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Why Do Men Have Nipples?


Why do men have nipples? This is what Scientific American has to say: "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.'"

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. 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? 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.

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. 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. We should not immediately assume that every trait has an adaptive explanation. 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.

why do men have nipples



In LiveScience, Laura Geggel gives us the following info about male nipples in the womb:

During the first several weeks, male and female embryos follow the same blueprint, which includes the development of nipples. However, at about six to seven weeks of gestation, a gene on the Y chromosome induces changes that lead to the development of the testes, the organ that makes and stores sperm and produces testosterone, according to the book "Exploring the Biological Contributions to Human Health: Does Sex Matter?" (National Academies Press, 2001). After the testes are formed, the male fetus begins producing testosterone at about nine weeks of gestation, changing the genetic activity of cells in the genitals and brain. But by then, those nipples aren't going anywhere.

But why do men keep this vestigial structure? (A vestigial body part does not serve an evolutionary purpose.) Furthermore, just because men don't need nipples, it's not exactly an evolutionary priority to get rid of them.

Carina Hsieh, in Cosmopolitan's "Why Do Men Have Nipples?" discusses the development and sexual aspect of men's nipples. Nipple development occurs about a week before sexual differentiation occurs in embryos, which is why both men and women have nipples. By the time the Y chromosome and hormone testosterone responsible for bringing about penis and testicle growth to a male embryo happens, the nipples have already developed at that point, so they remain.


Did you know?

  1. Men can get breast cancer because they have the same breast tissue as women, so doctors should keep an eye out for lumps or changes around nipples and areola in patients of all genders during skin exams.
  2. Men can lactate. If you give enough female hormones to men, there's a chance some could convert their existing breast tissue into lactating tissue.
  3. Here are facts about sex and nipples: Male nipples can be just as sensitive as nipples in women, because both men and women have the same nerve bundles in the nipple. It's even possible for men to experience a nipple orgasm. Here's how to achieve it: Circle the pigmented portion of the nipple with your fingers or mouth until the nipple becomes hard, and then stimulate each nipple with gentle rubbing or slight pinching. "The harder the nipples become, the more sensitive they are to direct stimulation," Rodgers says. At this point, you can stimulate both nipples at the same time, and incorporate pulling and sucking on his nipples as well. "Much like the moment before a woman orgasms, don’t stop but don’t keep going full steam, either.
  4. Men can develop breasts. Dr. Cruise, a leading expert in gynecomastia, explains that the condition is when breasts become enlarged due to excess breast tissue, excess fat, or both. While breast tissue is both squishy and firm, breast fat tends to be flabby. The cause of gynecomastia is multifactorial," Dr. Zuckerman explains, but the excess breast tissue can be caused by a hormonal imbalance, pituitary tumor, anabolic steroids, among other causes. As far as male breast reduction, this is completely different than female breast reduction. In men, gynecomastia surgery reduces an enlarged breast using liposuction or cutting out excess skin, whereas female breast reduction surgery opts to remove excess volume and heaviness in breasts.
  5. Men can have accessory nipples. With male accessory nipples, doctors should check to make sure there's no cancer developing in them, as the accessory nipples contain actual breast tissue.


Not everyone thinks nipples are necessary. This is what Signe Dean says in ScienceAlert: They're pointless. Why do men and other male mammals have nipples if they don't feed their young? What's the point? When we think about traits from an evolutionary perspective, we often think in terms of 'What is this thing good for?' It turns out that the case of male nipples somewhat contradicts that very question. It's not necessarily a feature produced by evolution - instead, it's more like a feature that evolution didn't bother to get rid of, or possibly even couldn't. All mammal embryos - male and female - start out looking exactly the same, with potential to develop into either sex. After the first few weeks a gene called SRY kicks in, triggering the genetic switch that sends the embryo down the male development path.

why do men have nipples



What about milk? Elise Andrew answers this question and more in IFLScience. 

  1. Men generally lack the necessary levels of prolactin to stimulate lactation and cannot produce milk.The answer comes down to timing of sex determination during embryonic development.
  2. Humans are mammals, which means they are warm-blooded, hairy vertebrates that breathe air and produce milk for babies.
  3. Up until genes on the Y-chromosome kick in after week 4 in development, however, male and female embryos develop identically. The primary formation of mammary glands and tissues are highly conserved across mammalian species and begin to form early in development, before the gender-specific processes take place.
  4. The embryo’s gonad appears around week 4 of development and is considered bipotential or indifferent, meaning that gender is not playing a role in development at that point.
  5. During week 8, germ cells start to undergo sex determination. Males will then secrete factors that block the development of female ducts and structures. Once the male embryo produces testosterone, the hormone can influence other sex-specific traits around the body.


