Why are female not competitive for reproduction like males?

I have wondered if competition for mate among males and the race among sperm cells would result in healthy offspring,

why no such mechanisms exist among females and egg cells?

(Even females are genetically responsible for the offspring.)

As @Dexter said, there are examples of species where female compete for the access to the males but most often it is the other way around; males compete for the access to the females.

The reason for this discrepancy is mainly explained by Bateman's principle. Citing wikipedia:

Bateman's principle suggests that in most species, variability in reproductive success, or "reproductive variance," is greater in males than in females. This is ultimately a consequent of anisogamy. Females, especially mammalian females, almost always invest more energy into producing offspring than males invest. Bateman's principle anticipated and is consistent with Robert Trivers's theory of Parental investment-in most species females are a limiting factor over which males will compete. This competition results in some males being more successful than others, leading to greater reproductive variance among males than females

Actually there are lot of examples in which females are competing for mating which also have evolutionary consequences. It is also widely spread in nature. This competition may arise at cellular, molecular, genetic, behavioral level. Quoting from (Stockley and Bro-Jørgensen 2011)

female competition is associated with many diverse adaptations, from overtly aggressive behaviour, weaponry, and conspicuous sexual signals to subtle and often complex social behaviour involving olfactory signalling, alliance formation, altruism and spite, and even cases where individuals appear to inhibit their own reproduction.

Following table taken from (Gwynne 1991) which shows lot of examples in which females are competing

There are many more you can find by just searching on pubmed!

Males in monogamous species don't compete much for females. According to "The selfish gene", monogamy is an evolutionary stable strategy. They tend to only produce the number of kids they can get enough food to rear because those that produce more kids almost certainly can't rear all of them and maybe even 2 of them will starve because of the extra energy of producing the extra kid and of the kids fighting with each other for food. If any of them fight for more than their fair share of food, those who fight back those who fight for more than their fair share will be selected for because there's nothing to lose in fighting back for more than their fair share let alone their fair share. After that, those that start a fight for more than their fair share will be selected against because of the possibility of losing the fight and getting even less food, which is why they don't start a fight for more than their fair share of food in the first. At an evolutionary stable population size, each individual can on average get enough food to rear 2 offspring. It's an evolutionary advantage for a female to pick a male that doesn't already have another mate because if she picks a male who already has another mate, there will be 3 parents so they can get enough food to rear 3 offspring but each one will only have a 50% chance of being her own, making it pretty impossible for a male to get a second mate. Since there are about the same number of males as females, almost all males get one mate with almost no left over males or females. Males who fight against any male who tries to take their mate away from them are selected for which in turn causes males of a monogamous species who fight another male for a second mate to be selected against.

According to, bonobos are also peaceful. All the members of a group have sex with each other all the time and no male knows which baby is his removing the advantage in males fighting. They're probably also peaceful for a similar reason, that those that fight back to not get less than their fair share of food are selected for which in turn causes those that fight to get more than their fair share to be selected against.

Competition Among Women: Myth and Reality

Women seem to have a reputation for being &ldquocatty&rdquo and competitive with other women, unlike how men behave with other men. This is a curious notion, especially since women are actually less competitive than men out in the world and less comfortable being competitive.

How can we make sense of this paradox?

Healthy competition and confidence are encouraged in boys but often seen as undesirable traits in girls. Team spirit and friendship provide the glue that strengthens and bonds men when competition prevails. Not surprisingly, men are typically comfortable with competition and see winning as an essential part of the game, rarely feeling bad for others after a victory, and maintaining camaraderie with their buddies.

Because women learn that they are not supposed to be competitive and win at others&rsquo expense, their natural competitive spirit cannot be shared openly, happily, or even jokingly with other women. In such situations, when aggression cannot be channeled into a healthy, positive edge, it becomes inhibited and goes underground. What could have been healthy competition becomes a secret feeling of envy and desire for the other to fail &ndash laced with guilt and shame.

Thus, what looks like hostile competition between women may instead mask feelings of insecurity, fear of success, and healthy aggression. Women, often experts at being tuned in and sensitive to others&rsquo feelings, may easily overidentify with other women&rsquos insecurities, projecting how they would feel in the other&rsquos shoes and then feeling bad about their own success. Women learn to feel guilty for feeling happy and successful &ndash and with their female friends who may not be having such luck, they may experience their own success as hurtful to their friend. This can make it uncomfortable for a woman to share and enjoy her accomplishments with her female friends.

In a common example, women may feel uncomfortable or self-conscious discussing their dieting success or weight loss with certain friends. They may even eat high-calorie foods they don&rsquot desire when with a friend who is struggling with her own weight but having trouble being disciplined with food. In such situations, women may succumb to what they experience as an instinctive pressure to protect their friend in this way, sabotaging themselves but insulated from becoming the object of envy and resentment.

Interestingly, in friendships with men, where men and women are often competing in different arenas, these issues of competition usually do not come into play. Women don&rsquot perceive men to be as vulnerable and sensitive as women, or threatened by success, and are therefore freed up from worrying about their feelings in this way. Further, women seek approval from men and often rely on them to validate their desirability, creating an interpersonal context in which success and confidence are rewarded. (Note that this &ldquosafer&rdquo dynamic with men applies to platonic friendships but is more complicated in romantic relationships, where women may diminish themselves with their partners as they do with other women.)

Women often rely on the approval of others to feel good about themselves.

Women often take care of people emotionally and rely on the approval of others to feel good about themselves. Women&rsquos fear of triumph over others may lead to keeping themselves down and even (conscious or unconscious) subversion. Dependency on other people to maintain self-esteem creates a double bind, impeding women from embracing and using their own edge to achieve success. Constrained by internal conflict and over-focus on others&rsquo reactions, many women endure the frustration of being unable to fulfill their true potential in terms of aggression, sexuality, and power.

Women&rsquos trepidation and ambivalence in the face of their own strength and power often underlies their mistrust of the power of other women. Discomfort with their own power can make women alternate between inhibiting themselves to protect a female friend, and feeling mistrustful and helpless in the face of another woman&rsquos perceived destructive power. A good example of this is when women whose husbands have had an affair blame the other woman more than they blame their spouse, holding the other woman more accountable &ndash and seeing men as helpless in the grips of a desirable woman.

