Making a butch mouse

ScientificAmerican.com August 05, 2007 Inside Every Girl Mouse Brain Is a Swaggering Boy Mouse—And It Doesn't Take Much to Bring Him Out All it takes is a little genetic manipulation or brain surgery to make female mice act like pelvic thrusting, rump-sniffing, aggressive males Female lab mice tend to be docile, passive creatures. But by either genetically shutting down or surgically removing their ability to smell pheromones, scientists transformed them into aggressive, pelvic-thrusting, vocalizing lotharios—without any significant rise in testosterone or other steroid hormones. "The female brain has the neuronal circuit both to control male and female behavior," says molecular neuroscientist Catherine Dulac of Harvard University. "What is sexually dimorphic is the switch that allows one to be silenced." The key to gender-specific behavior, in mice at least, is a cluster of receptors in their noses that allows them to smell pheromones, special chemicals that deliver information about sexual readiness, among other things, between members of the same species. Called the vomeronasal organ (VNO), it connects to the brain and registers the gender of other mice, triggering the appropriate response. But when the researchers genetically disabled the VNO, female mice began to chase their male peers, mount them and attempt to pelvic thrust [see video here ]. "From a behavioral standpoint you could not recognize the animal from being any different than the male," Dulac adds. "All the thinking until now was that female brains can produce feminine behaviors while male brains can produce masculine behaviors, with little or no cross talk between them," says Marc Breedlove, a neuroscientist at Michigan State University in East Lansing. "These results do suggest that, at least for mice, the brain retains circuitry to display both masculine and feminine behaviors into adulthood." Surgically removing the VNO also provoked the same behavior, the researchers report online in Nature. The full range of male mouse behaviors were on offer: from pelvic thrusting to high-pitched songs designed to woo mates. "Females never do this—ultrasound vocalizations," Dulac notes. "People thought that they didn't have the larynx." Although the females acted macho at times, they also retained the biological functions of their gender, ultimately becoming pregnant when paired with males (though only after aggressively trying to mount said males). Their maternal instinct was not as strong, however, as they wandered off from their pups after a few days, unlike normal mouse mothers. It remains unclear if disabling the VNO in males would make them act like females, because female lab mice are largely passive and do not exhibit any particularly distinct behaviors, Dulac says. But it is very clear female and male mouse brains are functionally identical. "It's easier to do it that way," Dulac adds. "If you have to imagine how to build two entirely different circuits, it's complicated." It also remains unclear how broadly the finding may apply. Fruit flies display similar behavior, but additional studies will be needed to assess the wider effect. "For other animals, we need to explore whether the VNO has this same dramatic effect," Breedlove says. "Is this true for all rodents? What about carnivores?" As for humans, we appear to lack VNOs and pheromones may play a less critical role in our mating rituals. "Humans are relying much more on visual cues," Dulac notes. But our brains may be wired similarly with "both circuits present and one is constantly inhibited." The key to gender-specific behaviors may have less to do with hormones and more to do with how various neural circuitry gets triggered.

Howstuffworks "How Women Work"

Howstuffworks "How Women Work"

How Women Work
How Women Work
by Tracy V. Wilson

Introduction to How Women Work
If you believe what you see on TV, women are inscrutable, conniving, hysterical and apt to change their minds without reason or warning. Some women's magazines perpetuate these stereotypes by offering advice on how to entrap men or keep them guessing. And some of the basic differences between men and women can seem a little confusing, depending on your point of view. So it's not surprising that one of the most requested articles in the history of HowStuffWorks is "How Women Work."

The irony is that from conception until the eighth week of gestation, men and women are almost exactly the same. The only difference is at the chromosomal level, deep inside the embryos' cells. Inside every cell of a person's body, DNA is tightly wound into pairs of structures called chromosomes. One pair of chromosomes determines whether the person is male or female. Except in the case of extremely rare abnormalities, a person with two X chromosomes is female, and a person with one X chromosome and one Y chromosome is male. For a few weeks, these chromosomes are all that differentiates male embryos from female embryos.

X and Y chromosomes
Image courtesy National Human Genome Resource Institute
A karyotype, or chromosome "map," for a normal
human male, showing X and Y chromosomes

Of course, by the time an embryo has grown into an adult woman, many attributes make her different from a man. On average, women are shorter and smaller than men are, although women have a higher percentage of body fat. Women have reproductive organs that can support a developing baby and nourish it after its birth. Their blood pressure is lower, and their heart beats faster, even when they're asleep [Source: FDA]. Women also have faster blood flow to their brains and lose less brain tissue as they age than men do [Source: Psychology Today].

And then, of course, there are hormones, which a lot of people view as a huge difference between men and women. But every person's body, whether it's male or female, uses hormones to regulate and control a wide range of processes. Hormones are the products of the endocrine system, which includes numerous glands located in various parts of the body. For example, two well-known hormones are adrenaline, which comes from the adrenal gland, and insulin, which comes from the pancreas. These and other hormones are vital to the lives and health of both men and women. To learn more about the endocrine system, watch this ADAM animation.

