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I'd be tempted to call nipples in men vestigial, but that suggests they have no modern function. They do have a function, of course, but only in women. So why do men (and all male mammals) have them?

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I concur with one of the answers in that it is evolutionary not very important to remove features that are not used, specially in the case of female/male traits. Another more dramatic example is that of fish that live at the bottom of the ocean, that have developed new sensory organs to adapt to life without light, but they still have eyes from when they where living in shallower waters where light was present. – 719016 May 6 '12 at 11:35
This is a typical case of if a trait is not advantageous, why does it exist. – WYSIWYG Oct 19 at 7:11

4 Answers 4

up vote 18 down vote accepted

The two key concepts here are

  1. sex-specific selection, and the fact that
  2. males and females share the majority of genes

1) sex-specific selection

Obviously any population where females lacked nipples would be in trouble. Men, on the other hand, have no evolutionary need for them, but they don't pay much either—there is no strong selection against men with nipples. So at first sight, it seems that nipples are positively selected in females while seem to be quite neutral in males.

2) Males and females share the majority of genes

If you consider two separate species where the two species undergo different selection pressures, you will just see one species evolve toward one optima while the other one will independently evolve toward the other optima.

However, males and females are not independent entities. The vast majority of our genes can be found in one sex as well as in the other sex. In other words, male phenotypes do not evolve independently of female phenotypes. As a result of this interdependence, you can end up with the trait that is selected in one sex present in the other sex.

Evolutionary equilibrium

This is all much more rigorously defined in terms of selection coefficients and evolutionary pressure. Without going into the math, the questions of who has the highest selection coefficient and How differential is gene expression for this trait are important questions to predict the equilibrium trait value in both sexes.

Lack of a strong selection pressure

Finally, any trait that is seemingly not-useful has to have a significant disadvantage on the fitness of the organism to be selected out (If a trait would be advantageous to an organism, why hasn't it evolved?). Even if a trait is useless for both males and females it may persist. The case of females needing the trait just makes its elimination in males even more difficult, as explained above. However, in some mammalian species the males do lack the nipples (Evolutionarily, why do male rats and horses lack nipples?).

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Sorry to spoil the fun here, but male nipples are not completely useless. With stimulation and hormones, they can be used to make milk. I don't have a great peer reviewed source for this, because it is sort of common knowledge. I heard about it at a Le Leche Leage meeting when my son was small.

This question could even be considered culturally biased

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This is interesting: are there any documented instances of males (in any mammalian species) actually providing nutrition to their young this way? Don't get me wrong; I certainly don't want to oppress men by suggesting that they shouldn't breastfeed if they want to, I've just never heard of them doing it. Maybe a question for SE.Parenting? – Shep Feb 2 at 0:17
I remember seeing a male breast feeding in a discovery documentary long time ago. Though I was very small then and don't remember the name of the program. – One Face Feb 2 at 1:12
It seems like it is more common in people and domesticated animals, but here is An Article about a bat that regularly suckles its offsrping, Here is a recent general article about this topic – axsvl77 Feb 2 at 1:56

No one has mentioned the Neutral Theory of Evolution, which explains mutations that are not necessarily motivated by increased "fitness".

Similarly (and more to the point for this question) there is no selective pressure with regards to mens nipples.

Men have nipples because they find a purpose on women, but for men there is no reason to not have them. From an evolutionary stand point it is simpler for men to have them. The more complex situation (Women have nipples but men do not) would most likely only occur if there was some selective pressure for men to not have nipples.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

No Preece, it is not saying the female body plan is the default one. It is saying that if a trait is necessary for one gender, it is simpler to have it for both genders as well, unless it hinders the gender that doesnt' require it. – Mew Jan 7 '13 at 14:09
I agree that males probably have nipples just because there is positive selection in females for having nipples while it is neutral in males (there is gender specific selection coefficient for having nipples). However, this is not exactly what the Neutral Theory of Evolution states! This is probably why no one mentioned the Neutral Theory of Evolution. Therefore, the answer seems a bit misleading. – Remi.b Feb 1 at 23:41
Also, the sentence mutations that are not necessarily motivated by increased "fitness" should better be replaced by the existence of alleles present at high frequency even though they don't confer a selective advantage (that is, they don't confer higher fitness to the carrier). – Remi.b Feb 1 at 23:43
I'm astonished that this answer has 20 upvotes and was accepted. The correct answer to this question has nothing to do with the neutral theory of evolution (which, as Remy notes, is not the same as observing that some traits are neutral) and everything to do evolutionary constraint (in this case, with the timing of sex determination in the developing embryo). – Corvus May 11 at 6:12
This answer is wrong. The most active users of Biology.SE are considering the problem here. – Remi.b Oct 20 at 18:05

I believe it is for this reason: the female body plan is the default one. Males are a variation upon that, in humans at least. Nipples are part of the basic body plan. For a man to not have them, he would need to actively evolve something that would prevent nipples from developing. There is no selective pressure for the development of such a thing, so it hasn't happened. Keep in mind that the code for the general body plan is shared between males and females. The Y chromosome modifies the development of that body plan so the person becomes male.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

I think your answer makes sense from the 4th sentence on. As for the first three sentences: why would you define the female body plan as the "default" when half the population is male? Consider the opposite stance: given that the Y chromosome has fewer genes, it seems just as reasonable to say male is "default", female a modification. The point isn't that either is default, only that referring to either as default is subjective at best. – Shep May 4 '12 at 11:12
Female is default because organs first develop as female in the embryo and fetus, then transform into male. Also bear in mind that both sexes have X but onlymale have Y. – Armatus May 4 '12 at 11:17
@Shep: Preece and Armatus are right, 'default' refers to sex determination. At the beginning of development, the cells have bipotential state. Then, the Sex-determining Region of the Y chromosome (SRY gene) express a transcription factor that initiates male development. In absence of Y chromosome, the human embryo develops as 'default' into a female. – Gianpaolo R May 4 '12 at 12:50
@Armatus, that is true, but suffers from the ergo hoc proptor hoc logic fallacy. It's reasonable to say that the process of organ development is simply a hold over from the trajectory of how mammals have evolved. – leonardo May 5 '12 at 18:14
Consider this. If a boy is born without the receptors for androgens, they will develop physically as a female. Check it out: – Preece May 6 '12 at 5:34

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