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18-May-1979
Dear Cecil:
As I understand it, the appendix at one time assisted in our
digestion. Today it is useless. Our wisdom teeth, I have learned,
helped our ancestors chew tough herbs and raw meat. Today they are
a nuisance. But there is a third biological mystery for which I
have no answer. So please explain, Cece: why do males have
nipples? --Susan L., Los Angeles
Dear Susan:
To tell you the truth, nobody really knows. The best explanation
I've been able to find (and frankly it doesn't explain very much)
is that nipples aren't a sex-linked characteristic. In other
words, nipples are just one of those sexually neutral pieces of
equipment, like arms or brains, that humans get regardless of sex.
As you may know, every human being gets a unique set of 23 pairs of
chromosomes at conception. These fall into two categories. One pair
of chromosomes determines sex--the XX combination means you become
female, the XY combination means you become male.
The other 22 pairs, the non-sex chromosomes (they're called
autosomes), supply what we might call the standard equipment that
all humans get. These 22 pairs constitute an all-purpose genetic
blueprint that in effect is programmed for either maleness or
femaleness by the sex chromosomes. The programming is done by the hormones secreted by the sex glands.
For example, the autosomes give you a voice box, while the
sex hormones determine whether it's going to be a deep male
voice or a high female voice. Similarly, the
autosomes give you nipples, and the sex hormones determine whether
said nipples are going to be functioning (in females) or not (in
males).
One interesting consequence of the developmental set-up just
described is that during the very early stages of fetal life,
before the sex hormones have had a chance to do their stuff, all
humans are basically bisexual. Among other things, you have two
sets of primitive plumbing--one male, one female. Only one set
develops into a mature urogenital system, but you retain traces of
the other for the rest of your life.
It's tempting, therefore, to say that male nipples are yet another
vestige of your carefree bisexual youth. Trouble is, male nipples
are hardly vestigial. They're full-sized and fully equipped with
blood vessels, nerves, and all the usual appurtenances of
functioning organs. Why this should be so nobody knows--in some
other mammals, such as rats and mice, male nipple development is
completely suppressed by the male sex hormones. (Incidentally,
don't start thinking that at one time our human male ancestors must
have suckled their young. So far as anybody knows, male lactation
has never developed in any mammalian species.)
Human nipples appear in the third or fourth week of development,
well before the sex characteristics. (The sex hormones start to
assert themselves at seven weeks.) As many as seven pairs of
nipples are arranged along either side of a "milk line," a ridge of
skin that runs from the upper chest to the navel.
Normally only one pair amounts to anything, but on about one baby
in a hundred you can detect some vestige of the other ones, usually
on the order of a freckle. There are cases of women who ended up
with an extra breast, which made them freak show candidates not so
many years ago. Luckily today the women can avail themselves of
corrective surgery while the rest of us can watch Jenny Jones.
Anyway, both male and female babies are born with the main milk
ducts intact--the gland that produces milk is there in the male,
but it remains undeveloped unless stimulated by the female hormone,
estrogen. Occasionally, a male baby is born with enough of his
mother's estrogen in his body to produce a bizarre phenomenon known
as "witches' milk," with the male glands, suitably stimulated,
pumping away at the moment of birth.
In the adult male, the dormant glands can still be revived by a
sufficient dose of estrogen. Actual lactation is rare--only a
couple cases have been recorded. But at least one writer (Daly,
1978) has suggested that the "physiological impediments to the
evolution of male lactation do not seem individually surmountable."
Meaning we may yet see the dawn of the truly liberated household.
--CECIL ADAMS
The Straight Dope / Questions or
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