Monday, February 13, 2006

Male nipples and round ligaments of the uterus

Why do men have nipples? In women, of course, the major function is clear: nipples provide a convenient milk-delivery device for a hungry infant to latch onto. Nature has even made it easier for babies to find the nipple by causing the areola to turn darker during pregancy. (At least that's one explanation; personally I think mothers are pretty good at guiding babies to the nipple with or without the additional color contrast.)

In men, however, the nipples serve no obvious function. They certainly have nothing to do with delivering milk. Lactation has never been observed in any healthy male mammal. I had to qualify that last statement with "healthy" because there are diseases, such as certain tumors of the pituitary gland, that can cause men to produce milk, an inconvenient condition called galactorrhea. An imbalance in the endocrine system can also cause gynecomastia, enlargement of the male breast. Still, these are rare exceptions to the rule. Nipples and breasts may have the potential to be useful in men, but in general they appear to be extraneous.

So why do we have them? Could male nipples be vestigial organs, evolutionary equivalents of the appendix? Darwin proposed that male mammals once shared the job of providing milk to their young. It's delightful conjecture, and not unreasonable, but it remains in the realm of just-so stories because (so far) there is no way to test its validity. If the story were true, you might expect the most anatomically primitive mammals - monotremes such as the duck-billed platypus and echidna - to have males with more highly developed nipples. In fact, we see the opposite: monotremes - male and female - have no nipples at all (but the females still lactate, expressing milk via little pores in the skin). I'm only aware of a couple mammal groups in which the female has nipples and the male doesn't (a feature we might call "mammillary sexual dimorphism"): horses and rodents. If male nipples are on their way out, they sure are tenacious.

Whether or not male nipples are a relic of evolution, they are almost certainly a relic of development. In the earliest weeks following conception, the male and female embryo follow a virtually identical developmental trajectory. Then at about 7 weeks, the production of testosterone kicks in and the male diverges anatomically from the female. By then it's too late: nipples have already formed in both sexes. Biologically it's conceivable that random mutations could reverse the continued growth of the male nipple, causing it to involute and disappear completely by the time the baby boy is born, but apparently there hasn't been pressure for such mutations to take hold, if they have occurred. There are occasional mutations that lead to the absence of one or both nipples (in both males and females), but they are typically associated with other defects such as missing muscles and sweat glands and webbing of the fingers.

So are male nipples utterly useless? It's hard to respond with an unqualified "yes," because someone can always come up with something plausible. In some men the nipple may be considered an "erogenous zone," but what part of the male anatomy isn't? Even the appendix, the poster child of vestigial organs, isn't totally useless: it contains an abundance of lymphocytes and other cells that fight infection. Still, as many appendectomy patients can attest, we can live perfectly well without it. The same goes for nipples in men.*

In the interest of gender equity, what about women? Do women have anything similar to a male nipple, an essentially useless part of their anatomy that reflects a developmental constraint? In a classic (1987) and controversial essay called "Male Nipples and Clitoral Ripples," the late paleontologist Stephen Jay Gould argued that the clitoris, along with the female orgasm, fits the bill. The argument is further elaborated in a recently published book by biologist and philosopher of science Elisabeth A. Lloyd: The Case of the Female Orgasm: Bias in the Science of Evolution. Evidently she makes a good case (click here for a review), but lingering doubts are understandable. I suspect that the average woman places a much, much higher value on her clitoris than the average man places on his nipples.

Instead of weighing in on that controversy, I'd like to propose a better female analogue of the male nipple: the round ligament of the uterus. The round ligaments are two slender ropes of connective tissue that run from the top of the uterus to the front side of the abdominal wall, pass through the inguinal canal (approximately at the level of the bikini line), and ultimately blend into the fatty connective tissue of the labia majora.

In the female fetus, you can trace the round ligament from the abdominal wall all the way up to the ovaries. At those early stages of development the round ligament is referred to as the gubernaculum, which means governor (same root as gubernatorial). The male fetus has a gubernaculum, too, except that it's attached to testes, not ovaries. As the fetus grows, the role of the gubernaculum is similar in both the male and female: it gently guides the gonads (i.e., testes or ovaries) during their descent from their birthplace in the upper part of the abdomen. As they descend, the gubernaculum gets shorter.

