Why do men have nipples?

Great thinkers, from Aristotle to Darwin, have pondered this question.

Topics: Evolution, Charles Darwin,

Why do men have nipples? 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?

There are some male mammals without nipples, a fact I was alerted
to by Aristotle, who wrote “Such, for instance, is the case with
horses, some stallions being destitute of these parts.”

Since Aristotle’s medical facts were sometimes a bit wobbly — he
said cabbage cures hangovers — I called an equine veterinarian. “I
have never seen a stallion with nipples,” she declared flatly. “And
I have looked around down there.” As far as I know, she’s never
seen a bald stallion, either, so that’s how they avoid being called

The veterinarian pointed out that a mare’s two nipples are located
toward the tail end of the body, as opposed to the chic head-end
location in humans. This, she daintily hinted, might be why
stallions don’t exhibit nipples. “There’s no room.”

These shocking facts sent me on a quest for other data on animal
nipples or, as medical types have long preferred to say, mammae.
Male nipples? Mammae masculinae. (If you need to be even more
obscure you can also call a nipple a mamilla or a thelium.)

My mother, when I told her of my research, may have been hinting
that there were more hard-hitting stories I could be working on by
bringing up the folk analogy “as useless as tits on a boar hog.” My
research appears to indicate that boar hogs do in fact have tits.
Which they are not known to use.

Not only do male platypuses not have nipples, neither do females.
The milk simply flows out through pores and is licked up by baby
platypuses. And while platypuses are not actually categorized as
reptiles, you’ll notice that people are always talking about how
“primitive” they are and making fun of their noses.

I would have assumed that nipples were only available in even
numbers had I not learned that female possums, for example, have
between seven and 25 nipples. The delightful Virginia opossum, which
inhabits the middles of American roads and highways, usually has
13, efficiently arranged in an open circle with one in the center.
This information should not tempt you to snicker and point the next
time you see a possum: They also have 50 teeth.

Most mammals, however, stick to even numbers of nipples, and often
the males get to have them too. In addition to boar hogs, dogs,
cats, all primates and many other animals feature the mamma

It seems that human embryos develop mammary tissue before they
bother to check on whether they’re going to be male or female and
start modifying the basic plan with surges of this or that hormone.
After only a few weeks, milk ridges form — two stripes of tissue
that start in the armpits, curve out over the chest, go straight
down the stomach and then veer in toward the groin, ending
somewhere high on each thigh. Later the milk ridges regress to some
extent, usually leaving us with just two nipples.

Quite a few people end up with an extra, or supernumerary nipple
somewhere along the trail of the milk ridge, however. (One man had
five.) Sometimes they can’t be mistaken for anything but a nipple,
and other times they look like a mole. In fact, many people with
supernumerary nipples don’t know they have them until some
officious and informative person starts examining their moles.
Extras often run in families — Darwin cites two brothers who each
had a supernumerary nipple. Anyone who thinks that’s weird should
immediately leave the room and go check his or her torso for moles. How
do you know you’re not head-to-foot extra nipples and we’ve all
just been too polite to mention it?

What of male nipples as erogenous zones? You know they are, or why
would they be banished from the chest of Ken? (To avoid inflaming
Barbie.) I have looked into the matter of G.I. Joe: I never owned a
G.I. Joe, though I recall liking his accessories, particularly the
canteen. (Don’t take that the wrong way. Sometimes a canteen is
just a canteen.)

I asked a friend, who indicated with some annoyance that her
childhood G.I. Joes were just as smooth-chested as Ken. But it
seems that over the years G.I. Joe bulked up, and from being an
average Joe with an average physique became an eerily burly
muscle man who apparently never leaves the gym except to go to the
rifle range. Somewhere along the line some G.I. Joes acquired
nipples to go with their superior muscle definition and popping
veins. The effect is not particularly erotic: I suspect they’re
just there to give the viewer a reassuring landmark among all the
unfamiliar ripples of the bodybuilder’s torso caused by out-of-control delts, pecs, abs, intercostals and other oddities.

(In addition to the mute testimony of dolls, many actual men state
emphatically that male nipples are erogenous zones.)

Of course, the principal reason for the nipple’s enduring
popularity is its function as a food delivery device. Ask any baby.
Ask any father who has held his child in his arms and suddenly had
said infant jerk its head to the side and latch optimistically onto
a nipple. After a moment, the baby gives the father the reproachful
look of an innocent child betrayed: You’re no fun!