Autonomy cannot be achieved when actions are based on fear, and without the self-protective capacity to experience anger and aggression, which are part of drive. Being able to experience and utilize these states adaptively is different from acting them out in hurtful ways. If women are frightened of aggression in themselves or others, and threatened by success, their experience of themselves will be muted, leading to depression. How can women feel comfortable with their own (and other women&rsquos) drive and power, without feeling threatened or worrying that their own success will hurt others?

Scientists investigate why females live longer than males

An international team of scientists studying lifespans of wild mammals have found that, just like humans, females tend to live significantly longer than their male counterparts.

The researchers looked at the lifespans of 101 different species, from sheep to elephants, and found that females lived an average of 18% longer than males for more than 60% of the species studies. In humans, females tend to live around 7.8% longer.

The study, led by scientists at University Lyon 1 and published in the journal, Proceedings of the National Academy of Sciences, found this was not due to the sexes aging at different rates but that females had an average lower risk of mortality in adulthood than males.

It was unclear from the data as to why females survive longer than males, however the authors suggest that it could be due to complex interactions between the local environmental conditions and sex-specific costs of reproduction.

Professor Tamás Székely, from the Milner Centre for Evolution at the University of Bath, was one of the authors of the study. He said: 'We've known for a long time that women generally live longer than men, but were surprised to find that the differences in lifespan between the sexes was even more pronounced in wild mammals than in humans.

"This could be either because females are naturally able to live longer, or that female mortality drops compared with males.

"For example, lionesses live at least 50% longer in the wild than male lions. We previously thought this was mostly due to sexual selection -- because males fight with each other to overtake a pride and thus have access to females, however our data do not support this. Therefore there must be other, more complex factors at play.

"Female lions live together in a pride, where sisters, mothers and daughters hunt together and look after each other, whereas adult male lions often live alone or with their brother and therefore don't have the same support network.

"Another possible explanation for the sex difference is that female survival increases when males provide some or all of the parental care. This is also true in birds. Giving birth and caring for young becomes a significant health cost for females and so this cost is reduced if both parents work together to bring up their offspring."

The researchers plan to compare the data on wild animals with that of captive zoo animals, which do not have to deal with predators or competition for food or mates. This will allow them to measure the extent to which biological differences between the sexes have an effect on life expectancy.

"By affecting males and females differently, harsh environmental conditions such as a high prevalence in pathogens, is likely to cause sex-differences in lifespan.

"Comparing the sex gap in lifespan and aging across several populations of the same species is definitely full of promises," said Jean-François Lemaître from the National Centre for Scientific Research (University Lyon 1, France) and coordinator of this study.

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I heard on TV that the male and female reproductive systems are very similar. The only difference is that the male's is located outside the body and the female's inside.

When I look at images of the two, it really does look like the male and female systems are inside out. Is this accurate? SteamLouis yesterday

@donasmrs-- Ovaries are where eggs develop. If the ovaries are removed, the woman will no longer have eggs which means that her periods will end and she won't be able to conceive children anymore.

Every month, an egg develops in the ovaries an moves into the fallopian tubes where it may or may not be fertilized. If it's not fertilized, menstruation occurs. If it's fertilized, then the fertilized egg moves into the uterus and develops into an embryo and eventually an infant.

Sometimes the uterus is removed along with the ovaries during surgery, sometimes it is left. Either way, the body will no longer produce estrogen which means that the woman enters menopause. donasmrs March 20, 2013

What happens if parts of a woman's reproductive system are removed?

My mom said that she is going to have surgery and the doctor will remove her ovaries. But I don't understand what this means.

Hierarchical differences

Female academics less likely to cross rank in collaboration, study finds

Scouring YouTube and the video archive of several international sports federations, researchers found hundreds of videos of tennis, table tennis, badminton, and boxing matches, and focused their attention not on the matches, but on their immediate aftermath.

“We watched carefully to see what happened after the match ended,” she said. “The requirement is that people touch after the match ends, but how do they touch? They can just touch hands quickly, or they can really shake hands or give a pat or even a hug.”

Researchers watched hundreds of matches, taking care to ensure no player was repeated in any match, and found clear sex differences in all four sports.

“Most people think of females as being less competitive, or more cooperative, so you might expect there would be more reconciliation between females,” Benenson said. “With their families, females are more cooperative than males, investing in children and other kin. With unrelated same-sex peers however, after conflicts, in males you see these very warm handshakes and embraces, even in boxing after they’ve almost killed each other.”

So why is it that women seem less willing to reconcile following conflict?

Part of the answer, Benenson and Wrangham believe, may be tied to traditional gender roles that stretch to earliest human history. Chimps and humans live in groups of both males and females, but while males cultivate large friendship networks, females focus more on family relationships and a handful of few close friends — partly, researchers believe, as a way to share the burden of raising children. The whole community gains when unrelated men successfully prevail against external groups. In contrast, women gain more from family members and one or two close friends who help with child care. It makes sense, in that light, that women would reconcile more with these individuals, and men with a larger number of unrelated same-sex peers.

Ultimately, Benenson said, the implications of the study could reach far beyond the boundaries of the playing field.

“What we’re talking about is women having a harder time when they have to compete with other women,” she said. “Studies have shown that when two females compete in the workplace, they feel much more damaged afterward. I think this is something human resources professionals should be aware of, so they can mitigate it.”

Female Reproductive System

Reproduction is the process by which organisms make more organisms like themselves. But even though the reproductive system is essential to keeping a species alive, unlike other body systems, it's not essential to keeping an individual alive.

In the human reproductive process, two kinds of sex cells, or gametes (GAH-meetz), are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system. When sperm fertilizes (meets) an egg, this fertilized egg is called a zygote (pronounced: ZYE-goat). The zygote goes through a process of becoming an embryo and developing into a fetus.

The male reproductive system and the female reproductive system both are needed for reproduction.

Humans, like other organisms, pass some characteristics of themselves to the next generation. We do this through our genes, the special carriers of human traits. The genes that parents pass along are what make their children similar to others in their family, but also what make each child unique. These genes come from the male's sperm and the female's egg.

What Is the Female Reproductive System?

The external part of the female reproductive organs is called the vulva, which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs inside the body.

The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia (which means lips) surround the vaginal opening. The clitoris, a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra (the canal that carries pee from the bladder to the outside of the body) and vagina. When girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.

