While there is a reasonable biological explanation as to why women have nipples—to feed babies—their function in men remains less clear.

The Darwinian theory of natural selection would seem to dictate that male nipples serve no real purpose and, as such, should have been bred out of the human species by now. Of course, they have not, and this has to do with the foundations of how a human begins developing in utero.

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Fetal Development

The answer is simpler than you think. During embryogenesis (the development of an embryo after fertilization), females and males will both start from the same genetic base, so to speak.

It is only in the latter part of the first eight weeks that the sex genes—called the X and Y chromosome—will dictate whether the baby will be female or male. The Y chromosome is the one that differentiates males (who will have one X and one Y chromosome) from females (who will have two X chromosomes).

During the first four to five weeks of gestation, there is no differentiation between sexes even as the embryonic cells continue to divide and specialize. At this point, nipples are already developing.

It is only by weeks six or seven that the Y chromosome will induce changes (by way of the SRY gene) that lead to the development of the testes and the male sex.﻿﻿

By contrast, female embryos, which are not under the influence of the Y chromosome, will undergo changes in the mammary cells, starting with the development of a pit at the center of each nipple. This pit will gradually form a depression that connects to a lactiferous (milk-producing) duct.

While this happens to some degree in males, it is far less profound and developed.

Function of the Male Nipple

While male nipples are sometimes thought to be vestigial—meaning that they have become functionless in the course of evolution (much like the appendix or wisdom teeth)—that is largely untrue. They may be more accurately described as remnants of fetal development, but even that suggests they serve no real purpose.

The nipple, in fact, contains a dense supply of nerves that function as a major stimulatory organ in both men and women.﻿﻿ As such, it can be considered a secondary sex characteristic along with pubic hair, enlarged breasts and widened hips in women, and facial hair and Adam's apple in men.

The male nipple is no less sensitive than the female nipple and can contribute significantly to sexual arousal when stimulated.

With that being said, the nerve network in the male nipple is much denser, meaning that sensory response tends to be more discreet. This response in men and women appears unique to the human species.

Male Nipple Abnormalities

There are characteristics associated with the female breast and nipple that can abnormally occur in the male breast and nipple as well. Some are the result of the dysregulation of hormones, while others may be triggered by genetics.

Galactorrhea

While men will not lactate under usual circumstances, the male breast can produce milk if under the influence of the hormone prolactin.

The condition, known as male galactorrhea, often occurs as a result of a medication or medical condition that triggers a drop in male hormones (primarily testosterone) and an associative increase in female hormones.﻿﻿

One such example is the drug Motilium (domperidone), which not only treats lactation problems in women but can be used in men to treat nausea, vomiting, gastroparesis, and Parkinson's disease.

Other causes include malnutrition, pituitary gland disorders, hypothyroidism (low thyroid function), and frequent nipple stimulation.

Gynecomastia

Gynecomastia is the enlargement of male breasts that commonly occurs in older men as testosterone levels progressively drop with age. In addition to the general swelling of breast tissue, gynecomastia can trigger the enlargement of the nipples and surrounding areolas.

Gynecomastia can also affect boys and younger men for any number of reasons. In some cases, the condition will be temporary, particularly in adolescent boys undergoing puberty.

Other common causes include:

Adrenal or pituitary gland tumor

Anabolic steroid use

Cancer chemotherapy

Hypothyroidism

Kidney failure

Prostate medications like Propecia (finasteride) and Aldactone (spironolactone)

Tricyclic antidepressants

Gynecomastia is associated with an increased risk of breast cancer in men. While rare, breast cancer in men is most commonly detected by the formation of a hardened lump under the nipple and areola.﻿﻿