Darwin, who thought about everything, naturally wondered about
nipples. He collected case reports of men and women with extra
nipples (which he called mammae erraticae), including the case of
a woman who allegedly nourished a child via an extra nipple on her
thigh. (Why? Why not use the ones on her chest? Pure showboating,
that’s my guess.) This led him to suspect that we are descended
from creatures with more than just the two mammae.

He also pondered male nipples. In “The Descent of Man,” Darwin
suggests the possibility that “long after the progenitors of the
whole mammalian class had ceased to be androgynous, both sexes
yielded milk, and thus nourished their young; and in the case of
marsupials, that both sexes carried their young in marsupial

Darwin defended mammae masculinae: “The mammary glands and nipples,
as they exist in male mammals, can indeed hardly be called
rudimentary; they are merely not full developed, and not
functionally active.” He suggested that ancestral males gave up the
practice of nursing, after a prolonged period, perhaps because
litters were smaller. When “the males ceased to give this aid,
disuse to the organs during maturity would lead to their becoming
inactive; and … this state of inactivity would probably be
transmitted to the males at the corresponding age of maturity. But
at an earlier age these organs would be left unaffected, so that
they would be almost equally well developed in the young of both

Surely this is why everybody loves Darwin. Who else was thinking up
ancestral father animals suckling pouches full of thirsty babies?

I asked mammalogist Douglas Long, collections manager for
ornithology and mammalogy at the California Academy of Sciences,
whether there’s any new thinking on this particular suggestion of
Darwin’s. “Unfortunately, the fossil record doesn’t give much of
a clue at all,” Long said. “It’s very intriguing.”

While there’s no evidence to refute or support Darwin’s hypothesis,
Long points out that of the thousands of species of living mammals,
“Not a single one has a male that is able to lactate in any way.”
Why all the male nipples, then? Long cites the embryologic process
that creates mammary tissue and also notes that, evolutionarily
speaking, “It’s a lot more difficult to lose an organ than develop
an organ … It could be that males still have nipples because
there’s nothing deleterious about nipples. There’s no real need to
get rid of them. Why do we still have toenails, for example? Other
animals use them for digging, scratching or fighting, but we
don’t. They’re useless but at the same time they don’t distract
from the business of living.”

Pigeons and a couple of species of fish do something similar to
suckling their young, a task they split down the middle. Male and
female pigeons and doves feed their nestlings “pigeon’s milk,” a
cheesy substance they manufacture in their crops. Discus and orange
chromide fish feed their young with a nutritious mucus from the
sides of their bodies.

(Which reminds me. I do not want to hear about the breast being
just a modified sweat gland one more time, OK? That was a long
time ago and it was a pretty radical modification. Milk isn’t
sweat. Do you ever hear people say “the sweat of human kindness,”
“She rode a sweat-white horse” or “got sweat?” There’s a reason:
Milk is different from sweat. Until I hear you describe your hand
as a modified flipper, there will be no more talk of sweat glands.)

Male humans look pretty unhelpful next to pigeons. Newborn babies,
still pumped full of maternal hormones, usually lactate slightly,
producing a few drops of “witch’s milk.” Medical conditions like
acromegaly (excess growth hormone) can induce male lactation.

Dr. Miriam Stoppard, author of “The Breast Book,” agrees with
Darwin that male nipples are more than rudimentary, cheerfully
suggesting that “men could develop fully functional breasts given
the right hormonal conditions.”

That’s right. If men would just submit themselves to an intense
barrage of hormone therapy, affecting every organ system of the
body in unknown ways, maybe they would be able to suckle their
young and throw off the charge of reptilianism once and for all.
But where is the research? Where is the funding? Where is the will?

Whither the male nipple? Is it ever likely to stomp off in an
evolutionary snit over not getting any respect (“Enough about boar
hogs!”) and leave male humans as smooth-chested as stallions or
bulls? It seems unlikely. They’ve managed to hang in there all
these millennia, and many guys speak well of their nipples and
would clearly vote to retain them. Ask any boar hog and he’ll
tell you the same.

Susan McCarthy is a San Francisco freelance writer and the author, with Jeffrey Masson, of "When Elephants Weep: The Emotional Lives of Animals."

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