A female's internal reproductive organs are the vagina, uterus, fallopian tubes, and ovaries.

The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. Because it has muscular walls, the vagina can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist.

The vagina serves three purposes:

  1. It's where the penis is inserted during sexual intercourse.
  2. It's the pathway (the birth canal) through which a baby leaves a woman's body during childbirth.
  3. It's the route through which menstrual blood leaves the body during periods.

A very thin piece of skin-like tissue called the hymen partly covers the opening of the vagina. Hymens are often different from female to female. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little (this usually causes little, if any, pain). Some women who have had sex don't have much of a change in their hymens, though. And some women's hymens have already stretched even before they have sex.

The vagina connects with the uterus, or womb, at the cervix (which means neck). The cervix has strong, thick walls. The opening of the cervix is very small (no wider than a straw), which is why a tampon can never get lost inside a girl's body. During childbirth, the cervix can expand to allow a baby to pass.

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls — in fact, the uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. When a woman isn't pregnant, the uterus is only about 3 inches (7.5 centimeters) long and 2 inches (5 centimeters) wide.

At the upper corners of the uterus, the fallopian tubes connect the uterus to the ovaries. The ovaries are two oval-shaped organs that lie to the upper right and left of the uterus. They produce, store, and release eggs into the fallopian tubes in the process called ovulation (pronounced: av-yoo-LAY-shun).

There are two fallopian (pronounced: fuh-LO-pee-un) tubes, each attached to a side of the uterus. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area wraps around the ovary but doesn't completely attach to it. When an egg pops out of an ovary, it enters the fallopian tube. Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus.

The ovaries (pronounced: OH-vuh-reez) are also part of the endocrine system because they produce female sex such as estrogen (pronounced: ESS-truh-jun) and progesterone (pronounced: pro-JESS-tuh-rone).

How Does the Female Reproductive System Work?

The female reproductive system enables a woman to:

  • produce eggs (ova)
  • have sexual intercourse
  • protect and nourish a fertilized egg until it is fully developed
  • give birth

Sexual reproduction couldn't happen without the sexual organs called the gonads. Most people think of the gonads as the male testicles. But both sexes have gonads: In females the gonads are the ovaries, which make female gametes (eggs). The male gonads make male gametes (sperm).

When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which remain inactive until puberty begins. At puberty, the pituitary gland (in the central part of the brain) starts making hormones that stimulate the ovaries to make female sex hormones, including estrogen. The secretion of these hormones causes a girl to develop into a sexually mature woman.

Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. About once a month, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes.

Unless the egg is fertilized by a sperm while in the fallopian tube, the egg leaves the body about 2 weeks later through the uterus — this is menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche (pronounced: MEH-nar-kee).

It's common for women and girls to have some discomfort in the days leading to their periods. Premenstrual syndrome (PMS) includes both physical and emotional symptoms that many girls and women get right before their periods, such as:

PMS is usually at its worst during the 7 days before a girl's period starts and disappears after it begins.

Many girls also have belly cramps during the first few days of their periods caused by prostaglandins, chemicals in the body that make the smooth muscle in the uterus contract. These involuntary contractions can be dull or sharp and intense.

It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days.

What Happens If an Egg Is Fertilized?

If a female and male have sex within several days of the female's ovulation, fertilization can happen. When the male ejaculates (when semen leaves the penis), a small amount of semen is deposited into the vagina. Millions of sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube. It takes only one sperm to fertilize the egg.

About 5 to 6 days after the sperm fertilizes the egg, the fertilized egg (pronounced: zygote) has become a multicelled blastocyst. A blastocyst (pronounced: BLAS-tuh-sist) is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium (pronounced: en-doh-MEE-tree-um) to become thick and rich with blood. Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation.

As cells from the blastocyst take in nourishment, another stage of development begins. In the embryonic stage, the inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby. The cells multiply thousands of times and move to new positions to eventually become the embryo (pronounced: EM-bree-oh).

After about 8 weeks, the embryo is about the size of a raspberry, but almost all of its parts — the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin — have formed.

During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. The fetus (pronounced: FEE-tis) floats in amniotic (pronounced: am-nee-AH-tik) fluid inside the amniotic sac. The fetus gets oxygen and nourishment from the mother's blood via the placenta (pronounced: pluh-SEN-tuh). This disk-like structure sticks to the inner lining of the uterus and connects to the fetus via the umbilical (pronounced: um-BIL-ih-kul) cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.

Pregnancy lasts an average of 280 days — about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. Mucus has formed a plug in the cervix, which now loosesn. It and amniotic fluid come out through the vagina when the mother's water breaks.

When the contractions of labor begin, the walls of the uterus contract as they are stimulated by the pituitary hormone oxytocin (pronounced: ahk-see-TOE-sin). The contractions cause the cervix to widen and begin to open. After several hours of this widening, the cervix is dilated (opened) enough for the baby to come through. The baby is pushed out of the uterus, through the cervix, and along the birth canal. The baby's head usually comes first. The umbilical cord comes out with the baby. It's clamped and cut close to the navel after the baby is delivered.

The last stage of the birth process involves the delivery of the placenta, which at that point is called the afterbirth. After it has separated from the inner lining of the uterus, contractions of the uterus push it out, along with its membranes and fluids.

Description – Scope, Organization, and Access:

The Scope of the Topics and Materials. We know a lot about gender inequality – its history, how people experience it in their lives, the ways it varies in intensity and form across time and place, the beliefs that make it seem natural, and much more. The outpouring of research and commentary on gender inequality over the past half century has been extraordinary. Unfortunately, despite all this, our understanding of what causes gender inequality remains troubled. Both ordinary people and experts (such as scholars) commonly fluctuate between simplistic explanations that founder under close scrutiny and throwing up their hands in frustration over what can seem an enigma beyond human comprehension. Here we will seek to surmount this dilemma. We will explore diverse facets of gender inequality and varied ideas about what causes might be decisive. We will also look carefully at the ways we can identify and verify the causes of social phenomena. Through these efforts we will aim both to enhance our understanding of what produces gender inequality and to improve our general ability to do causal social analyses effectively.

