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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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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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this explanation is much more to the point than any other answers. –  Shep Jun 4 '12 at 7:20
    
@Shep And yet it misses the most crucial piece of information, while simply restating other things that have been said. In fact, his second point is almost an exact restatement of something I said. The female body plan is the default one, and that fact is the heart of a real answer. –  Preece Jun 4 '12 at 13:35
    
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
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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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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
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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
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@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
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Consider this. If a boy is born without the receptors for androgens, they will develop physically as a female. Check it out: en.wikipedia.org/wiki/Androgen_insensitivity_syndrome –  Preece May 6 '12 at 5:34
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Yes, I read it: "The gonads are testes regardless of phenotype due to the influence of the Y-chromosome. A 46,XY female thus does not have ovaries or a uterus, and can neither contribute an egg towards conception nor gestate a child." This means that if you have XY chromosomes, you will never be a normal woman, even if you insensitive to androgens. –  Anixx May 7 '12 at 9:55
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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.

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This is because genetic information is shared between males and femeles.

The share of large amount of genetic information between sexes if evolutionary advantageous. The most selective pressure is put on men by the natural selection. Thus men need muscles, flexible body, good vision, speech abilities and so on to get food and to compete with other men in warfare.

Most of these advantages get transferred to women even though the selective pressure on them is much smaller. Women have no less vision or ability to speak than men do.

Similarly women undergo the maximum pressure from sexual selection, especially regarding the appearance. These traits are also somewhat transferred to men, otherwise men would possibly appear much uglier than they are now because good external appearance is not what is necessary in warfare and work.

There are other examples of shared genetic information: for example our left and right hands anatomically nearly identical despite having different functions. This is because advantages developed for right hand also used in the development of left hand. If there was no such share, our hands would be very much different with the right one much more advanced.

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This kind of trainwreck can only result from someone who only has a very tenuous grasp on evolution. It is just broad stroke evolutionary anecdotes pulled from pure imagination. –  Preece May 7 '12 at 9:49
    
What's wrong with this answer? –  Anixx May 7 '12 at 9:56
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It has no scientific basis. It's just random conjecture. –  Preece May 7 '12 at 9:59
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You clearly show that you do not yet understand evolution. Two examples: 1) 'Selective pressure'. Scientists don't agree what exactly selection acts on (genes, individuals, groups,...), but no serious scientist would ever claim that it acts on sexes. 2) You are using the point of view that things evolve because they are needed. Even school biology covers that this is not true. Your understanding of what 'sharing genetic information' means also needs improvement. I suggest en.wikipedia.org/wiki/Introduction_to_evolution –  Armatus May 7 '12 at 10:18
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Also you seems that you hever heared about intralocus sexual conflicts en.wikipedia.org/wiki/Intralocus_sexual_conflict –  Anixx May 7 '12 at 10:37
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