Andrew M. Simons, a professor of biology at Carleton University in Ottawa, Ontario, explains.
Like all "why" queries, the question of why men have nipples can be addressed on many levels. My four-year-old daughter, always suspicious of a trick when asked such obvious questions, answered: "because they grow them." In search of the trick answer, she quickly added that "chests would also look pretty funny with just hair."
Evolutionary biologists, whose job it is to explain variety in nature, are often expected to provide adaptive explanations for such "why" questions. Some traits may prove—through appropriate tests—to be best explained as adaptations; others have perfectly good evolutionary, but nonadaptive, explanations. This is because evolution is a process constrained by many factors including history, chance, and the mechanisms of heredity, which also explains why particular attributes of organisms are not as they would be had they been "designed" from scratch. Nipples in male mammals illustrate a constrained evolutionary result.
A human baby inherits one copy of every gene from his or her father and one copy of every gene from his or her mother. Inherited traits of a boy should thus be a combination of traits from both his parents. Thus, from a genetic perspective, the question should be turned around: How can males and females ever diverge if genes from both parents are inherited? We know that consistent differences between males and females (so-called sexual dimorphisms) are common--examples include bird plumage coloration and size dimorphism in insects. The only way such differences can evolve is if the same trait (color, for example) in males and females has become "uncoupled" at the genetic level. This happens if a trait is influenced by different genes in males and females, if it is under control of genes located on sex chromosomes, or if gene expression has evolved to be dependent on context (whether genes find themselves within a male or a female genome). The idea of the shared genetic basis of two traits (in this case in males and females) is known as a genetic correlation, and it is a quantity routinely measured by evolutionary geneticists. The evolutionary default is for males and females to share characters through genetic correlations.
The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them. Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them. In a sense, male nipples are analogous to vestigial structures such as the remnants of useless pelvic bones in whales: if they did much harm, they would have disappeared.
In a now-famous paper, Stephen Jay Gould and Richard C. Lewontin emphasize that we should not immediately assume that every trait has an adaptive explanation. Just as the spandrels of St. Mark's domed cathedral in Venice are simply an architectural consequence of the meeting of a vaulted ceiling with its supporting pillars, the presence of nipples in male mammals is a genetic architectural by-product of nipples in females. So, why do men have nipples? Because females do.
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21 Comments
Add CommentIt seems to me that all fetuses, male and female, initially develop as females, which is the only prevailing sex hormone around. Hence the nipples. When the fetus' pituitary develops and estradiol can convert to testosterone, then the male characteristics beging to show. Had there been no testosterone, the child would probably be a hermaphrodite. However, all humans have both estrogen and testosterone, so this is rarely a problem. The same chemical (FSH or follicle stimulating hormone) floods the secondary sex hormone. However, the conversion to primarily testosterone is what causes the male characteristics.
Reply | Report Abuse | Link to thisAre we the only males in the mammal kingdom that have nipples?
Reply | Report Abuse | Link to thisNo, almost all mammals have nipples in both the male and females. Proving this isn't just a human thing but takes place in all species that use this form of reproduction.
Reply | Report Abuse | Link to thisNo, almost all males in mammals have nipples. From Armadillos to Zebras. Suggesting that this is not a human phenom but a mammal one. Perhaps our reproductive systems.
Reply | Report Abuse | Link to thiswhen formation embryo in first stage it is not asign sex , but it my be function of many thing that effects embryo , nipple is one of the main structural in body if any respond came to change the sex during that stage there is no any structure, what will happened ,
Reply | Report Abuse | Link to thisdamage to embryo .
It's because male mammals a just a historical afterthought. The primal human stock was probably female.
Reply | Report Abuse | Link to thisCleva
Reply | Report Abuse | Link to thisPlease. We are trying to discuss science here. Not superstition
To the biblical creationist: Your 'argument' is ridiculous because, in the typical tunnel vision of a religious zealot, you neglected the FACT that ALL male mammals have nipples. Are you saying that God created animals in his image too? Or that he created every mammal from Adam? Or are you saying that God was limited in his scope and had to create every single male mammal first, like Adam, in order to make a 'prototype' from which to use a rib for making a female. Pathetic attempt, you get an "E".
Reply | Report Abuse | Link to thisI did not like the explanation in the article (although it was aimed at the lay audience) because it was not very straightforward to grasp. Here is mine: from a genetic standpoint, since nipples development requires the activities of so many genes located throughout the genome, there is no way for these structures to have disappeared in male mammals, caused by the accumulation of errors in those genes, because this very same process of gene abrogation would also make the females nipple-less. The latter scenario would be, obviously, under a strong negative selection. So, in short, we could not say "Good bye!" to nipples because the females use the same genes to build them as we males do!
Reply | Report Abuse | Link to thisUm, I think he was just talking about Humans. For female animals, nipples are also needed. So that would simply mean that the same thing happens with animals. No, animals did not come from Adam. Nipples are developed just as an arm or brain, and all males have both testosterone and estrogen,implying that Eve could come from Adam but still be a female. Nipples don't have anything to do with sex, they're just "there", same with animals. They can only be activated in females after puberty.