The class organization and goals. In this class, each week's work will be organized around an analytical task, as well as a set of readings. Rather than focusing on discussion of the readings, the analytical tasks involve attempting a causal analysis of some aspect of gender inequality related to the week's issue, building on the materials we read (in brief papers of a couple pages). The approach in this class seeks to develop analytical skills as well as understandings of the relevant literature by stressing doing actual analyses of gender inequality. (Note: this class does not have an exam nor a final paper.)

All class meetings are organized as discussions. Part of our class discussions will be on the common readings and part on students' efforts to explore the analytical tasks each week. We will adjust the time devoted to these two goals according to our experiences over the class. Every week, students will initiate discussions on readings and papers. To make this work, each week's papers will be exchanged (electronically) with enough lead time that we can all read all the papers prior to the class meetings.

Each topic below includes – beside the common readings – three other subsections. These are: an analytical task, recommended readings, and related readings. The analytical task is the writing assignment for the week. Everyone should read the common readings while doing the analytical task (and be prepared to discuss them). In each of these papers – always brief papers – students will try out causal ideas related to the week's topic. Recommended and related readings are optional materials useful for those who want to dig deeper into a topic. To simplify navigating through the syllabus, these subsections are hidden until the viewer clicks on the subsection heading, then they will appear.

Most of our readings will be articles available for downloading. The links will appear in the online version of the course syllabus. Excerpts from Down So Long . . .: The Puzzling Persistence of Gender Inequality (book manuscript by RMJ not yet published) will similarly be available for downloading from the class web site. (As we will read selections from Jackson's book Destined for Equality [Harvard U Press] throughout the course, you might want to buy it or borrow it.)

Any student unfamiliar with the study of gender, can (and probably should) pick up the basics from a standard textbook in the area – I recommend Michael Kimmel's Gendered Society (which I use in my basic general undergraduate class on gender, so used copies should be easy to find).

For further relevant sources, my reading lists/syllabi for two graduate courses might be valuable. The one most directly related is What Causes Gender Inequality: Analytical Foundations a more general class, What Causes Inequality: Analytical Foundations , may provide materials for broader questions about different kinds of inequalities and how to think about gender inequality in relationship to them.

A note on the "hidden" material below : Each section of this guide includes – beside the common readings – three subsections, one for an analytical task, one for recommended readings, and one for related readings. To simplify navigating through the course guide, only the headings for these subsections are initially visible. The contents of all these subsections are hidden (so that the beginning appearance of the page is similar to a standard syllabus) until the viewer clicks on a subsection heading, then its contents will appear. While this organization is helpful for negotiating the page most of the time, it can become an obstacle if we want to search the page (for example, for a particular article) as searches will ignore the hidden material (that is, if you search a page you are viewing in an internet browser, the search will only examine what is shown on the page at that time). To overcome this limitation, you can "open" all the hidden sections to show everything on the page by clicking the § symbol at the top of the page. (To restore the page to the normal condensed view, simply reload the page which will collapse all the "hidden" sections to their usual look). The table of contents at the top of this page will aid speedy navigation to any topic, which is particularly helpful if you reveal all the "hidden" material.

Why Don't More Women Want to Work With Other Women?

Most Americans don't care about the gender of their coworkers, but those who do prefer men by a wide margin.

A recurring issue in glass-ceiling debates revolves around whether women are, either directly or indirectly, excluded from high-level jobs. Many offices tend to promote people according to a concept called "homosocial reproduction"—essentially, the spoils go to the workers who move in the right social circles. If your workplace is a boy's club where the high-performers smoke cigars and play golf together, this theory would dictate that the next big promotion is likely not going to the quiet female analyst who knits during lunchtime.

That's why this one fact, from a Pew survey released last month, is so devastating.

Pew asked 2,002 people if they would prefer to work with men or women. Most—78 percent of men and 76 percent of women—said they didn't care. But for the 22 percent who did have a preference, "it’s men who get the nod from both sexes by about a 2-1 margin," Pew's Rich Morin writes. In fact, more women said they'd rather work with men than men did.

When the results are broken down by generation, workers who were born between 1925 and 1942 were most likely to say they prefer working with men (21 percent), and Millennials (born in the 1980s and 1990s) were least likely to (11 percent). But from there, the percentage doesn't track with age. In fact, more workers from Generation X, who are closer in age to Millennials, said they'd prefer to work with men (19 percent) than did the middle-aged Boomers (16 percent).

Millennials may seem gender-neutral by comparison, but in other parts of the same survey, their responses were equally bleak. Far more Millennial women than men (59 percent versus 19 percent) said being a working parent makes it hard to advance in a job, and fewer Millennial women said they aspired to become managers.

The results are similar in other surveys on who Americans would prefer to work for, rather than with. As Derek Thompson wrote in November, Gallup reported that all of 23 percent of U.S. employees now say they'd prefer a female boss—and that's a record high. And once again, more men than women said they either preferred a female boss or had no preference. We've apparently come a long way, though: That number was only about 5 percent until the 1970s, which helps explain some of Peggy Olson's endless suffering.

There's no easy answer for why both sexes lean slightly toward a male-dominated workplace. Among women, some studies make it seem as though this preference is the professional extension of "all my best friends are guys." As in, "I'm a woman, but I have an overall negative perception of female personalities and thus prefer to surround myself with men." For example, female respondents told one British pollster that male bosses are "more straight-talking" and "less prone to moods."

Social scientists who have been poking at this can of worms for decades have found that women sometimes exclude other women from opportunities in order to gain a competitive edge. Women tend to mainly only bully other women in the workplace, while men are equal-opportunity harassers.

Much of this can be chalked up to decades of workplace and societal gender discrimination. Women tend to face the so-called "backlash effect," the theory that women can only advance at work if they act more like men, but they then face social penalties for behaving in unfeminine ways.

But perhaps the statistic to focus on isn't the quarter of workers who care about the gender of their colleagues. It's the 77 percent of workers who say they don't care. More of that, and we won't have to worry about boy's clubs or glass ceilings.

Why are female not competitive for reproduction like males? - Biology

Prevention efforts will founder until they are based on the understanding
that rape evolved as a form of male reproductive behavior


(1) A friend of ours once told us about her rape. The details hardly matter, but in outline her story is numbingly familiar. After a movie she returned with her date to his car, which had been left in an isolated parking lot. She was expecting him to drive her home. Instead, the man locked the car doors and physically forced her to have sex with him.

