Why do Men have Nipples?
Human body has small projections, two to be precise, one on each breast. Nipples play an important role in the anatomy of a woman as it produces milk for babies. But, nipples do not perform any function in the male anatomy, it is redundant. According to the theory of evolution, where changes occur in the species through time only due to the environmental need for survival, men should not have nipples as they are purposeless in their bodies but the fact is that they do; each and every one of them. Often, this question has been asked and discussed at various levels.
Despite of the sex of the embryo in the womb, it has similar tissues and body parts. In fact, the embryo follows a female anatomy as default. The sex of the embryo is determined by their genes. The Y chromosome and hormone testosterone causes the changes an embryo into a male baby. The growth of the penis and testicles is caused by testosterone. Since, nipples are already formed before these changes occur due to the default genetic template of a female and the male embryos already have nipples and male babies are born with it. There are no functions for the nipples and breast tissues in the male anatomy except for maybe protecting the heart and lungs.
In the womb, after three to four weeks of conception, all embryos develop mammary ridges called milk lines. Up till that time, the twenty third chromosomes, XX or XY has not initiated the development of the reproductive system which differentiates between male and female. As the embryo grows, those milk lies extending from top of the chest to lower abdomen regress and leave behind a pair of nipples, milk producing glands called lobule and fatty tissues. The breast in female bodies grow during puberty due to estrogen while in male remains so due to minimal amounts of estrogen.
Male body contains a certain level of the female hormone estrogen. As a result, hormonal imbalance or disease can cause the breast tissue to grow in the male body and produce milk. This condition is called gynecomastia. It usually occurs in adolescence as there is a lot of fluctuation in hormones. It can also occur in men suffering with liver disease and alcoholism.
The topic of men having nipples often stirs up a discussion about evolution. It is the job of an evolutionary biologist to explain such anomalies. There are, sometimes, traits that make perfect evolutionary explanation but are not adaptive to the environment while others have adaptive explanations but does not make evolutionary sense. Evolution is defined as a process of gradual change in the feature and characteristic of specie influenced by multiple factors such as heredity, history, coincidence, environment etc. to ensure survival. Due to such factors, the anatomy of organisms does not appear as they would if designed entirely from scratch with no influences from external factors. Evolutionary biologist contributes the presence of nipples in men to this theory.
An embryo receives genes from both of its parent ‘“ a male and a female. Fifty percent of the genetic material is handed down from the father and fifty percent from the mother. Hence, the genetic material of the embryo is a combination of its parent. And it is genes that dictate the development of the embryo into a baby. Each embryo has half male and half female genes so it is interesting as in how the two distinctively different sexes emerge from the same combination of genes. The difference in male and female occur as genes for certain trait becomes uncoupled at the genetic level. This occurs when a trait is influenced by different genes in the two sexes or when it is controlled by genes located in sex chromosomes.
The sharing of genetic material of traits in males and females is called genetic correlation. According to evolution, there are default traits that are to be shared between the two sexes through genetic correlations. Uncoupling is not expected when an attribute is important to both sexes or if it is important to one but not the other. Nipples are the example of the latter. The function of nipples in female is obvious; nursing of the babies. But there is no apparent function in male body. However, due to the genetic correlation, males and females share characteristics such as nipples. They are best explained by lack of selection against them rather than selection for them.
According to evolutionary biologist, it is unwise to assume that every trait and feature in human body as an adaptive explanation. Consider an analogy of the spandrels of St Mark’s cathedral. The spandrels are merely an architectural consequence of joining of a vaulted ceiling with support pillars, nipples in men are a genetic consequence of nipples in females.
Conger, Cristen (N.D.), Why Do Men Have Nipples?, Retrieved from http://health.howstuffworks.com/human-body/parts/why-men-have-nipples.htm
Kennard, Jerry (2014), Why Do Men Have Nipples, Retrieved from http://menshealth.about.com/od/conditions/a/Nipples_Men.htm
Scientific American (2003), Why Do Men Have Nipples, Retrieved from http://www.scientificamerican.com/article/why-do-men-have-nipples/