Why do men have nipples?

[phenylalanine]

Why do men have nipples?

[nautica]

They are only modified sweat glands. Some people can actually see the other 4. Look down your chest in line with your nipples and you might be able to spot the other 4. They probably look like freckels.

Nautica

[Eppy]

Nautica is right about them being glorified sweat glands. If you're wondering why both men and women have them, both have mammary glands due to being mammals--though they are generally vestigal in males. It is physiologically possible for male humans to lactate, and apparently children have been breast-fed this way. (Here's a brief explanation of male lactation.) (http://en.wikipedia.org/wiki/Male_lactation) I found out about this rather odd phenomenon browsing around just now, I didn't know male lactation was possible before this. Someone knowledgable about the development of humans in utero could tell us what happens as far as developmental divergence between the sexes and what causes the mammary glands to develop in females but not males.

[Brennen]

i believe that gender is undecided in a developing foetus until after the basic body formation has occured, complete with, you guessed it, nipples. so a foetus will have nipples before it is certain to be male or female. im pretty sure thats why...

[Moonbear]

Brennen, that may not be the case. That has been the thinking for a long time, that the sexes develop identically until a critical stage during gestation when males produce a small testosterone surge that triggers differentiation into a male, and females are sort of the "default." However, some very recent work suggests this may be incorrect, and that there are indeed genes that start determining male and female much earlier in development, and female is determined, not just default.

However, development from nipples to breasts seems to be regulated hormonally beginning at puberty...in the presence of higher estrogens, breasts form in girls, and in the absence of this, they do not form in boys. Evidence that this is something that depends on hormones and is not pre-determined genetically is that male to female transexuals can develop breasts when placed on hormonal therapies to develop more feminine characteristics.

This is one of those examples of natural selection processes that shows if something doesn't hurt, there's no reason for it to go away.

[cepheid]

I remember reading in an article that an increased concentration of estrogen in the water supply due to artificial supplements being taken (much of which are not absorbed fully) has led to abnormalities in male children...but I could be getting the details wrong....