LAB Medicine - file 02
Men's Nipples, What are you doing?
In biology, in theory, evolution should properly work for the “survival of the fittest” (Herbert Spencer) as evolutionary optimization. The invisible hands of nature should be flawless. And we humans have some vestigial organs, such as the Coccyx (tailbone), for the evidence.
If natural selection should be perfect throughout the path of evolution, what purposes in the World do men’s nipples exist for? In principle, breastfeeding is a role exclusive to female mammals. Are men's nipples nature's certain intention, or one of nature's rare errors?
We ALL are female in the beginning
In biology, the vestigial organs are NOT technically leftovers from an ancestor that once used the organs properly functioned, but rather a structure that has lost All, or most of, its original functions through evolution.
The growth of the embryo in the uterus is fantastic, as if All the various creatures, including us Humankind, trace nearly the millions-year history of evolution from the primordial form! Actually, in our mother's womb, human embryos (up to the 8th week) follow the default blueprint for several weeks.
Between Weeks 4 and 6 of gestation, mammary tissues and nipples begin to develop. And then, until about Week 7, the Sex-determining region Y protein (SRY) gene on the Y chromosome doesn't yet start triggering male hormonal development. It means that until Week 7, we ALL had been female.
By the time our body begins producing male hormone, Testosterone, to develop male sex organs, our nipples are already built for us ALL. Evolution hasn't removed men’s nipples because they don't hurt any male’s chance of survival or reproduction.
Men's nipples are harmless, but useless?
While men’s nipples don't serve the primary biological purpose, lactation, they aren't altogether inert.
Men's nipples are highly innervated. Dense networks of sensory nerve endings are gathered within and directly under the skin of men’s nipples as much as women's. As some study found, simulations on men’s nipples enhance sexual arousal in approximately 52% of men. But it's a shame that I myself am one of the exceptional men of 48%...
Moreover, men's breasts contain the same basic components, mammary ducts and glands, as women's. So under some exceptional hormone conditions, for example, certain hormonal medications, starvation recovery, or pituitary tumors inside the brain, etc., men can actually produce and secrete milk. In medical terms, this condition is called Male Galactorrhea. So men's nipples are equipped with the proper mechanism to work.
In evolutionary biology, a trait that is an unintentional byproduct of the development of another characteristic, rather than a direct product of adaptive selection, is often called “Spandrel.” So men's nipples are a consequential “spandrel” of the human evolutionary blueprint that only becomes functional with female hormones.
Why haven't men's nipples disappeared in evolution?
Evolution is NOT always about perfection, but rather about efficiency. In principle, the logic of evolution is usually that if NOT broken, it is NOT necessary to fix it. So if harmless, let it be! Following this logic, evolution has made men's nipples remain part of the male anatomy.
Lack of Selective Pressure
For a trait to disappear through natural selection, it usually needs to be detrimental. For example, if having nipples caused males to die younger or to fail to find a mate, the harmful nipples would (or should) eventually be removed. But in reality, at least so far, nipples inflict NO damage on men's bodies like an accessory, and moreover, require zero metabolic energy to maintain. So men's nipples are cost-FREE, and have presented NO fatal pressure to persuade evolution to get rid of them.
Genetic Conservation
The genes for nipples are located on autosomes, which are Non-sex chromosomes. This means that both males and females have inherited the common blueprint to build nipples. So even if evolving to turn OFF nipple development ONLY on males, it would require a specific complicated genetic mutation to erase nipples ONLY for males. Since there is NO benefit to losing nipples for men, such a cumbersome mechanism hasn't been favored by evolution. So men's nipples exist by natural selection to let them be.
Biological Spandrel
As I’ve mentioned above about the “Spandrel” in evolutionary biology, in this sense, men's nipples are a typical spandrel, an unintended byproduct of how we develop in the womb in the early stage. Because we ALL are female for the first several weeks, the cost of removing the initial step ONLY for males out of our common blueprint is significantly more expensive than simply letting harmless nipples remain. So this is about evolutionary efficiency.
Men's nipples might be a backup plan for future pregnancy for men?
Evolutionary preparation usually involves maintaining a basic structure that can be scaled up for probable future necessity. In terms of this principle, men's anatomy completely lacks the hardware for gestation, even though equipped with nipples and mammary ducts for lactation.
