The 12 days of evolution. #7: Why do males have nipples?

December 27, 2015 • 11:15 am

The seventh video in the 12-episode series produced by PBS/It’s Okay to be Smart (I’m putting them up in order) is about a question that always excited my undergraduate students: why do male mammals have nipples? One undergraduate whom I taught about two decades ago told me recently that he’d forgotten almost everything I taught in introductory evolution, but he never forgot the case of male nipples. Such is the fate of teachers.

One thing is certain, though: every male carries genes for female traits like breasts, vaginas, ovaries, and the like. People often don’t realize that, but, after all, a male has the same genome as a female except for the absence of one of the two X chromosomes and the addition of a Y chromosome. Yet all the genes for being female are in that Y-containing genome. What happens is that the Y switches development onto the male track, silencing those genes producing female traits and activating the ones producing male traits.

Likewise, females carry nearly all the genes for male traits: beards, penises, higher aggression, and so on. (There are very few genes on the Y; it acts largely as a switch to turn on or off genes on the other chromosomes.) Male nipples are simply the expression of some genes that, in females, produce an adaptive trait.

But why are they expressed in males? This video gives an explanation:

There are two problems with this video. First, it’s far more confident in its explanation for male nipples than the evidence warrants. There are at least three explanations for the trait:

  1. Male nipples don’t impose any reproductive burden on that sex, so evolution hasn’t removed them. They are simply “neutral” features that remain, and the genes that produce them have stayed active.
  2. If selection were to remove male nipples, it would require rewiring the developmental system in a way that can’t be done. That is, perhaps the formation of nipples in males acts as some kind of inducer or link in the rest of development, and to eliminate them would involve—at least temporarily—screwing up development. This explanation assumes that there is some reproductive cost to producing male nipples, but there is a greater cost to getting rid of them.
  3. Male nipples are adaptive or were adaptive in our ancestors. I’ve heard it said, for instance, that, at least in the millions of years of evolution before we wore clothes, they called attention to a man’s manly chest, acting as a way for women to assess male physique.

Now we don’t know which explanation is correct, though I’m extremely dubious about #3 and think the answer may be a combination of numbers 1 and 2.  This video, however, confidently asserts that #2 is the answer. It would be better to give all three explanations and say that we simply don’t know. After all, we have no idea how the development of male nipples is related to the rest of male development.

Further, the video confuses male nipples with Steve Gould’s concept of “exaptations”, which are features originally built by selection for one function, and then coopted later by selection for a different function. (As the video notes, the swimming function of penguin’s “wings” is an exaptation.)

But implying that male nipples are exaptations is an explicit acceptance of explanation #3: that they have a new function in males coopted from their original function as suckling outlets for females’ milk. Yet the video also claims they are neither advantageous nor deleterious.

All in all, the video is okay, but just. Again, I wish they’d consulted some other evolutionary biologists before making this series.

34 thoughts on “The 12 days of evolution. #7: Why do males have nipples?

  1. #3 is presumably BS since all male mammals have nipples, not just human, and I’ve not heard tales of e.g. a sow inspecting a boar’s undersides to evaluate his nipple endowment.

  2. When the word “spandrels” popped up, I flinched. What does that have to do with something he described as vestigial? I’ve noticed in a number of episodes they seem to overly complicate things that could be stated more succinctly.

    Still, it is a nice series as a whole.

  3. I agree that the likely reason for male nipples is a combination of #1 and #2. They are likely neutral, or nearly neutral in males, and so are effectively invisible to natural selection. But messing with their development will generally not be neutral b/c mammary structures begin during the embryonic period and tinkering with gene circuitry that operates during that time is not easy and so there better be significant selective pressure for it. So this explains why male and female embryos look ~ identical(not just in the mammary area but elsewhere), and it also is related to why vertebrate embryos look remarkably similar across species.

    1. Yes, I’ve always hypothesized that the maintenance of these sorts of seemingly unnecessary traits rests on their being “one less thing to go wrong.” The developmental pathways are already fraught with enough important sex-specific modifications at critical times that must occur for reproductive reasons. Why evolve the need for more crucial developmental “intersections” for characters that are essentially neutral?

      I also see this as one answer to those who want to claim that women are genetically inclined to be less intelligent than men. Seems to me the chance that that pathway might go haywire in some men would be evolutionarily less fit than the level playing field is.

      (Not to mention some other fine reasons! Foremost, of course, the advantage of intelligence for all humans.)

    2. Explanation Number 1 doesn’t seem to be an explanation; “explaining” why natural selection “hasn’t removed them” doesn’t address why males HAVE them in the first place. The maintenance of a trait doesn’t get at its origin. Without using jargon, the video could have introduced the fact that there are often inescapable pleiotropies between the sexes: gene expression cannot always be completely decoupled between the sexes, despite the different hormonal backgrounds. In many polygynous birds, males are much showier but females may have “highlights” of the same colors, with no adaptive function. Similarly, if you perform artificial selection for increased body size on female fruit flies (but choose males randomly), male size will also get bigger, as thee genetic correlation between the sexes is not zero. This pleiotropic effect might be considered a “spandrel” but I view it as a genetic constraint.

  4. “man’s manly chest”

    Hmmm, men don’t have “manly chests” unless they engage in a lot of bench pressing. Ape males have saggy, middle-aged man chests.

    That’s one of the many problems with the depictions of Tarzan.

  5. #3 runs against my own speculation that bare breasts are a turn-on for men even though they have no direct role in reproduction because they act as a sort of “flag” that the owner is a woman. If I’m right then male nipples would only reduce the effectiveness of that “flag.”