Sex hormones, on the other hand, work a little differently in men's and women's bodies. In men, the testes produce the hormone testosterone, which regulates sperm production and causes masculine secondary sex characteristics. In women, the ovaries produce hormones like estrogen and progesterone, which regulate reproductive processes. Men's bodies convert a little testosterone into estrogen, and women's bodies make small amounts of testosterone, so neither hormone is exclusive to one sex or the other.

estrogen molecule

A man's testosterone levels can fluctuate throughout the day as his body regulates its production of sperm. But a woman's sex hormone levels fluctuate as part of her reproductive cycle, which takes about a month to complete. During a woman's childbearing years, the recurring changes in her hormone levels can cause symptoms like irritability and moodiness, known as premenstrual syndrome (PMS). When a woman reaches perimenopause, her body slows down its production of sex hormones. During the process, her levels of estrogen and progesterone can vary significantly, causing symptoms like hot flashes and trouble sleeping.

Sex hormones can affect a woman's emotions and physiology throughout most of her life. But contrary to some people's perceptions, they're not responsible for every facet of her behavior. In this article, we'll look at some other common perceptions and stereotypes about women as we examine how they work.

Boy Babies and Girl Babies
Research suggests that testosterone in a woman's body may contribute to the sex of her children. It may encourage eggs to allow fertilization from a sperm carrying a Y chromosome, resulting in the birth of a boy [Source: Psychology Today].

Women and Emotions
A 2001 Gallup poll asked American adults whether a series of qualities applied more to men or to women. Ninety percent of those surveyed said that the characteristic "emotional" applied more to women. The survey didn't ask about particular emotions or specify positive or negative connotations for the word "emotion." But it seems likely from the results that most Americans view women as either able to experience or prone to experiencing a wider, more intense range of emotions than men do.
Confusing on Purpose?
One common stereotype is that women give mixed signals, especially when it comes to romantic involvement. A study at the University of Texas at Austin suggests that the sexes misunderstand one another and that there are evolutionary forces involved. The confusion may come from early human men trying to have more descendants while early human women tried to protect themselves from deception [Source: Psychology Today].

Are women more emotional than men are? Do they cry more?
The perception that women cry more than men is pretty widespread. But as babies and children, boys and girls cry about the same amount on average. Only during puberty do girls begin to cry more than boys do. According to a 2005 New York Times article, by age 18, women cry four times as much as men.

A possible explanation for this is the hormone prolactin, which contributes to how much people cry. Prolactin is present in blood and tears, and it's more prevalent in women than in men. Women's tear ducts are also shaped a little differently from men's, which could be either a cause or an effect of increased crying [Source: New York Times]. In addition, people who are depressed may cry four times as much as people who are not, and two-thirds of people diagnosed with depression are women [Psychology Today].

Of course, another common explanation is that some societies encourage women to cry while discouraging men from crying. In the United States, an exception to this standard seems to be the business world. In some businesses, crying is discouraged -- a woman who cries in the office may be viewed as weak or ineffectual [Source: New York Times].

Are women more stressed out than men are?
Women sometimes have a reputation for being worriers. According to a 2005 Gallup poll, women are more worried about a range of social issues than men are. Significantly more women than men answered that they worried "a great deal" about seven of the 12 issues in the survey.

Studies show that, in addition to worrying more often, women may be physiologically prone to experiencing more stress. For example, the amygdala of the brain processes emotions like fear and anxiety. In men, the amygdala communicates with organs that take in and process visual information, like the visual cortex. In women, though, it communicates with parts of the brain that regulate hormones and digestion. This may mean that stress responses are more likely to cause physical symptoms in women than in men [Source: Live Science].

Brain with amygdala highlighted

In addition, women's bodies produce more stress hormones than men's bodies do. Once a stressful event is over, women's bodies also take longer to stop producing the hormones. This may be a cause or an effect of women's tendency to replay stressful events in their minds and think about upsetting situations [Source: Psychology Today].

Are women more jealous than men are?
In some people's minds, women are more jealous and possessive than men are, especially in the context of romantic relationships. But research suggests that women aren't more jealous than men -- they're just jealous about different situations.

In one German study, researchers showed participants images of several scenarios. The participants used a computer to describe which of the scenarios would be more upsetting. The results suggest that, across cultures, women find emotional infidelity more upsetting than sexual infidelity. Men's responses varied across cultures, but in general they were jealous of sexual infidelity [Source: Human Nature].

On the other hand, a study at the University of California at San Diego measured participants' blood pressure and heart rate rather than asking them to describe their responses. Men had greater physical reactions to physical infidelity, while woman reacted with about the same intensity to both scenarios. Women who were in committed relationships were more upset by physical infidelity than those who were not. However, 80 percent of the women in the study thought emotional infidelity would be more upsetting to them than physical infidelity [Source: Psychology Today].

Next, we'll look at some common perceptions of how women learn and communicate.

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