There the similarities end. The testes have much farther to go. While the ovaries drop down into the relatively well-protected pelvic cavity (the space surrounded by the hip bones), the testes travel onward, punching a tunnel (i.e., the inguinal canal) through the abdominal wall and ending up suspended in an outpouching of the abdominal wall called the scrotum. Click here for a little animation of the testes squeezing through the abdominal wall (the greenish band is the gubernaculum).

The different fates of the ovaries and testes are reflected in the gubernaculum. In the male, each gubernaculum shortens as much as possible and leaves little or no remnant in the scrotum. In the female, the middle of the gubernaculum fuses with the top of the uterus, forming what appear to be two separate ligaments: (1) the ligament of the ovary, which connects the ovary to the uterus, and (2) the round ligament of the uterus, which connects the uterus to the abdominal wall. See the illustration below.

The ligaments of the ovary may serve a useful function: each one appears to maintain the proper distance between the ovary and the uterus, so that the fallopian tube can receive eggs from the ovary during ovulation.

But the round ligaments? As far as I can tell, they're useless. One popular (and generally trustworthy) online resource (The Interactive Body Guide) suggests that the "round ligaments hold the uterus anteverted (inclined forward) over the urinary bladder." Seems reasonable, until you realize that something like 20-30% of women are born with a uterus that is retroverted (inclinded backward). The retroverted configuration is considered a perfectly normal variation that has no effect on fertility. In other words, the round ligaments aren't very good at holding the uterus forward because there's no good reason for them to be.

Not only are round ligaments unnecessary, they can be a real pain - literally. As the uterus grows during pregnancy, the round ligaments stretch like rubber bands and tug on the abdominal wall, often causing round ligament pain. Fortunately the pain can usually be relieved with simple measures such as a hot bath, a shift of body position, or Tylenol. Like male nipples, the round ligaments of the uterus are relatively minor anatomical flaws, and any inconvenience they cause pales in comparison to the many anatomical marvels of the human body.

*More resources on male nipples:

20 Comments:

At 2/16/2006 6:25 PM, Blogger NathanGWright said...

I've actually read that book, "Why Do Men Have Nipples?...". It is a great read to obscure questions like the one in the title. It's best for places in your home where you have 5 or 10 minutes to read some interesting snippets of entertaining and devoid of use information.

 
At 4/22/2006 8:14 PM, Blogger Urban Shaman said...

Firstly love your blog. Secondly I was watching a documentry that said that essentially we default to female as a species. When the hormone cocktale that creates males fails to take, these would be males become infertile females with the Y chromosone. So I guess we have nipples so we could revert to females at the last minute even if an infertile one. Does that work for you?

 
At 4/23/2006 8:13 PM, Blogger Brad said...

Sounds like you're talking about androgen insensitivity syndrome, a genetic condition in which a genetically male fetus doesn't respond to the male hormones that it produces during development, and therefore ends up developing as a female, at least externally. Internally, these intersex individuals don't have fallopian tubes or a uterus; the vagina ends in a blind pouch.

Do men have nipples so that people with androgen sensitivity syndrome can look more like normal females? I doubt it, but I'm not sure how you'd go about proving it one way or the other....

 
At 4/26/2006 6:58 PM, Blogger Urban Shaman said...

Sorry Brad I wasn't very clear. What I meant was that if we know that there are cases in which a male can become a female at least externally, possibly there could be cases where a male becomes a female internally as well. I don't imagine it would be reported because there would be no reason for the person to suspect they would have been a male.

 
At 4/29/2006 2:51 PM, Blogger Brad said...

I'm not aware of any cases where an individual with XY chromosomes is a completely normal female anatomically. Swyer syndrome (XY gonadal dysgenesis) comes close: http://en.wikipedia.org/wiki/Swyer_syndrome

 
At 5/26/2006 6:44 PM, Blogger I am a Milliner's Dream, a woman of many "hats"... said...

FASCINATING...the charts available through childbirthgraphics.org which I use in my childbirth classes make the round ligament appear to be part of a system suspending, supporting and grounding the uterus as it grows great with child.

No?

Hh

 
At 5/26/2006 6:46 PM, Blogger I am a Milliner's Dream, a woman of many "hats"... said...

So I re-read what you wrote...and they are like the appendix--useless, perhaps unless they cause a problem.

We are taught in CB ed training, and even in nursing school the theory I just proposed above.

Hh

 
At 1/16/2007 9:28 AM, Anonymous Anonymous said...

