It surprises many people to learn that it is actually much more common for male mammals to have nipples, or something that resembles them, than it is to be without them. It is only quite a small group of male warm-blooded animals – including mice, oddly enough – that loses this trait.
A simple way that is sometimes used to explain male nipples is that we all start off female, and male embroyos develop away from this while retaining some of the female characteristics that they started with. To look at this point in more detail: humans, along with other mammals, develop mammary tissue in the early embryo stage. This is the tissue from which the nipple eventually develops, with its nerves and milk-producing glands. When an embryo is three or four weeks old it will start to grow what are called milk lines, a kind of ridge on the chest that later will give place to the milk producing gland and the nipple from which milk can flow. Therefore an unborn male has the same milk producing equipment as a female; and this applies to most mammals except those where something occurs to stop it (embryo male mice, for example, produce a protein that blocks the growth of milk glands).
In young children the chest area with its visible nipples looks much the same in both boys and girls. It is only later that puberty in girls, with the production of oestrogen, will create the changes that mean she will be able to produce milk.
The reason most commonly given why men continue to have this characteristic when it doesn’t seem to offer a practical advantage is simply that there is no reason why they shouldn’t – in other words, having nipples doesn’t harm their chances of survival even if it doesn’t do much good, so there is no reason why there should be any evolutionary pressure to lose them. It’s true that men do sometimes develop medical problems around this area, but generally they do no harm – it’s a bit like the appendix, which is no longer of any use, but doesn’t cause problems enough to represent a threat to our survival.
However, it can be argued that the male nipple is of more use than the appendix. It is a sensitive area of the body, and there have also been documented cases of men actually being able to produce milk. The well known and reliable cases of this have involved a phenomenon called galactorrhea (from the Greek word for milk, gala, which also formed part of the name of the original Greek Earth goddess figure, Galatea). In galactorrhea, either males or females can produce milk but it doesn’t have anything to do with childbirth or pregnancy – it just happens by itself. Usually this is harmless, though a big surprise to the person concerned, but occasionally it is a sign of an underlying health condition so it should always be checked out if it occurs. It has also been known to happen when a person is extremely malnourished.
As for men actually producing milk in response to a baby’s need to be fed, and this being of sufficient quality to nourish an infant: there have been many stories of this and scientists from Darwin onwards have speculated on it. It does seem that there have been cases where men have succeeded in feeding an infant, usually after the baby has tried to feed from the man a number of times – it may be that the suckling action triggers milk production. However, not enough is yet known about the phenomenon to be sure of how it works, or if it really can work exactly as it would with a female.