(2) Our friend was emotionally scarred by her experience: she became anxious about dating, and even about going out in public. She had trouble sleeping, eating and concentrating on her work. Indeed, like some war veterans, rape victims often suffer from post-traumatic stress disorder, in which symptoms such as anxiety, memory loss, obsessive thoughts and emotional numbness linger after a deeply disturbing experience. Yet gruesome ordeals like that of our friend are all too common: in a 1992 survey of American women aged eighteen and older, 13 percent of the respondents reported having been the victim of at least one rape, where rape was defined as unwelcome oral, anal or vaginal penetration achieved through the use or threat of force. Surely, eradicating sexual violence is an issue that modern society should make a top priority. But first a perplexing question must be confronted and answered: Why do men rape?

(3) The quest for the answer to that question has occupied the two of us collectively for more than forty years. As a purely scientific puzzle, the problem is hard enough. But it is further roiled by strong ideological currents. Many social theorists view rape not only as an ugly crime but as a symptom of an unhealthy society, in which men fear and disrespect women. In 1975 the feminist writer Susan Brownmiller asserted that rape is motivated not by lust but by the urge to control and dominate. In the twenty-five years since, Brownmiller's view has become mainstream. All men feel sexual desire, the theory goes, but not all men rape. Rape is viewed as an unnatural behavior that has nothing to do with sex, and one that has no corollary in the animal world.

(4) Undoubtedly, individual rapists may have a variety of motivations. A man may rape because, for instance, he wants to impress his friends by losing his virginity, or because he wants to avenge himself against a woman who has spurned him. But social scientists have not convincingly demonstrated that rapists are not at least partly motivated by sexual desire as well. Indeed, how could a rape take place at all without sexual motivation on the part of the rapist? Isn't sexual arousal of the rapist the one common factor in all rapes, including date rapes, rapes of children, rapes of women under anesthetic and even gang rapes committed by soldiers during war?

(5) We want to challenge the dearly held idea that rape is not about sex. We realize that our approach and our frankness will rankle some social scientists, including some serious and well-intentioned rape investigators. But many facts point to the conclusion that rape is, in its very essence, a sexual act. Furthermore, we argue, rape has evolved over millennia of human history, along with courtship, sexual attraction and other behaviors related to the production of offspring.

(6) Consider the following facts:

  • Most rape victims are women of childbearing age.
  • In many cultures rape is treated as a crime against the victim's husband.
  • Rape victims suffer less emotional distress when they are subjected to more violence.
  • Rape takes place not only among human beings but also in a variety of other animal species.
  • Married women and women of childbearing age experience more psychological distress after a rape than do girls, single women or women who are past menopause.

As bizarre as some of those facts may seem, they all make sense when rape is viewed as a natural, biological phenomenon that is a product of the human evolutionary heritage.

(7) Here we must hasten to emphasize that by categorizing a behavior as "natural" and "biological" we do not in any way mean to imply that the behavior is justified or even inevitable. Biological means "of or pertaining to life," so the word applies to every human feature and behavior. But to infer from that--as many of our critics assert that we do--that what is biological is somehow right or good, would be to fall into the so-called naturalistic fallacy. That mistake is obvious enough when one considers such natural disasters as epidemics, floods and tornadoes. In those cases it is clear that what is natural is not always desirable. And of course much can be, and is, done to protect people against natural threats--from administering antibiotics to drawing up emergency evacuation plans. In other words, the fact that rape is an ancient part of human nature in no way excuses the rapist

(8) Why, then, have the editors of scholarly journals refused to publish papers that treat rape from a Darwinian perspective? Why have pickets and audience protesters caused public lectures on the evolutionary basis of rape to be canceled or terminated? Why have investigators working to discover the evolutionary causes of rape been denied positions at universities?

(9) The reason is the deep schism between many social scientists and investigators such as ourselves who are proponents of what is variously called sociobiology or evolutionary psychology. Social scientists regard culture-- everything from eating habits to language--as an entirely human invention, one that develops arbitrarily. According to that view, the desires of men and women are learned behaviors. Rape takes place only when men learn to rape, and it can be eradicated simply by substituting new lessons. Sociobiologists, by contrast, emphasize that learned behavior, and indeed all culture, is the result of psychological adaptations that have evolved over long periods of time. Those adaptations, like all traits of individual human beings, have both genetic and environmental components. We fervently believe that, just as the leopard's spots and the giraffe's elongated neck are the result of aeons of past Darwinian selection, so also is rape.

(10) That conclusion has profound and immediate practical consequences. The rape-prevention measures that are being taught to police officers, lawyers, parents, college students and potential rapists are based on the prevailing social-science view, and are therefore doomed to fail. The Darwinian theory of evolution by natural selection is the most powerful scientific theory that applies to living things. As long as efforts to prevent rape remain uninformed by that theory, they will continue to be handicapped by ideas about human nature that are fundamentally inadequate. We believe that only by acknowledging the evolutionary roots of rape can prevention tactics be devised that really work.

(11) From a Darwinian perspective, every kind of animal--whether grasshopper or gorilla, German or Ghanaian--has evolved to produce healthy children that will survive to pass along their parents' genetic legacy. The mechanics of the phenomenon are simple: animals born without traits that led to reproduction died out, whereas the ones that reproduced the most succeeded in conveying their genes to posterity. Crudely speaking, sex feels good because over evolutionary time the animals that liked having sex created more offspring than the animals that didn't.

(12) As everyone knows all too well, however, sex and the social behaviors that go with it are endlessly complicated. Their mysterious and tangled permutations have inspired flights of literary genius throughout the ages, from Oedipus Rex to Portnoy's Complaint. And a quick perusal of the personal-growth section of any bookstore--past such titles as Men Are from Mars, Women Are from Venus and You Just Don't Understand --is enough to show that one reason sex is so complicated is that men and women perceive it so differently. Is that the case only because boys and girls receive different messages during their upbringing? Or, as we believe, do those differences between the sexes go deeper?

