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:
- 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.
- 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.
- 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.