|MadSci Network: Medicine|
Question: ‘Why do men have nipples?’
I can do no better than to direct you to the following book: Bully For Brontosaurus - Reflections In Natural History by Stephen Jay Gould, 1991 - Random House or Penguin paperback. The relevant section is Chapter 8 - ‘Male Nipples and Clitoral Ripples’ This chapter is also published in the smaller, and cheaper, Penguin 60s’ edition, Adam’s Navel and Other Essays, by Stephen Jay Gould.
One oft-suggested answer, that in prehistoric times males could suckle their young, is unlikely, unproven and probably nonsense. The mistake we often make about human (and mammalian) anatomy is in assuming that every piece of us has to have a logical, practical, useful reason for its existence. Gould argues that many of our bits are simply accidental left-overs of our evolutionary past. Nature takes the path of least resistance, so once an item of anatomy is in place it tends to remain, unless it is decidedly evolutionary disadvantageous. Ear lobes are an example; they are there because they are there. As for nipples, despite the way we act sometimes, men and women are of the same species, and the basic biological blueprint for a human being is female. Males are a variation on the theme. That should surprise and upset male chauvinists, but I am speaking physiologically here, not culturally or socially.
While our gender is (in most cases) determined at the moment of conception, it is damnedly difficult to identify the sex of a fetus until it is quite advanced. In a young fetus, to quote Gould: ‘The clitoris and penis are one and the same organ, identical in every form, but later enlarged in male fetuses through the action of testosterone.’ The same relationship exists between the labia majora of women and the scrotal sac of men.
A reverse variation on your question would be: ‘Why do female hyenas (appear to) have penises?’ This puzzling phenomenom was very evident in one of David Attenborough’s ‘Wild Screen’ documentaries. The answer is the same. The female hyena’s ‘penis’ is, in fact an extremely elongated clitoris, and it can be no coincidence that hyena society is strictly matriarchal - female dominated. This is covered in detail in another S.J. Gould book, ‘Hen’s Teeth And Horse’s Shoes’ - Chapter 11 ‘Hyena Myths And Realities.’
Long answer; here is the short one. Gould again:
‘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 to their development builds precursors in all mammalian fetuses, enlarging the breasts later in females but leaving them small (and without evident function) in males.’
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