    1. And I, having also speculated about that, opine that bare breasts are a turn-on for males because they are one visual marker for gender, not that they developed for that purpose.

  6. Actually, Stephen J. Gould touched on this in one of his essays. In “Male Nipples and Clitoral Ripples” he writes that the male nipples need no adaptive explanation as; “Males and females are not separate entities, shaped independently by natural selection. Both sexes are variants upon a single ground plan, elaborated in later embryology. Male mammals have nipples because females need them – and the embryonic pathway builds precursors in all mammalian fetuses…”

  7. I like my nipples, but I like my wife’s more. 🙂

    I think they are also a function of sexual stimulation for both sexes, no?

      1. That nipples would be sensitive in both sexes follows from their lactation function in females.

        A cross cultural study could deal with the sexual connotations. Are all cultures using breasts as sexual signals? I doubt it, many do not clothe or ornament them especially.

    1. This is as good a place to hang this little “chestnut” (or chestnipple… more apt.)

      Q: Why do males have nipples?

      A: for fun.

      This only works for certain values of “why”, though. …and whenever “titty-twisters” are not involved.

    2. ,,, they are also a function of sexual stimulation for both sexes, no?

      There’s a massage parlor down at the strip-mall, ’round the corner from the Pep Boys auto-parts shop, next to the one-hour-Martinizing dry-cleaners. They can show you there (by the minute or by the hour).

      You’re welcome.

  8. While we are discussing this, people might be interested in knowing that ‘lower down’, a mammal embryo develops the primordia of both male and female reproductive plumbing. In ALL normal mammal embryos, there are a pair of tubes for the male epididymus and vas deferens and another set of tubes for the female fallopian tubes and a uterus! During fetal development, which follows, one set of ‘plumbing’ continues their development while the other withers away. A picture of this combined reproductive system is shown here.

    1. Does that mean when someone tells you to go procreate with yourself (so to speak), they aren’t basing their epithet on a complete anatomical impossibility?

  9. People often don’t realize that, but, after all, a male has the same genome as a female except for the absence of one of the two X chromosomes and the addition of a Y chromosome. Yet all the genes for being female are in that Y-containing genome. What happens is that the Y switches development onto the male track, silencing those genes producing female traits and activating the ones producing male traits.

    Is it possible this is a spelling error, and all the genes for being female are in the X chromosome? What I understand the Y chromosome hasn’t the complete set of genes, but the gender switch mechanism.

    By the way, since supposedly in the average female genes on the X chromosomes are randomly silenced to make expression the same as in males, are these silencing mechanisms the same? Or is every other X chromosome silenced by some other mechanism?

    – Signed, “Gender Confused”

    1. “…Is it possible this is a spelling error, and all the genes for being female are in the X chromosome? What I understand the Y chromosome hasn’t the complete set of genes, but the gender switch mechanism.”

      I, too, first read that wrong, but note that it says that “…all the genes for being female are in that Y-containing genome“, not chromosome.

    2. It’s a somewhat awkward way of phrasing it, but what Jerry wrote is correct: all the genes for building a female body are present in the male (“Y-containing”) genome.

      1. I 2nd this statement. It is common to assert that the ‘sex chromosomes’ determines ones’ sex, but that is not always true. It depends on the chromosome.
        In mammals, the Y chromosome does determine sex since it contains a gene called SRY (Sex Determining Region on the Y chromosome). When this gene is expressed during fetal development, the embryo develops into a male. This typically occurs in XY individuals, and these are the average male.
        Females are usually XX, and they develop as female not so much b/c of the X chromosome but b/c of the absence of the Y chromosome. The X chromosome does not really determine ones’ sex. But I suspect it does carry genes that are ‘feminizing’. My evidence for that is that humans with Turner syndrome have just one sex chromosome. They are ‘XO’, with the O indicating that they are missing a chromosome. These are female b/c they have a female reproductive system, but they are not fully feminized. They have poor breast development, for example. In contrast, humans that are ‘XXY’ have Klinefelter syndrome. These develop as males, but they are feminized males and this may be b/c of the extra X chromosome. They typically have some breast development plus a few other female characteristics.
        That said, most of the genes for male and female characteristics, including genes for the reproductive organs, are located on non-sex chromosomes. The program of male vs female development is started by the presence vs absence of SRY.

  10. I like this bit: “One thing is certain, though: every male carries genes for female traits like breasts, vaginas, ovaries, and the like. People often don’t realize that, but, after all, a male has the same genome as a female except for the absence of one of the two X chromosomes and the addition of a Y chromosome. Yet all the genes for being female are in that Y-containing genome. What happens is that the Y switches development onto the male track, silencing those genes producing female traits and activating the ones producing male traits.”

    In light of this fact I wonder how christians reconcile their mantric belief that if we, humans, are indeed made in the image of God, then how could God be anything other than transgender? Just sayin’.

    1. There are some who are surprised to learn that men and women have the same # of ribs. We have a long way to go with some of them!

  11. … females carry nearly all the genes for male traits: beards, penises, higher aggression …

    Accounts for my mother-in-law …

  12. My understanding is that all traits that aren’t selected for will be degraded and eventually eliminated. But I have heard mention of neutral traits that linger in spite of not being selected for. So I wonder about these latter, “Won’t mutations eventually do away with neutral traits?” And wasn’t Darwin clear on this, that for a variant to persist it had to give a bump up to fitness?

  13. I remember Elisabeth Badinter explaining in an interview that the X chromosome is the basic chromosome upon which all mammals are built, and that the Y chromosome is a variant of the X chromosome. This explains that.

    I often tease misogynist men by telling them that the Y chromosome is simply an X chromosome with one leg missing! 😉

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