What is the Difference Between Female and Male Mammary Glands?

A news story reporting that the NYPD re-mentioned to their officers that women could legally go topless in public has gone viral.

Everyone has an opinion!

There are many countries around the world where both men and women will be out in public topless, but since the US has traditionally only accepted males to go topless, this has caused quite a stir.

This post is less of a debate on topless laws, and more of an examination of this specific area of male and female anatomy.

So, what is the difference between a female and male mammary gland?

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Woman Reading ( The topless woman), Felix Vallotton.

Not that much.

During the onset of puberty, male or female hormones either inhibit or signal branching differentiation of the ductal tree. ( -source)

Also during puberty, estrogen signals fatty tissue to form around the breast area in females. In addition to Cooper’s ligaments, this is what gives female breasts their shape.

Why do men have mammary glands and nipples?

There are a couple of theories to answer this question. The most widely accepted is that men have mammary glands and nipples simply because females do. The trait would have to evolve and “uncouple” in order for men to not have them:

“The only way such differences can evolve is if the same trait (color, for example) in males and females has become “uncoupled” at the genetic level. This happens if a trait is influenced by different genes in males and females, if it is under control of genes located on sex chromosomes, or if gene expression has evolved to be dependent on context (whether genes find themselves within a male or a female genome). The idea of the shared genetic basis of two traits (in this case in males and females) is known as a genetic correlation, and it is a quantity routinely measured by evolutionary geneticists. The evolutionary default is for males and females to share characters through genetic correlations.” – Andrew M. Simons, professor of biology at Carleton University

 

meme1Basically, there isn’t enough reason for deselection of this trait in males for nipples or mammary glands to go away. The trait persists not only because of lack of selection against it, but there is strong advantage to males keeping their mammary glands – they have the ability to lactate.  Although rare, male to female transexual parents are now using this once thought of vestigial part of their anatomy to breastfeed their children.

Why do human females have perennially enlarged breasts?

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Check out any of the other Great Apes (or any other mammal) and you’ll find enlarged breasts in pregnant and lactating females only.

Unfortunately, no one knows exactly why this is, but there are, of course, very probable theories as to why this might be.

Bipedalism:

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Australopithecus afarensis (“Lucy”)

Evolution and natural selection works to our advantage, as it makes almost every part of our body have a function. When our species began walking upright, we had an entire area (the chest area) that served no helpful purpose in attracting a mate.

The theory that human females have permanently enlarged breasts suggests that this area of the body now could help attract a male, as this area was now focal when scanning a group of females, in addition to the buttocks.

Fat is Fetching:

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In addition to the fact that this bare chest area of bipedal humans could be useful in attracting a mate, the reason behind enlarged breasts rather than brightly colored or other eye-catching features that could have adapted onto this area is because fat signals a high reproductive value for males.

Breast, buttocks, and hips are typical areas where human females store fat. There is a theory that these areas (which are unique to human females only for fat storing) were selected traits humans adapted due to male preference. (–”Breast asymmetry, sexual selection, and human reproductive success” by Moller, AP, Soler, M and Thornhill R)

It is hypothesized that male humans would prefer the fat stored in these areas because it appeared to give women a wider pelvis (less complications in childbirth) and larger breasts would signal larger mammary glands (which at one time did signal better lactation ability).

Symmetry:

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Breasts could just be another area of the body that symmetry can be displayed to attract a mate. Humans find symmetrical features more attractive, and asymmetrical breasts have been shown to be less attractive to males.

Breasts Allowed for Female Freedom.

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Human females have concealed ovulation. Other primates like bonobos, baboons, and common chimpanzees have external signs of estrus to alert a mate of reproductive worth.

However, enlarged breasts typically are not seen as attractive to other primate species as it shows the female is either pregnant or lactating, which makes of very little reproductive value to the male.

So, as a benefit of concealed ovulation, the breast area adapted to have fat around it as a confusion signal to males, allowing them to be less closely guarded and available to multiple sexual partners for social, material, or genetic benefits. (-source)

The Nubility Hypothesis

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This hypothesis suggests that permanently enlarged breasts in hominid females evolved as an honest signal of age, and thus, reproductive value.

This idea goes into detail about how pre-pubescent and post-menopausal women are infertile, and size and shape of breasts indicate to a male whether or not the female he is observing is of high reproductive value because her breasts give some signal of her age.