(13) Over vast periods of evolutionary time, men and women have confronted quite different reproductive challenges. Whereas fathers can share the responsibilities of child rearing, they do not have to. Like most of their male counterparts in the rest of the animal kingdom, human males can reproduce successfully with a minimal expenditure of time and energy once the brief act of sexual intercourse is completed, their contribution can cease. By contrast, the minimum effort required for a woman to reproduce successfully includes nine months of pregnancy and a painful childbirth. Typically, ancestral females also had to devote themselves to prolonged breast-feeding and many years of child care if they were to ensure the survival of their genes. In short, a man can have many children, with little inconvenience to himself a woman can have only a few, and with great effort.

(14) That difference is the key to understanding the origins of certain important adaptations--features that persist because they were favored by natural selection in the past. Given the low cost in time and energy that mating entails for the male, selection favored males who mated frequently. By contrast, selection favored females who gave careful consideration to their choice of a mate that way, the high costs of mating for the female would be undertaken under circumstances that were most likely to produce healthy offspring. The result is that men show greater interest than women do in having a variety of sexual partners and in having casual sex without investment or commitment. That commonplace observation has been confirmed by many empirical studies. The evolutionary psychologist David M. Buss of the University of Texas at Austin, for instance, has found that women around the world use wealth, status and earning potential as major criteria in selecting a mate, and that they value those attributes in mates more than men do.

(15) Remember, none of the foregoing behavioral manifestations of evolution need be conscious. People do not necessarily have sex because they want children, and they certainly do not conduct thorough cost-benefit analyses before taking a partner to bed. As Darwin made clear, individual organisms merely serve as the instruments of evolution. Men today find young women attractive because during human evolutionary history the males who preferred prepubescent girls or women too old to conceive were outreproduced by the males who were drawn to females of high reproductive potential. And women today prefer successful men because the females who passed on the most genes, and thereby became our ancestors, were the ones who carefully selected partners who could best support their offspring. That is why, as the anthropologist Donald Symons of the University of California, Santa Barbara, has observed, people everywhere understand sex as "something females have that males want."

(16) A dozen roses, romantic dinners by candlelight, a Tiffany engagement ring: the classic courtship ritual requires lots of time, energy and careful attention to detail. But people are far from unique in that regard: the males of most animal species spend much of their energies attracting, wooing and securing sexual partners. The male woodcock, for instance, performs a dramatic display each spring at mating time, soaring high into the air and then tumbling to the ground. Male fireflies are even flashier, blinking like neon signs. The male bowerbird builds a veritable honeymoon cottage: an intricate, sculpted nest that he decorates with flowers and other colorful bric-a-brac. Male deer and antelope lock antlers in a display of brute strength to compete for females.

(17) Once a female's interest is piqued, the male behaves in various ways to make her more sexually receptive. Depending on the species, he dances, fans his feathers or offers gifts of food. In the nursery web spider, the food gift is an attempt to distract the female, who otherwise might literally devour her partner during the sex act. The common thread that binds nearly all animal species seems to be that males are willing to abandon all sense and decorum, even to risk their lives, in the frantic quest for sex.

(18) But though most male animals expend a great deal of time and energy enticing females, forced copulation--rape--also occurs, at least occasionally, in a variety of insects, birds, fishes, reptiles, amphibians, marine mammals and nonhuman primates. In some animal species, moreover, rape is commonplace. In many scorpionfly species, for instance--insects that one of us (Thornhill) has studied in depth--males have two well-formulated strategies for mating. Either they offer the female a nuptial gift (a mass of hardened saliva they have produced, or a dead insect) or they chase a female and take her by force.

(19) A remarkable feature of these scorpionflies is an appendage that seems specially designed for rape. Called the notal organ, it is a clamp on the top of the male's abdomen with which he can grab on to one of the female's forewings during mating, to prevent her escape. Besides rape, the notal organ does not appear to have any other function. For example, when the notal organs of males are experimentally covered with beeswax, to keep them from functioning, the males cannot rape. Such males still mate successfully, however, when they are allowed to present nuptial gifts to females. And other experiments have shown that the notal organ is not an adaptation for transferring sperm: in unforced mating, the organ contributes nothing to insemination.

(20) Not surprisingly, females prefer voluntary mating to mating by force: they will approach a male bearing a nuptial gift and flee a male that does not have one. Intriguingly, however, the males, too, seem to prefer a consensual arrangement: they rape only when they cannot obtain a nuptial gift. Experiments have shown that when male scorpionflies possessing nuptial gifts are removed from an area, giftless males--typically, the wimpier ones that had failed in male-male competitions over prey--quickly shift from attempting rape to guarding a gift that has been left untended. That preference for consensual sex makes sense in evolutionary terms, because when females are willing, males are much more likely to achieve penetration and sperm transfer.

(21) Human males obviously have no external organ specifically designed for rape. One must therefore look to the male psyche--to a potential mental rape organ--to discover any special-purpose adaptation of the human male to rape.


(22) Since women are choosy, men have been selected for finding a way to be chosen. One way to do that is to possess traits that women prefer: men with symmetrical body features are attractive to women, presumably because such features are a sign of health. A second way that men can gain access to women is by defeating other men in fights or other kinds of competitions--thereby gaining power, resources and social status, other qualities that women find attractive.

(23) Rape can be understood as a third kind of sexual strategy: one more way to gain access to females. There are several mechanisms by which such a strategy could function. For example, men might resort to rape when they are socially disenfranchised, and thus unable to gain access to women through looks, wealth or status. Alternatively, men could have evolved to practice rape when the costs seem low--when, for instance, a woman is alone and unprotected (and thus retaliation seems unlikely), or when they have physical control over a woman (and so cannot be injured by her). Over evolutionary time, some men may have succeeded in passing on their genes through rape, thus perpetuating the behavior. It is also possible, however, that rape evolved not as a reproductive strategy in itself but merely as a side effect of other adaptations, such as the strong male sex drive and the male desire to mate with a variety of women.

(24) Take, for instance, the fact that men are able to maintain sexual arousal and copulate with unwilling women. That ability invites inquiry, according to the psychologist Margo Wilson of McMaster University in Hamilton, Ontario, and her coworkers, because it is not a trait that is common to the males of all animal species. Its existence in human males could signal that they have evolved psychological mechanisms that specifically enable them to engage in forced copulation--in short, it could be a rape adaptation. But that is not the only plausible explanation. The psychologist Neil M. Malamuth of the University of California, Los Angeles, points out that the ability to copulate with unwilling women may be simply a by-product of men's "greater capacity for impersonal sex."