Reply | Report Abuse | Link to thisI like the way you put that, EvoLover.
Reply | Report Abuse | Link to thisTo the creationists
Reply | Report Abuse | Link to thisThe above discussion shows me why men have nipples. I believe in evolution and to someone like me the fact that men have nipples is in accord with evolution. But if you are a creationist you don't believe in evolution and you believe that man was created in God's image. So if Adam had nipples and he was made in God's image then God must have nipples. We men got our nipples from the genes of our mothers but God didn't have a mother did he? So why would God have nipples? Why would he need them. Explain that one!
Let me give you another one. Why would God have legs? Couldn't he just be where ever he wanted to be? I mean he's God isn't he - the boss of all he sees. Why would he want something so slow and cumbersome as legs. I could sort of understand it if he had wings but he didn't even have wings poor thing.
If he was really God he would have given us wings just like the birds. That would be so handy and so much fun.
All this is circumstantial proof for the non-existence of God wouldn't you think eh?
Yea my comment to the Adam and Eve thing...Do you think it's at all possible that humans, being as smart as we are, could make up a story as believable as the Bible. How do you know for certain that your story wasn't written by some MAN who was trying to make a buck. I mean...that would be the only time in history has ever done that right?
Reply | Report Abuse | Link to thiscoprogirl - interesting theory however is it feminist or fact?
Reply | Report Abuse | Link to thisray stasionis - i hope you intended the question rhetorically.
Mr.Cox21 - it feels good to "know" the answer doesn't it?
olipoli - coprogirl
worrixx cleva witbooi - took a wrong turn at Albuquerque huh?
Mona Lisa - good point.
In a natural selection sense it seems interesting that the male nipple has survived as "visible" as it is today. The female of our species seems to prefer males with smaller, more benign nipples, or more generally speaking, a chest that is less feminine (lacking the presence of breasts.) It is sensible to speculate that historically the female has sought after a masculine male, or one that is clearly lacking feminine physical traits.
I would speculate that the male nipple would have, through natural selection, become less "present" on the male (becoming smaller, etc.) over time (into the future) had it not been for a byproduct of the modern age: the preservation of all members of the species without regard to genetic viability or survivability. Since food is plentiful in the developed world, and all level of genetic suitability now procreates, those traits previously in the process of evolving into nonexistence are now preserved. There no longer is an appreciable "survival of the fittest" quality to our species. It is possible that in generations to come, the male nipple, or even chest, becomes more feminine for a variety of factors too complex to elaborate on in this post.
Is it possible that maybe both male and female mammals at one time were able to nurse their young? I mean we can attempt to relate other characteristics to adaptation such as this, why not on this subject? I know you cannot really "prove" it fact, but I don't see why it can't be a theory. I guess I'm just curious about the evidence against this.
Reply | Report Abuse | Link to thisHey I know these comments are old but I just don't understand why most people are so cruel and patronizing to those who believe in Intelligent design. It is not irrational or illogical and many scientists have admitted that. It seems that one who swings completely to the extreme of evolution and one who swings completely to the side of no science are the ones who are most lost in their reasoning. Not all who believe in Intelligent design discount science and certainly not microevolution...but when you find yourself all the way to one extreme is when you will be lost. Finding the balance and being open minded to TRUTH is when you will see clearly, but let your emotions and arrogance cloud your vision and you will never find it.
Reply | Report Abuse | Link to thisAdam and Eve were not actual people. The Catholic church has confirmed that. Science and religion are both looking for the same answer, but they go about it two different ways, which is perfectly fine! Everybody needs something to believe in; some choose science, and some choose a God. It is YOUR own personal choice.
Reply | Report Abuse | Link to thisThe nipples were a major evolutionary milestone for earthly creatures. This appendage addition to a group of earth organisms is perhaps second only in importance to the introduction of the vertebrae. Nipples are the single, most defining feature that separates mammals from other animal kingdoms. It enables us to care for our young in a direct and superior way to other creatures. Being able to produce milk gives mammals an evolutionary edge since frail babies would no longer have to eat dead worms or dead rats or poisonous nuts as other species do. Also, milk wraps the baby in an invincible shield of antibiotic resistance to diseases.
Reply | Report Abuse | Link to thisSaying men should not have nipples or to question why men have nipples is similar to saying why do women have vertebrae since they evolutionarily didn't hunt? Nipples are present in ALL mammals be they male, female, hermaphrodite, or asexual because it is such a defining feature of who we are as a species.
Fingers would be rendered evolutionarily obsolete before the nipples do.
I find it funny how no one has mentioned male lactation.
Reply | Report Abuse | Link to thisI thought it was pretty well determined that since males can lactate, in extreme circumstances having the equipment is a survival advantage.
Reply | Report Abuse | Link to thisThere is an overlooked fact here: male rats & mice do NOT have nipples. the question is why?
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