A female's uterus is a highly specialized organ for gestation. The womb does Not Only hold a baby, but also manages the proper immune system, lest the body mistakenly rejects the fetus as an antigen, a foreign invader.
A female's vascular system is also specialized for gestation. A female's pregnant body needs to increase its blood volume by about 50%, and also create massive NEW arterial pathways to the placenta in order to provide enough nutrition and fresh oxygen for a baby. But male anatomy does NOT have the pre-existing wiring to support such a vascular system without a uterus.
Female Anatomy as the Default Blueprint
Men's nipples have already existed since embryonic time, using the common blueprint of Humankind for the first 6 weeks of Life. After that stage, once the Y chromosome (male sex chromosome) has triggered male development, the body actively begins suppressing the female reproductive system as a whole, including the uterus and fallopian tubes, by secreting the Anti-Müllerian Hormone (AMH). This hormone is a key to diverging the way for sex differentiation. Therefore, if nature wanted to keep a slight chance for male pregnancy as a future backup alternative, evolution must NOT have installed such a powerful OFF switch.
Anti-Müllerian Hormone (AMH)
Anti-Müllerian Hormones (AMH) play important roles in sex differentiation in males in the embryonic time, as well as the formation of ovarian follicles in female bodies.
You may have heard of the hormone in gynecological (or obstetric) clinics, since the Anti-Müllerian Hormone (AMH) test is one of the common blood tests for women's health to examine the ovarian reserve (egg quantity, NOT quality) or assess reproductive aging. The glycoprotein hormone is produced by ovarian follicles.
In male mammals, the Anti-Müllerian Hormones (AMH) are produced by Sertoli cells in the fetus, and inhibit the development of the Müllerian ducts, which is the precursor of the female reproductive tract, growing into the uterus, fallopian tubes, and other Müllerian organs. On the other hand, females don't produce the Anti-Müllerian Hormones (AMH) in the fetal stage, so that female reproductive organs develop without being repressed.
While its primary developmental role is in the male embryo, in adult women, the Anti-Müllerian Hormones (AMH) are secreted by granulosa cells in the ovaries to regulate the development of ovarian follicles. The hormone inhibits the recruitment of primordial follicles into the growing pool to prevent the premature exhaustion of the ovarian reserve.
Male Mammals, who don't have Nipples — Diversity is Important
Since the vast majority of Mammals share the same early embryonic development process with Humankind, male Mammals in general have nipples mirroring the placement of their females.
Having said that, amazing nature is magnanimous, embracing weird exceptions… This is one of the key features why genuine diversity important to adapt to variable environments.
Here are some male Mammals who miss nipples:
● Stallions (male Horses)
Male horses do NOT have nipples. Because, during the stage of their fetal development, the Anti-Müllerian Hormones (AMH), as the same OFF switch as Humankind, work far more aggressively to stop growing nipples than other Mammals. So Stallions are inherently more masculine than men!
● Male Rats and Mice
While female Rats and Mice have nipples, ONLY male Rats and Mice do NOT. But it's a little confusing,... as most other Rodents, such as Guinea Pigs, have nipples in both females and males as the same as Humankind and other Mammals.
● Platypuses and Echidnas (Monotremes)
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| Platypus (above) and Echidna |
Neither their males nor females have nipples! Both Platypuses and Echidnas, Monotremes indigenous to Australia and New Guinea, are very unique and cute Mammals, but lay eggs! Nevertheless, their female nurses young with milk. The females sweat (secrete) milk from their mammary gland pores (mammary patches) on their skin, and then the young lick off the milk. Echidna’s females form a temporary pseudo-pouch by contracting abdominal muscles to nurse and feed the milk to the young from the mammary patches inside.
How many Nipples do Mammals have?
In terms of the number of nipples in Mammals, there is an interesting rule of thumb, called the one-half rule. This rule explains that most of Mammals typically have a number of nipples equal to twice the average litter size. That is, each baby mammal has 2 nipples in average. This evolutionary rule of thumb ensures a few spare nipples for every baby lest even one misses a chance in the struggle.
First, it's We… Please note that here, in these typical cases, I don't count some exceptional cases, but the principles, regardless of human twins, triplets, or more.
