of skin carrying the openings of 15 to 20 milk ducts arranged in a ring around its tip, set within the usually darker areola. Across marsupials and placental mammals, nipples typically come in bilateral pairs, anywhere from 2 to 19 of them. In animals the term teat is common, and the same word covers the rubber mouthpiece of a baby's bottle. Etymologically, nipple probably began as a diminutive of the Old English neb, meaning beak, nose or face, while teat and tit descend from a shared Germanic root, one arriving through Old French and the other directly.
The skin here is densely wired with four kinds of touch receptors, some adapting slowly to steady pressure and others rapidly to change, including Merkel, Ruffini, Meissner and Pacinian endings. Most of its nerve supply comes from branches of the fourth intercostal nerve, and anatomists use it as a landmark for the T4 dermatome, roughly level with the diaphragm. About 75 percent of lymph from the nipple and breast drains to nodes near the armpit. In both sexes the nipple responds to sexual stimulation, and many cultures sexualise the female nipple and set rules about covering it.
Its main job is delivering milk. When an infant suckles, signals prompt the hypothalamus to release oxytocin, which squeezes small muscles around the ducts and pushes milk forward in what is called the let-down reflex. The same hormone makes the uterus contract after birth, clamping its arteries and helping prevent heavy bleeding. Supply follows demand: the more the baby nurses, the more milk the glands make. A shallow latch near the tip gives weak stimulation and is a common cause of sore, cracked nipples, which lead many women to stop breastfeeding.
Nipples change naturally across a lifetime, and the average female nipple projects a little more than 9.5 millimetres. Clinicians nonetheless pay attention to some changes, such as a newly inverted or puckered nipple or scaly skin, because they can occasionally reflect disease underneath, including a rare skin cancer called Paget's disease.
Source: Nipple