I just wonder whether the ligament of the ovary is playing some major role in holding the ovary in place relative to the fallopian tube. I'd say the position of the ovary is mostly determined by its attachment to the broad ligament, namely the mesovarium.

Tom

 
At 1/29/2007 8:51 AM, Blogger Brad said...

Hi Tom - sorry for the delayed reply - I'd say that both the ovarian ligament and the mesovarium play a role in properly positioning the ovary. But the real question is: why doesn't the fallopian tube simply grow around and fuse with the ovary during development? Even though they have separate developmental origins, why do they have to remain apart? This unfortunate arrangement can lead to endometrial tissue in the peritoneal cavity (a form of endometriosis) and, occasionally, an abdominal pregnancy (1-2% of ectopic pregnancies). I don't have a good explanation.

 
At 12/17/2008 3:40 AM, Anonymous Nippits said...

I haven't come across a very publicized issue of some guy with breast cancer. It really piques my interest.

 
At 6/19/2009 2:42 AM, Blogger kriscrash said...

Maybe women have always had preferences against men with no nipples and they never reproduced? I wouldn't consider it a flaw, even if mostly ornamental it wouldn't be the only body part to be so.

 
At 3/25/2010 9:59 AM, Blogger Nany said...

Hello friend congratulation very interesting and professional post about Male nipples and round ligaments of the uterus and I would like to know if you have any post about inverted nipple and when you add information on this blog please send me a message

 
At 4/05/2011 9:48 AM, Anonymous cheap viagra said...

Wow interesitng, I already knew it why man have nipples the ligaments of the uteros were amazing, well I saw many woman have births of 4 boys 6 and 8 which is very awesome.

 
At 5/02/2011 8:34 AM, Anonymous Generic Viagra said...

this is a interesing and educative theme, in fact could be used as a good theme from some homework for children of course, and to me as a general knowledge.

 
At 6/23/2011 11:04 AM, Anonymous Acai Berry Select Reviews said...

wow great i have read many articles about this topic and everytime i learn something new i dont think it will ever stop always new info , Thanks for all of your hard work!

 
At 6/24/2011 3:01 PM, Anonymous SENuke X said...

I value the article. Really thank you! Really Cool.

 
At 6/02/2012 8:27 AM, Anonymous Anonymous said...

I'll tell you why men have nipples; Vestigial or not - a GREAT NUMBER of men I know - including ME - have extraordinarily sensitive nipples. They are sexual stimulation zones....as postulated in the initial article. No - this is not uncommon or rare: just observe male-to-male sensual activity: the male nipples become obvious in their function: they are CRITICAL for most men in achieving total sexual satisfaction and prolonged sexual enjoyment: FULL BODY enjoyment comes from the stimulation of a man's nipple(s) - be it by himself or with a partner.

That said: THAT is why men have nipples. Two more extra-sensitive parts on his body....to go along with his lips, tongue, penis, testicles, perineum - and his ever so sensitive anus and inner rectum.

End of story, folks. It's a guy thing...and boy...are we lucky.

 
At 8/20/2013 12:13 PM, Blogger Steve Reinagel said...

I’ve had a lingering notion and based on your unique anatomical posts, you might be the just person to address it to. My question deals with orgasms and religion (just to mix things up a bit!). The male orgasm is necessary for the propagation of a species, but the female orgasm is not. So, speaking from an evolutionary perspective, why would the female have developed the capability to have an orgasm if there is nothing in nature to prompt the need? And taking it one step further, can any females of the animal kingdom produce an orgasm, and if not, why do only human females have the ability? Would this in fact provide any evidence that humans are not part of the animal kingdom, but are a separate creature (that’s the religious part)?

 
At 9/10/2013 1:02 PM, Anonymous Yonah Z said...

Great article! Regarding the function of the round ligament, here's an interesting peer reviewed research paper that suggests that it's a muscle that helps during childbirth. (I might add that this is somewhat supported by it's ability to contract in embryo, so it definitely was a functioning muscle at some point.)

http://onlinelibrary.wiley.com/store/10.1111/j.1471-0528.1964.tb04295.x/asset/j.1471-0528.1964.tb04295.x.pdf;jsessionid=DFB881CCD5B77AFEC94B938E8FB0138A.d02t01?v=1&t=hlff4of6&s=082149c06c50d2857afee2497755e77d92e6ffe4

 
At 10/06/2013 12:23 AM, Blogger Trentent Silver said...



Good blogging.I like it..............
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