The amazing things the human body can do are normally not limited to one sex. Obviously some things can only be done by women including providing the nourishing space where babies to grow. Other things are the only available to men like, well, finding humor in fart jokes after their 39th birthday. Some body processes, like producing milk, are normally reserved for women but can be performed by men should those men have the inclination, money, and access to a willing medical professional. While not likely, this is entirely possible due to the fact that men and women produce the same hormones in their bodies, just not normally in the same concentrations. Change the concentration of a few of those hormones and interesting things can happen.
This explains not only the unusual situation of male lactation, but also a few other things as well. For example, you may have noticed that men have nipples. Upon noticing this you may have also noticed that, unlike women, they don't feed babies with them (unless, as previously noted, they really want to). Once you have noticed both of these things (and by the way, congrats on all the noticing) you will probably have asked yourself the age old question of "why are they there if they don't normally serve any function?"
I have heard many different responses to this question ranging from the completely scientific and well researched to the less scientific and more bro-tastic, "Dude - chests would look weird without them so, you know...nipples." The real reason men have nipples actually quite simple: because women have them.
That isn't a very satisfying answer, so lets get a little more detailed. To start with, lets work with an analogy.
My sons have a Lego set that allows them to build 3 different things. Follow one set of instructions and you get a car, another set and you get a boat, while yet another produces a plane. All of the pieces for each structure are included in the kit and you simply have to selectively ignore the pieces you don't need. One of the slick parts of this set is that the first bit of the instructions for all three things is the same. It is only after page 20 that you are forced to make a decision as to which structure you are going to build.
The different human sexes are built pretty much the same way as our Lego set. Lots of individual pieces are assembled and come together to make up a human and males and females are essentially two separate structures built from the same kit. For the model with the uterus, continue on to page 21. For the model with the dangly bits and the fart jokes, skip to page 45.
To put it slightly more scientifically, the vast majority of instructions (genes) used to build a male are also found in the female body and vice-versa. To drive this point home a bit, in the not too distant future it is going to be technically possible to make male and female clones of yourself. Flip a few genetic switches in the developing embryo and viola, female clones from male donors.
Two things of note before we continue. First, all humans have 46 chromosomes - half come from mom and half from dad. Due to some interesting genetic and evolutionary issues that are well beyond the scope of this post, the half that come from mom are pretty much identical to the half that come from dad. This means that both moms and dads carry around instructions for how to build nipples.
Second, all babies start out female and it is only with a well timed release of testosterone that the baby begins its transformation into a little dude. So to go back to the Lego analogy, the first 20 pages of instructions for human babies includes building the basic body parts and setting the stage for a little female baby. Everyone, even Dwayne "The Rock" Johnson, started off as a little girl.
"What does this have to do with nipples?" you might ask. Should you ask that question I would respond, "Everything."
You see, ever since mammals evolved to produce milk from modified sweat glands, nipples have been a fairly important part of the body plan for most of them. And because half the genes to produce those nipples come from mom and half from dad, this means that dads had to carry around nipple genes whether they wanted to or not. Since the dads had these nipple genes and it seems the nipple building directions were in the first 20 pages of our instruction manual, they built nipples even though they were not going to produce any milk.
These genes and their nipple instructions are only part of the reason men have them. The other part is evolution. To oversimplify a relatively complex concept, evolution works by natural selection. This means that those critters that are well adapted and have no major physical issues will survive better and make more babies than those with severe problems. In the grand scheme of things, nipples are not really that detrimental to the male who has them which means that he is still going to survive and he is still going to get to mate and have babies. If he survives and has babies, his babies will then also have nipples.
To sum up a bit, human embryos are basically identical until 8-9 weeks of development. At this time the genes that determine maleness will turn on in males and those same genes will stay off in females. During this initial 8-9 weeks of development the basic body shape happens including two arms, two legs, head, heart, internal organs, skin (nipples), etc. It is only after this part that embryos branch off to become male or female. So men have nipples because they have genes that produce them and these genes and their resulting superfluous nippleage don't seem to get in the way of survival or mating.
By the way, "Superfluous nippleage" would make a great name for a punk band.
As a quick aside, a few other interesting things can happen due to the similarities in hormones and genetics. One of the most common issues is male breast tissue. This normally develops during puberty and can come as quite a shock to boys who were not exactly expecting breasts, even if they are super small. No one expects this because no one ever tells boys that this is even a possibility. The sex talk in 6th grade and health class in high school often tell you that you will develop a deeper voice, hair in several novel places, and muscles due to hormone surges. They inconveniently leave out the whole, "you might get small breasts" thing, probably because no one wants to tell a 14 year old boy that he and his 11 year old sister are experiencing something in common.
The male breast tissue (normally called "breast buds") is often sensitive and can be painful if you happen to be a goalkeeper and someone hits a soccer ball into your chest at approximately 400 miles an hour. That may or may not have happened to me in high school, but here's a hint: It definitely did. Typically you will notice a is a small lump under one or both nipples which is completely unnoticable by everyone except, of course, the young man in question. They develop because during puberty the normally delicate balance of hormones can be thrown out of whack resulting in a little bodily confusion. Your male body recieves a dose of some hormones and thinks to itself, "Hey - I've got these nipples here, may as well do something with them." After a few weeks, months, or years, the hormomes will go back to normal and the breast tissue will disappear leaving only fond memories.
Human bodies are amazing, strange little things, aren't they?
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