● Humankind / other Primates
● Sheep / Goats / Horses(ONLY Mares)
● Cats
● Dogs
● Pigs
12 to 24 nipples. While the domestic pigs average litter is 8 to 12 piglets, modern commercial breedings often see averages of 12 to 14, or higher. Because they are artificially domesticated to bear more young. But the number of nipples does NOT evolve so rapidly in decades.
● Opossums
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| Opossum and the young (source: Oceana Conservation District) |
● Tenrecs
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| Tenrec (credit: Markus Fink) |
The Milk Lines — the Draft for Nipples’ Positions
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| Mammary ridges (milk lines) (source: the Geneva Foundation for Medical Education and Research) |
In almost ALL Mammals, nipples develop along 2 vertical lines, running from the armpits to the groin. These virtual lines are called the Mammary Ridges (or Milk Lines). In Humankind, our nipples ONLY develop at the 2 tops of the milk lines. However, occasionally, some people are born with more accessory nipples anywhere along these 2 symmetry lines. This is just evidence of evolutionary history from our multi-nippled mammalian ancestors in our DNA!
The female's body is amazing and built with great elaborations and full of mystery beyond male’s imagination!
The Creation is amazing beyond our imagination! We can't stop thinking of the Creator who is designing nature with such fantastic ingenuity. Who is She/He? — So we named the Creator “the God,” whether believing or not in whatever religion.
Diversity is a bunch of strategies for survival. When the climate or natural environment has drastically changed with any disasters or human errors, at least some groups MUST survive even in the middle of harsh conditions.
For example, among 10 people, there are 5 people who are resistant to the scorching heat and the other 5 who are resistant to the extreme cold. If the climate suddenly went back to the ice age, at least 5 cold-resistant people could survive, while even the other 5 would easily die. Or otherwise, we can NEVER fend off the worst scenario, complete eradication that misses a chance to reflourish in the future.
Therefore, weird creatures or extraordinary people are NOT extra out of majority, but significant for our survival.
Further reading (sponsored by Amazon):
“The Body: A Guide for Occupants” is a Must-Read owner’s manual for every body! “The Body: A Guide for Occupants” will lead you to a deeper understanding of the miracle that is Life in general and YOU in particular. Let’s take a head-to-toe tour of the marvel!
1 How to Build a Human
2 The Outside: Skin and Hair
3 Microbial You
4 The Brain
5 The Head
6 Down the Hatch: The Mouth and Throat
7 The Heart and Blood
8 The Chemistry Department
9 In the Dissecting Room: The Skeleton
10 On the Move: Bipedalism and Exercise
11 Equilibrium
12 The Immune System
13 Deep Breath: The Lungs and Breathing
14 Food, Glorious Food
15 The Guts
16 Sleep
17 Into the Nether Regions
18 In the Beginning: Conception and Birth
19 Nerves and Pain
20 When Things Go Wrong: Diseases
21 When Things Go Very Wrong: Cancer
22 Medicine Good and Bad
23 The End
Afterword to the Anchor Books Edition (2020)
Notes on Sources
Bibliography
Illustration Credits
Illustrations
“Woman: An Intimate Geography” remains an essential guide to everything from organs to orgasms and hormones to hysterectomies! With her characteristic clarity, insight, and sheer exuberance of language, bestselling author Natalie Angier cuts through the still prevalent myths and misinformation surrounding the female body, that most enigmatic of evolutionary masterpieces.
Preface
Introduction
Unscrambling the Egg
The Mosaic
Imagination
Default Line
The Well-Tempered Clavier
Suckers and Horns
Mass Hysteria
Circular Reasonings
Holy Water
A Gray and Yellow Basket
Greasing the Wheels
Venus in Furs
Mindful Menopause
There’s No Place Like Notoriety
Wolf Whistlers and Hyena Smiles
Spiking the Punch
Cheap Meat
Labor of Love
Of Hoggamus and Hogwash
A Skeptic in Paradise
APPENDIX
Biologically Correct
The Scientific Method: “Do Straw Men Have DNA?”
Opposites Attract? Not in Real Life
Skipping Spouse to Spouse isn’t Just a Man’s Game
References
Acknowledgments
Index
About the Author
Footnotes
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