(25) More research is needed to decide the question of whether rape is an adaptation or merely a by-product of other sexual adaptations. Both hypotheses are plausible: one of us (Thornhill) supports the former, whereas the other (Palmer) endorses the latter. Regardless of which hypothesis prevails, however, there is no doubt that rape has evolutionary--and thus genetic--origins. All traits and behaviors stem from a complex interplay between genes and the environment. If rape is an adaptation, men must possess genes that exist specifically because rape increased reproductive success. If rape turns out to be merely a side effect of other adaptations, then the genes involved exist for reasons that have nothing to do with rape. Either way, however, the evolutionary perspective explains a number of otherwise puzzling facts about the persistence of rape among human males.

(26) For example, if rape is evolutionary in origin, it should be a threat mostly to women of childbearing age. And, in fact, young adult women are vastly overrepresented among rape victims in the female population as a whole, and female children and post-reproductive-age women are greatly underrepresented.

(27) By the same token, if rape has persisted in the human population through the action of sexual selection, rapists should not seriously injure their victims--the rapist's reproductive success would be hampered, after all, if he killed his victim or inflicted so much harm that the potential pregnancy was compromised. Once again, the evolutionary logic seems to predict reality. Rapists seldom engage in gratuitous violence instead, they usually limit themselves to the force required to subdue or control their victims. A survey by one of us (Palmer), of volunteers at rape crisis centers, found that only 15 percent of the victims whom the volunteers had encountered reported having been beaten in excess of what was needed to accomplish the rape. And in a 1979 study of 1,401 rape victims, a team led by the sociologist Thomas W. McCahill found that most of the victims reported being pushed or held, but that acts of gratuitous violence, such as beating, slapping or choking, were reported in only a minority of the rapes--22 percent or less. A very small number of rape victims are murdered: about .01 percent (that figure includes unreported as well as reported rapes). Even in those few cases, it may be that the murder takes place not because the rapist is motivated by a desire to kill, but because by removing the only witness to the crime he greatly increases his chance of escaping punishment.

(28) Rape is more distressing for women than are other violent crimes, and evolutionary theory can help explain that as well. In recent years research on human unhappiness informed by evolutionary theory has developed substantial evidence about the functional role of psychological pain. Such pain is thought to be an adaptation that helps people guard against circumstances that reduce their reproductive success it does so by spurring behavioral changes aimed at preventing future pain [see "What Good Is Feeling Bad?" by Randolph M. Nesse, November/December 1991]. Thus one would expect the greatest psychological pain to be associated with events that lower one's reproductive success, and, indeed, emotionally traumatic events such as the death of a relative, the loss of social status, desertion by one's mate and the trauma of being raped can all be interpreted as having that effect.

(29) Rape reduces female reproductive success in several ways. For one thing, the victim may be injured. Moreover, if she becomes pregnant, she is deprived of her chance to choose the best father for her children. A rape may also cause a woman to lose the investment of her long-term partner, because it calls into question whether the child she later bears is really his. A variety of studies have shown that both men and women care more for their genetic offspring than for stepchildren.

(30) One of us (Thornhill), in association with the anthropologist Nancy W. Thornhill, has conducted a series of studies on the factors that contribute to the emotional pain that women experience after a rape. Those studies confirmed that the more the rape interfered with the women's reproductive interests, the more pain they felt. The data, obtained from the Joseph J. Peters Institute in Philadelphia, came from interviews with 790 girls and women who had reported a sexual assault and who were subsequently examined at Philadelphia General Hospital between 1973 and 1975. The subjects, who ranged in age from two months to eighty-eight years, were asked a variety of questions designed to evaluate their psychological responses to the rape. Among other things, they were asked about changes in their sleeping habits, in their feelings toward known and unknown men, in their sexual relations with their partners (children were not asked about sexual matters), and in their eating habits and social activities.

(31) Analysis of the data showed that young women suffered greater distress after a rape than did children or women who were past reproductive age. That finding makes evolutionary sense, because it is young women who were at risk of being impregnated by an undesirable mate. Married women, moreover, were more traumatized than unmarried women, and they were more likely to feel that their future had been harmed by the rape. That, too, makes evolutionary sense, because the doubt a rape sows about paternity can lead a long-term mate to withdraw his support.

(32) Among the women in the study, psychological pain rose inversely to the violence of the attack. In other words, when the rapist exerted less force, the victim was more upset afterward. Those findings, surprising at first, make sense in the evolutionary context: a victim who exhibits physical evidence that sexual access was forced may have less difficulty convincing her husband or boyfriend that what took place was rape rather than consensual sex. In evolutionary terms, such evidence would be reassuring to a pair-bonded male, because rape is a one-time event, whereas consensual sex with other partners is likely to be frequent, and thus more threatening to paternity.

(33) Finally, women of reproductive age reported more emotional distress when the assault involved sexual intercourse than when it involved other kinds of sexual behavior. Among young girls and older women, however, penile-vaginal intercourse was no more upsetting than other kinds of assaults. Again, the possibility of an unwanted pregnancy may be a key factor in the degree of trauma the victim experiences.

(34) For all those reasons, the psychological pain that rape victims suffer appears to be an evolved defense against rape. The human females who outreproduced others--and thus became our ancestors--were people who were highly distressed about rape. Their distress presumably served their interests by motivating them to identify the circumstances that resulted in the rape, assess the problems the rape caused, and act to avoid rapes in the future.

(35) If women today are to protect themselves from rape, and men are to desist from it, people must be given advice that is based on knowledge. Insisting that rape is not about sex misinforms both men and women about the motivations behind rape--a dangerous error that not only hinders prevention efforts but may actually increase the incidence of rape.

(36) What we envision is an evolutionarily informed program for young men that teaches them to restrain their sexual behavior. Completion of such a course might be required, say, before a young man is granted a driver's license. The program might start by inducing the young men to acknowledge the power of their sexual impulses, and then explaining why human males have evolved in that way. The young men should learn that past Darwinian selection is the reason that a man can get an erection just by looking at a photo of a naked woman, why he may be tempted to demand sex even if he knows that his date truly doesn't want it, and why he may mistake a woman's friendly comment or tight blouse as an invitation to sex. Most of all, the program should stress that a man's evolved sexual desires offer him no excuse whatsoever for raping a woman, and that if he understands and resists those desires, he may be able to prevent their manifestation in sexually coercive behavior. The criminal penalties for rape should also be discussed in detail.