    Men having nipples doesn’t really have any evolutionary advantage, but it usually doesn’t hurt anything either. As a result, the trait was never selected against. Developing those structures must also not be very energetically costly in the grand scheme of things. Most of the work with developing breast tissue and mammary gland function in females happens during puberty, while prolactin levels aren’t ramped up until pregnancy. Men account for less than 1% of all breast cancer cases, but it can happen. Risk factors include estrogen levels, obesity, alcohol consumption, and liver disease.

    In the BBC article “Future - Why Do Men Have Nipples?” we learn more about men and nipples. During the early stages of embryonic development we are generic, neither female or male. We are following a female blueprint until the testosterone kicks in. As far as animals go, male Dayak fruit bat produce milk - not certain whether they help feed the young.

    A mammal usually has twice as many nipples as it needs in order to feed it’s off spring. That’s why we don’t have one or three. Most but all mammals have an even number of nipples. They begin from the same genetic blueprint. After about 6 weeks a Y chromosome gene kicks in some changes to the fetus. -- testes int he male -- after two more weeks the genetic activity of cells in the genitals and brain begin to change and by that stage the nipples are here to stay. Male lactation can be stimulated by simply letting a baby suckle for several weeks, though it's not the biological intention. But, the presence of male nipples is not a threat to the survival of the species. Nature is unpredicable and messy with strange quirks. A great resource about breastfeeding is The Tender Gift by Dana Rafael.


    Nikhil Swaminathan expounds upon men's lactation in Scientific American's “Strange but True: Males Can Lactate.” article. He tells us there have been countless literary descriptions of men miraculously breast-feeding, from The Talmud to Tolstoy, where, in Anna Karenina, there is a short anecdote of a baby suckling an Englishman for sustenance while on board a ship. The little anthropological evidence documented suggests it is possible. And in her 1978 book The Tender Gift: Breastfeeding, medical anthropologist Dana Raphael claimed that men could induce lactation simply by stimulating their nipples, and iinsists that in order to produce milk, a hormone spike must occur.


    Newman explains that medical disruptions involving prolactin, the hormone necessary to produce milk, have resulted in spontaneous lactation. Lactation is listed as a possible side effect of the heart medication digoxin. A pituitary tumor could also induce milk production. In addition, starvation—which inhibits the functioning of hormone-producing glands as well as the hormone-absorbing liver—can cause spontaneous lactation, as observed in survivors of Nazi concentration camps and Japanese POW camps in World War II. In short, men may not have full-fledged breasts but they certainly can lactate, under extreme circumstances.

    It’s more likely in human men than in the vast majority of mammals. That said, don’t worry, fellas. You’re highly unlikely to start sprinkling milk out of the blue. You might suspect, the cause is genetic—22 of the 23 pairs of human chromosomes are identical, with the 23rd pair the only set that differs between the sexes. “The genes on chromosome 23, acting in concert with genes on other chromosomes, ultimately determine all differences between our sexes,” wrote Pulitzer Prize winner and one-time physiologist Jared Diamond in a 1995 article for Discover.


    Those differences, of course, include not only the possession of ovaries as opposed to testes but also the post-adolescent differences in beards, pitch of voice, and breast development. In this article, Diamond provided several cases “in which [not pregnant] women succeeded in nursing an infant by repeatedly placing it at the breast.” Most adoptive mothers, for instance, begin lactating within three or four weeks of adoption, suggesting mere stimulation can induce enough hormonal action to produce milk. Another example includes a 38-year-old man in Sri Lanka who, in 2002, nursed his two daughters through infancy after his wife died during childbirth, according to an Agence France-Presse report.

    All male and female mammals have mammary glands, which is the organ that produces milk. When mammary glands mature fully in females during puberty, they develop into a state where a hormonal spike—most notably of prolactin—can easily induce lactation. For males, the gland generally doesn’t mature to that level. The difference is a matter of hormonal influence, first during puberty and then again after pregnancy. Men do have the necessary equipment, but usually they don’t naturally produce the necessary levels of hormones to use it. Provide the right hormones, though, and male lactation is absolutely possible. Medical intervention may be needed by introducing both estrogen and prolactin into the system, often by injection. Some drugs, like Thorazine—an antipsychotic popular in the mid-20th century—and the heart medication digoxin have induced milk from men as a side effect.

    “The remaining obstacle will then no longer be physiological but psychological: Will all you guys be able to get over your hang-up that breast-feeding is a woman’s job?”

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