And if homosexuality prevents reproduction, why is it common in so many different cultures around the world—especially in men? For that matter, why is it common in so many species: sheep do it, monkeys do it, even killer whales do it. One of the biggest costs of sex is that it exposes individuals to the possibility of sexually transmitted infections STIs.
As we set out to explore how sex works and examine all the fascinating differences between male and female sexuality, keep this in mind: ovaries and testes—and the sexual organs most likely to give us pleasure when stimulated, the penis and the clitoris—started out in the very same place, from the very same parts.
The first menstruation, called menarche, is only one in a series of events that mark the transformation of girls into young women, but it is one that has been loaded with cultural significance throughout human history. For a long time, menarche was thought to coincide with the onset of fertility. Now we know that most girls do not ovulate with menarche; in fact, it can take a year or two after their first menstruation before ovulation even becomes regular. Something strange is affecting the age of menarche, making parents and researchers take note and wonder alike.
The average age of menarche has crashed from a traditional seventeen to just twelve, in the evolutionarily brisk span of just years. So what is turning young girls into young women so quickly? There are lots of theories, but no clear answers. According to the psychosocial acceleration theory , the root cause is increased stress, and a few studies have, in fact, found a correlation between increased stress and earlier menarche.
In earlier eras, stress was usually the result of circumstances that threatened survival, such as conflicts or famine. In those situations, there might be an evolutionary benefit to earlier menarche, because it would give an individual a chance to reproduce faster, perhaps before succumbing to local threats. But as far as your brain and body are concerned, stress is stress and it produces the same result.
A large study involving 1, college women, published in in the American Journal of Human Biology , indicated that both the absence of biological fathers and the presence of half brothers and stepbrothers had an impact on earlier menarche. And how is that signal sent? Well, it might be by scent. Another theory that has been garnering considerably more weight over the last few years revolves around the skyrocketing rates of childhood obesity.
One recent study that looked at weight and age of menarche was conducted by Joyce Lee at the University of Michigan. Lee tracked girls from the time they were three until they were twelve. She found that there was a clear link between extra weight and early puberty. In her study, obese girls—twenty-two pounds or more overweight—had an 80 percent chance of developing breasts before they were nine years old and reaching menarche before they turned twelve.
According to William Lassek, a researcher at the University of California at Santa Barbara: What our findings suggest is that menarche is likely to occur when girls have stored a certain minimal amount of fat in the hips and thighs, and that girls who tend to store more fat around the waist—who have abdominal obesity—are likely to have delayed menarche. Scientists know that sufficient fat stores are key to the onset of menarche. And as Lassek points out, fat located in the lower part of the body is chock full of the omega-3 fatty acids that are so important for fetal brain development.
This fat is protected from everyday use like money deposited in a bank, says Lassek. You are not allowed to withdraw it until late pregnancy. This long period of mismatch is very confusing for young people, says Peter Gluckman of the University of Auckland, New Zealand.
How Sex Works: Why We Look, Smell, Taste, Feel, and Act the Way We Do - Sharon Moalem - كتب Google
According to Gluckman, with the advent of agriculture the overall level of nutrition dropped, resulting in a decrease in nutritional health, and increase in the age of menarche. Early menstruation creates a mismatch for some girls. They may be physically ready, but emotionally and intellectually unable to handle the responsibilities of adult sexuality. Menarche is essentially the culmination of puberty in girls. Puberty itself is the incredible collection of physiological processes that transform children into adults, with sexually mature bodies capable of reproduction.
For example, scientists have recently discovered a protein called kisspeptin named in honor of Hershey Kisses by researchers at the Penn State College of Medicine , which plays an important role as a biological signal in starting both puberty and ovulation.
But even though the physical transformation into a sexually mature human being is more or less on biological autopilot, adult sexuality is anything but just biological. Modern sexuality is the intersection of biology, society, and history.
What we need, what we want, what we like, and how we like it are all shaped by a combination of evolutionary imperatives, cultural training, and individuality. Evolution, of course, has a keen interest in encouraging us to have sex, even if it comes at a significant cost. The guiding imperative for any species is survival. At least for us, and for most vertebrate animals, no sex means no babies, and no babies means extinction.
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Having an interest in our sex lives clearly has evolutionary advantages. In girls, this means breasts and rounded hips, usually accompanied by more body hair; especially the growth of hair under the arms and in the pubic area. Within about two to four years of the onset of puberty, most girls have breasts that are nearly mature. Human breasts are unique among primates: we are the only species in which breasts enlarge at puberty and remain enlarged throughout life.
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Among apes and our other primate cousins, breasts only swell when a female is nursing. But human breasts have two functions: one parental and the other sexual, and both functions appear to have played an evolutionary role. Human breasts are composed of fat and modified sweat glands, called mammary glands. The fat is what makes them noticeable.
The mammary glands can produce milk, which is a specially configured mixture that includes carbohydrates, protein, fat, vitamins, minerals, and hormones—exactly what the baby needs. Besides facilitating mother-child bonding, breastfeeding also provides babies with antibodies, which are not found in commercial formula and can provide crucial protection against infections. The ducts of the mammary glands terminate at the nipple, which is surrounded by a modified circle of darker skin called the areola.
Most countries today recognize the importance of providing human milk for infants, but a few nations, such as Norway and Sweden, go one step further to make sure that babies receive age-matched milk. They have developed extensive milk donor programs, essentially, milk banks that provide human milk for infants whose mothers cannot breast-feed.
Donor mothers are screened for diseases that can be transmitted through breast milk, such as HIV and hepatitis B, and then their milk is collected. The donated milk is usually pasteurized and frozen to reduce the chance of contamination. I first came across milk banks during a research trip to Sweden a few years ago and was really surprised at just how passionately doctors and parents believed the milk banks improved the health of infants.
There are a few milk banks in the United States, but nowhere near the same scale. The color of the areola is especially variable, ranging from quite dark to very pink in some fair-skinned individuals. Large areolae are important visual cues, creating the illusion of a larger breast.
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Book Description Condition: New. Sharon Moalem. Publisher: HarperCollins , This specific ISBN edition is currently not available. View all copies of this ISBN edition:. From the Back Cover : Why are women biologically driven to find Mr. Find out the answers to such provocative questions as: Can the birth control pill influence the type of men women are attracted to? What do men and honeybees have in common when it comes to sex? Why do hourglass-shaped women tend to be especially fertile?