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In course of evolution (possibly), mammary glands became vestigial in male mammals, but became fully developed in females. Is there any plausible explanation for this characteristic?

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    $\begingroup$ what exactly do you mean by plausible explanation? $\endgroup$ – WYSIWYG Dec 2 '14 at 12:19
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    $\begingroup$ Male mammary glands aren't vestigial, they're embryologically related to female mammary glands. So the question is "why are mammals sexually dimorphic" which gets tautological because the clade is defined in terms of mammary glands in the first place. $\endgroup$ – Ryan Dec 2 '14 at 13:17
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    $\begingroup$ @Ryan Don't provide answer in comments. You can post it as another answer (saying "addition") if you provide some extra points that are not mentioned in previous answers. Use comments only for requesting clarification or providing suggestion related to post (not content). $\endgroup$ – WYSIWYG Dec 2 '14 at 13:31
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    $\begingroup$ Perhaps due to the close hormonal interplay between pregnancy and lactation it is more efficient to have the females generate milk. To let a male start to lactate a new mechanism needs to be developed in evolution, because a continuous 'just-in-case' lactation is energetically cumbersome. Developing new characteristics is not favored in evolution. Rather, coupling of a lactating mechanism to pregnancy (breast tissue growth) and labor (start of lactation) seems an evolutionary more parsimonious way of regulating lactation. $\endgroup$ – AliceD Dec 3 '14 at 1:06
  • $\begingroup$ Just a note - they're not fully vestigial. Male lactation occurs in some species, including humans (the same mechanism that starts lactation in females still works for males). The Dayak fruit bat in particular has full blown lactating males. But yeah, taking mammals as a whole, these are relatively tiny exceptions. If you care about humans, though, yup, males can lactate just fine (though I have no idea about the amount or quality of milk so produced). $\endgroup$ – Luaan Mar 14 '16 at 8:46
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It comes down to investment in offspring. Males invest little in offspring only a little sperm whilst females have to develop the more expensive egg and the energy used through its development. In the mammalian line, this investment has increased in amount, initially through viviparity and then through the secretion of feeding fluids.

In species, such as humans, where there is a lasting pair bond and both male and female invest in rearing the young then I suppose there could be a situation where it makes equal sense for both males and females to nurse - although the cost of mammaries on other functions may favour specialisation anyway - but remember that, in many mammals, it is the mother alone that rears the offspring.

You may also find the answer to this question useful: Do any birds beside the family Columbidae (or any reptile or mammal) feed their young “Crop Milk”. What I hadn't realised is that there actually two species of bats where the male lactates.

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    $\begingroup$ It is not generally so that males invest little in offspring, especially not in humans. In fact, it is hypothesized that the rapid growth in brain volume in homo was made possible, at least in part, through the contribution of males in rearing the offspring (by gathering food etc.). Moreover, the 'more expensive egg' can be debated, as the vast amount of sperm cells generated during a male's lifetime vastly outnumbers the resources needed to ripen the relatively small amount of eggs during a female's life. $\endgroup$ – AliceD Dec 3 '14 at 0:59
  • $\begingroup$ I addressed your first point in the second paragraph. Male investment is high in humans, it is not in mammals in general and the dimorphism in mammaries did not evolve in humans. As to your second point, it's certainly true that males invest in reproduction to similar levels to females but in the individual offspring the investment is higher in the female. $\endgroup$ – Jack Aidley Dec 3 '14 at 10:14
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The mammary glands in human males is not vestigial but rudimentary.

Human females have XX chromosomes and males have XY chromosome,here the "Y" chromosome is provided by father and "X" by the mother. At the time of fertilization, the father can provide either X or Y chromosome. If he provides Y chromosome the child will be male and if he provides X then child will be female. The mother always provides X chromosome. So the Y chromosome in the male child is responsible for the development of male sex organs and due to the presence of X chromosome the female sex organs are suppressed. Note that the female organs are suppressed but they are not eliminated. At the time of puberty males don't receive hormones which initiate the development of female sex organs. As a result the mammary glands stay in males as nipples but the hormone effects are only present in females and cause the formation of large amounts of fatty tissue near the nipples and develop as breasts.

A distinguishing characteristic of the class Mammalia is the presence of mammary glands. The mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Only mammals produce milk. Mammary glands are most obvious in humans, as the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts. http://en.wikipedia.org/wiki/Female

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While estrogens are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sexual characteristics, such as breasts. http://en.wikipedia.org/wiki/Estrogen

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    $\begingroup$ The last sentence in your wikipedia quote is contradicting the first sentence in your answer. $\endgroup$ – fileunderwater Dec 2 '14 at 13:26
  • $\begingroup$ @fileunderwater: I saw it but it may be because they just used "vestigial" as an inappropriate organ(literal sense)they may not be thinking the controversy in using "vestigial" and "rudimentary". $\endgroup$ – Jayachandran Dec 2 '14 at 13:31
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    $\begingroup$ I rather see it as a reason to have healthy scepticism towards unreferenced statements in Wikipedia. But the reason is probably, as you say, how they define 'vestigial' (I don't think human nipples should be labelled 'vestigial' using normal definitions of the word though). It makes for a contradiction in your answer though. $\endgroup$ – fileunderwater Dec 2 '14 at 13:43
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    $\begingroup$ @fileunderwater:I just edited the answer and removed the portion, but I think its unfair to remove that portion, but it is for a good reason.Also you are thinking that it shouldn't be labelled as "vestigial". I have learned many things from your past comments and I am hoping your assistance will improve my knowledge and my approach towards SE.Thank you !! $\endgroup$ – Jayachandran Dec 2 '14 at 13:51
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    $\begingroup$ I think this answer, while of decent quality, does not actually address the question the OP is asking. This is the mechanics of how there is a difference not the reasons why there is a difference. $\endgroup$ – Jack Aidley Dec 2 '14 at 16:12

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