Information culled from about.com says:
"The Darwinian natural selection process would seem to dictate that male nipples really should not be there. So what's the deal? Why do men have nipples?

The answer is that as embryos men and women have similar tissues and body parts. If anything the embryo follows a 'female template'. That is why nipples are present in both sexes. It is the effect of the genes, the Y chromosome and the hormone testosterone that brings about the changes and masculinises the embryo. Testosterone promotes the growth of the penis and testicles. Because nipples are there before this process begins the nipples stay!

Nipples and breast tissue have no function as such except for perhaps protecting the heart and lungs from injury.

Male Breasts Can Produce Milk and Get Breast Cancer
A certain level of the female hormone estrogen is present in all men. If, as a result of disease or a condition affecting hormones, breast tissue in men can grow (gynecomastia- abnormal enlargement of breasts,) and men can produce milk....
Men can get breast cancer as well as women."

A 1979 article found on the straightdope.com website says:
"To tell you the truth, nobody really knows. The best explanation I've been able to find (and frankly it doesn't explain very much) is that nipples aren't a sex-linked characteristic. In other words, nipples are just one of those sexually neutral pieces of equipment, like arms or brains, that humans get regardless of sex.

As you may know, every human being gets a unique set of 23 pairs of chromosomes at conception. These fall into two categories. One pair of chromosomes determines sex--the XX combination means you become female, the XY combination means you become male.

The other 22 pairs, the non-sex chromosomes (they're called autosomes), supply what we might call the standard equipment that all humans get. These 22 pairs constitute an all-purpose genetic blueprint that in effect is programmed for either maleness or femaleness by the sex chromosomes. The programming is done by the hormones secreted by the sex glands.

For example, the autosomes give you a voice box, while the sex hormones determine whether it's going to be a deep male voice or a high female voice. Similarly, the autosomes give you nipples, and the sex hormones determine whether said nipples are going to be functioning (in females) or not (in males)."

A 1999 article found on the salon.com website says:
"To prove they're mammals, obviously. The distinguishing features of mammals, from whales to mice, are two: having hair and suckling their offspring. This gives us the notorious sentence that demonstrates why our pronouns need overhauling: "Man is an animal who suckles his young."

Clearly, if men didn't have nipples, to demonstrate their theoretical membership in the La Leche League, we could only identify them as mammals by their hairiness. And where would that leave bald guys? What are they, reptiles?"

A 2003 article (quoting a professor of biology) found on the sciam.com website explains:
"Like all "why" queries, the question of why men have nipples can be addressed on many levels. My four-year-old daughter, always suspicious of a trick when asked such obvious questions, answered: "because they grow them." In search of the trick answer, she quickly added that "chests would also look pretty funny with just hair...
A human baby inherits one copy of every gene from his or her father and one copy of every gene from his or her mother. Inherited traits of a boy should thus be a combination of traits from both his parents. Thus, from a genetic perspective, the question should be turned around: How can males and females ever diverge if genes from both parents are inherited? We know that consistent differences between males and females (so-called sexual dimorphisms) are common--examples include bird plumage coloration and size dimorphism in insects. The only way such differences can evolve is if the same trait (color, for example) in males and females has become "uncoupled" at the genetic level. This happens if a trait is influenced by different genes in males and females, if it is under control of genes located on sex chromosomes, or if gene expression has evolved to be dependent on context (whether genes find themselves within a male or a female genome). The idea of the shared genetic basis of two traits (in this case in males and females) is known as a genetic correlation, and it is a quantity routinely measured by evolutionary geneticists. The evolutionary default is for males and females to share characters through genetic correlations."

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