Men have nipples because they would look pretty silly without them (think about it, who else finds Action Man dolls strangely creepy?). But if that answer is not enough to satisfy then keep reading.
Men could have their nipples surgically removed and their function on this Earth would be pretty much unaltered (although most women would find them a little strange and probably not want anything to do with them). So why are male nipples still nipples?
To answer this we need to begin with evolution. For something to be selected for in a population it needs to offer some sort of advantage to an individual that enables them to pass their genes to the next generation (i.e. helps them to survive long enough to have babies and makes them attractive to the opposite sex). But as odd as nipple-less men would look, I'm sure they would still manage to find mates. The truth is that nipples give men no real advantage. But importantly they also give no disadvantage. Evolution works both ways - for something to be selected against there needs to be a good reason why it prevents someone from passing ontheir genes. And nipples don’t make any difference either way.
Let’s think a bit more about men and women. All humans are based on the same default genes – such as those which give us our upright skeleton, eyes in the front of our heads, same arrangement of internal organs etc. The differences between males and females can be thought of as modifications of this genetic default state. That means that the certain traits that differ between the sexes have to effectively become uncoupled from the basic blueprint that makes a person.
The differing characteristics of men and women are down to certain genes being located on sex chromosomes that are different between the sexes - females have two X chromosomes, and males have an X and a Y. In addition, whether certain genes are switched on or off will differ depending on whether they find themselves in a female (XX) or male (XY). So the same gene can be present in a man and a woman but how it is switched on can differ between the sexes.
So what does this have to do with male nipples? Well, this uncoupling of males and female traits that I just talked about occurs when it is important to the reproductive success of the sexes to possess different versions of the same trait. You may be aware that the male and female genitals are significantly different (maybe check out a biology text book if not!). Both the male and female versions are vital for reproduction to occur so uncoupling from the normal blueprint occurs in this case. However, this uncoupling does not occur when either the same trait is required by both sexes or if a particular attribute is important to one sex but completely unimportant to the other.
This is the case with nipples. It goes without saying that they are pretty important for females. The fact that they aren't important to men means that this trait has never become separated between the sexes. Why would evolution select for something that has no selective advantage? If it isn’t causing a problem, there is no need to fix it.
During the first few weeks of foetus development in the womb,
the baby will be female and it isn't until about week 8 that the
male Y chromosome kicks into action and starts to change the foetus
to male. By this point, early versions of breasts have been formed
in both male and female babies. Because it doesn't do a man any
harm to have nipples, there is no reason why this would change.
So men have nipples became women do, and evolution just hasn't
bothered getting rid of them.