(37) Young women also need a new kind of education. For example, in today's rape-prevention handbooks, women are often told that sexual attractiveness does not influence rapists. That is emphatically not true. Because a woman is considered most attractive when her fertility is at its peak, from her mid-teens through her twenties, tactics that focus on protecting women in those age groups will be most effective in reducing the overall frequency of rape.

(38) Young women should be informed that, during the evolution of human sexuality, the existence of female choice has favored men who are quickly aroused by signals of a female's willingness to grant sexual access. Furthermore, women need to realize that, because selection favored males who had many mates, men tend to read signals of acceptance into a woman's actions even when no such signals are intended.

(39) In spite of protestations to the contrary, women should also be advised that the way they dress can put them at risk. In the past, most discussions of female appearance in the context of rape have, entirely unfairly, asserted that a victim's dress and behavior should affect the degree of punishment meted out to the rapist: thus if the victim was dressed provocatively, she "had it coming to her"--and the rapist would get off lightly. But current attempts to avoid blaming the victim have led to false propaganda that dress and behavior have little or no influence on a woman's chances of being raped. As a consequence, important knowledge about how to avoid dangerous circumstances is often suppressed. Sure-ly the point that no woman's behavior gives a man the right to rape her can be made with-out encouraging women to overlook the role they themselves may be playing in compromising their safety.

(40) Until relatively recently in Europe and the United States, strict social taboos kept young men and women from spending unsupervised time together, and in many other countries young women are still kept cloistered away from men. Such physical barriers are understandably abhorrent to many people, since they greatly limit the freedom of women. But the toppling of those barriers in modern Western countries raises problems of its own. The common practice of unsupervised dating in cars and private homes, which is often accompanied by the consumption of alcohol, has placed young women in environments that are conducive to rape to an extent that is probably unparalleled in history. After studying the data on the risk factors for rape, the sex investigators Elizabeth R. Allgeier and Albert R. Allgeier, both of Bowling Green State University in Ohio, recommended that men and women interact only in public places during the early stages of their relationships--or, at least, that women exert more control than they generally do over the circumstances in which they consent to be alone with men.

(41) An evolutionary perspective on rape might not only help prevent rapes but also lead to more effective counseling for rape victims. A therapy program explaining that men rape because they collectively want to dominate women will not help a victim understand why her attacker appeared to be sexually motivated, why she can no longer concentrate enough to conduct her life effectively, or why her husband or boyfriend may view the attack as an instance of infidelity. In addition, men who are made aware of the evolutionary reasons for their suspicions about their wives' or girlfriends' claims of rape should be in a better position to change their reactions to such claims.

(42) Unlike many other contentious social issues, such as abortion and homosexual rights, everyone has the same goal regarding rape: to end it. Evolutionary biology provides clear information that society can use to achieve that goal. Social science, by contrast, promotes erroneous solutions, because it fails to recognize that Darwinian selection has shaped not only human bodies but human psychology, learning patterns and behavior as well. The fact is that men, relative to women, are more aggressive, sexually assertive and eager to copulate, and less discriminating about mates--traits that contribute to the existence of rape. When social scientists mistakenly assert that socialization alone causes those gender differences, they ignore the fact that the same differences also exist in all the other animal species in which males offer less parental investment than females and compete for access to females.

(43) In addressing the question of rape, the choice between the politically constructed answers of social science and the evidentiary answers of evolutionary biology is essentially a choice between ideology and knowledge. As scientists who would like to see rape eradicated from human life, we sincerely hope that truth will prevail.

Randy Thornhill is an evolutionary biologist at the University of New Mexico in Albuquerque. Craig T. Palmer is an evolutionary anthropologist at the University of Colorado at Colorado Springs. This article was adapted from their forthcoming book A Natural History of Rape: Biological Bases of Sexual Coercion , which is being published in April 2000 by MIT Press.

Why Do Men Have Nipples?

Male mice could be considered more highly evolved, so to speak, than human males in one anatomical regard: nipples. Whereas men develop a pair of nipples — and sometimes more — that serves little purpose other than offering topographical variety on the chest, male mice exit the womb with no such markings, their furry bellies smooth and nipple-free. Alongside stallions and male platypuses, mice are among a small cluster of mammalian species that don't go ahead and sprout nipples as though in symbolic solidarity with their female kin [source: McCarthy].

During early pregnancy, both male and female mice embryos form mammary tissue, the foundational tissue that becomes the full-fledged nipples, nerves and glands that later facilitate female milk production. In 1999, Yale University researchers published a study identifying the protein responsible for lobbing off those nipple precursors in male mice. A few days after mice mammary tissue starts to form, it produces a protein known as PTHrP. In male mice, PTHrP signals the mammary cells to form male hormone receptors, and those hormones effectively destroy the tissue, leaving the he-rodents nipple-less [source: Lawrence].

Male humans meanwhile undergo a similar embryonic process, minus the mammary tissue takedown. After three or four weeks, all embryos develop parallel mammary ridges called milk lines before the 23rd chromosomes, XX or XY, have a chance to tinker with sexual dimorphisms, or physical traits that distinguish biological males from females. As fetuses grow, those milk lines that extend from the top of the chest to the lower abdomen recede and typically leave behind a pair of nipples, along with milk-producing glands called lobules, ducts and the fatty tissue in between [source: Adams]. Without a protein like PTHrP stepping into action to trigger hormonal roadblocks as it does in mice, human embryos, male and female alike, are given the same set of internal plumbing in the breast region. Then, during puberty, estrogen in girls spurs breast tissue and mammary gland development.

Therefore, the simplest explanation for men having nipples is that all human embryos start out with them, and evolution didn't get around to selecting against their existence on the male bust. Since nipples and healthy breast development are so closely linked with female reproductive success, evolutionary biologists suppose that the adaptive pressure to wean out male nipples wasn't strong enough [source: Simons].

And though the purpose of male nipples rarely extends beyond the decorative, to lump them with the appendix, wisdom teeth and other vestigial anatomy would be a technical mistake. How could, after all, men's nipples be relegated to the evolutionary rubbish bin when they can -- oh, yes, they can — produce milk?