The reason the fat would specifically be in the mammary gland area was that it already coincided with menstrual cycle and aided in the signal of puberty.

No protrusion of breasts would indicate pre-pubescent (anovulatory) and not a reproductive match for a mating male.

Protruding firm breasts would indicate a mature (ovulatory) and reproductive match.

Sagging and thin-skin on breasts would suggest post-menopausal (anovulatory) and again, not a good reproductive match.

It seems, in this hypothesis, there is a trade-off of gaining maximum reproductive value during peak fertility, with less desired mates toward the end of her reproductive years.

Frank Marlow of the nubility hypothesis also offers information when given the argument of male cross-cultural breast size preference (it varies), but female breasts are an erotic stimulus for many cultures.

“Even though men’s interest in breasts may increase where women are clothed, the interpretation that men in nude cultures do not find breasts erotic is erroneous. Based on numerous interviews I conducted with the Hadza [ethnic group located in north-central Tanzania around the central Rift Valley], men find breasts erotic, even though women usually do not cover their breasts. They fondle breasts during intercourse [...]Such fondling is reported in numerous other nude cultures (Ford and Beach 1951). The contention that eroticism only occurs where breasts are covered would imply that men do not find vaginas erotic in completely nude cultures, and that there are no sexual signals among other species.” – Frank Marlow

 


4 comments

  1. I don’t think most men know they have mammary glands and can lactate. I think they don’t realize they have “breasts” (that’s what they are). I’m somewhat fascinated by the way our society is scared to even use the word “breast.” That word always garnered a snicker from my elementary school class when we had to use it in a hymn once in our music lesson. Kids didn’t want to even say it. Now we have men and women alike (ugh!) who call breasts all sorts of ridiculous names. I’m just wondering why? When we go to a restaurant do we ever ask for “boobies of chicken” or “chicken tits”? Nope. Chickens even get more respect than we do. I personally hate the whole “boob” thing. I feel like it’s some kind of shaming, like we should not say “breast”. (Why not?)

    I’m not enthused with the theories that women’s bodies have evolved to attract males. Certainly there must be more to it than just putting the burden on women to attract a mate — and what’s to say that attracting males who like bigger breasts even equals a “better” male? (Or do women not care who they have kids with from a scientific POV?) And why, if we are now walking upright and our buttocks are no longer necessary to attract a mate, do we have these buttocks and why are men still attracted to them? (Evolution, you have some ‘splaining to do!)

    • Although I definitely prefer it to other terms, I actually find it ironic that we use the term breast for human mammary tissue, because that term is NOT used in other mammals. In birds it refers to the anatomical region we would call the chest, but of course birds don’t have any mammary tissue, so that connotation is utterly (or “udderly” :) lacking.

      I would think of the anatomical dimorphism less as how women attract a mate than how they signal reproductive capacity to attract suitors from which they can then choose (generally based on protector/provider traits). It is relatively rare in the animal kingdom for females to be ‘burdened’ with attracting a mate, since gestation is almost always the reproductive limiting factor in mammals (i.e., one male can produce more offspring than one female in a given year). For that reason, female animals generally care a lot MORE about mate choice than males — their gametes are a bigger investment, even in species that don’t gestate and lactate.

    • Oh, and in terms of buttocks, remember that species will only LOSE a trait if it is a hindrance to survival and reproduction, so we have lots of vestigial anatomy and behavior like that. Also note that both sexes have them, although females store more fat there. So I would imagine buttocks are still pretty useful for things like sitting down. To quote one of my daughter’s favorite picture books (by Fran Manushkin): “Life would not be cushy, if you didn’t have your tushy!”

  2. I’ve wondered this too. Someone did a little experiment on Google+ where they posted a topless photo (not of themselves, at least I don’t think so) and it hung around for however long. Let’s say a week. Then they posted the entire photo, uncropped, which revealed that the topless figure lying down was an adult woman. The photo was taken in such a way, I guess, that it was tough to tell without seeing the face and hair…and as soon as it was revealed it was a woman it was flagged and then removed. It’s always been so strange to me. It’s even stranger that bare male bottoms get into family-friendly films but bare female bottoms practically imply pornography, but that might be a whole other topic.

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