So how does human semen compare? The evolution of the human SEMG2 gene has been found to be midway between chimpanzees and gorillas, just like testicular size. In fact, research published in 2008 found that Eastern Lowland gorillas (Gorilla beringei graueri) are so confident when it comes to paternity that their SEMG2 has become a pseudogene, a piece of defunct genetic currency that the cells can no longer cash in to form a usable (functional) protein.
It’s important to note, that when faced with competition from other males, not all species have simply, over time, increased the number of sperm they ejaculate. Scientists have observed that in some species, such as the European bitterling fish (Rhodeus sericeus), the number of sperm are actively reduced when there is too much competition. The reason for this is conservation: if the chances of success go down (because there are too many males), then it may be a waste of resources to spend too much energy on sperm. But who knows—some male fish do get lucky, so it may still be worth a shot, even if it’s with less sperm.
NOT ALL BOYS experience nocturnal emissions or “wet dreams.” For those who do, they can actually be a positive sign that everything is coming together, from semen production to the complex wiring of the nervous system that allows for arousal and culminates in ejaculation and orgasm. Like a girl reaching menarche, the first time a boy ejaculates, he’s far from fully fertile; the concentration of sperm in his ejaculate is still very sparse, and it will be anywhere from one to three years before he reaches full fertility. The average ejaculate of a mature male contains between 40 and 150 million sperm, while a young man’s first ejaculate may contain significantly less—but he’s getting there. None of which means he’s incapable of impregnating someone, of course.
Sperm are little marvels. Impossibly tiny—they’re among the smallest cells in the human body—they are also fast, efficient, and adaptable. Sperm are just one five-thousandth of an inch long; egg cells (ova), on the other hand, are among the largest human cells, about thirty times larger than the heads of sperm.
Sperm make up around 1 percent of ejaculatory fluid, but when it comes to semen it’s really all about them. The followers of the Greek thinker Pythagoras believed that semen was a “clot of brain containing hot vapour,” but in fact 99 percent of it is composed of sugars, fats, proteins, and alkaline fluids that variously serve to provide energy, security, assistance, or safe passage to sperm as they set off on their journey to penetrate an egg.
Here’s what happens when a man ejaculates: sperm move through the vas deferens, which carries them up into the body toward the prostate gland. Right before they reach the prostate, the vas deferens merge with ducts from a pair of glands called the seminal vesicles to become the ejaculatory ducts. As the ducts merge, the sperm are combined with fluid from the seminal vesicles that includes amino acids, vitamin C, sugars to provide energy for the sperm, and prostaglandins, compounds designed to mildly suppress the female immune system to prevent it from attacking the sperm. The fluid from the seminal vesicles makes up about 60 to 70 percent of semen.
The ejaculatory ducts then pass through the prostate gland, which secretes additional fluid that is highly alkaline, which will help to neutralize the natural acidity of the vaginal canal, ensuring better conditions for sperm to survive as they enter the urethra. This prostatic fluid is also rich in zinc, and makes up about 25 to 30 percent of semen. From there the combined sperm and seminal fluid pass by the opening to the bulbourethral glands. These glands have already done most of their work, emitting some of the clear liquid known as pre-ejaculate that clears the way for semen by cleaning up any traces of urine or acid in the urethra. Finally, the sperm is pleasurably shot out of the penis, searching to fertilize an egg.
For most sperm, of course, that’s an impossible dream. The numbers alone make that clear—with 150 million related competitors and one (maybe two) eggs if—and it’s a big if—your partner is ovulating, the odds are pretty stiff. On top of that, most sperm just aren’t up for the job. Dr. Harry Fisch, a urologist at Columbia University Medical Center and a specialist in male fertility, states that “only a perfectly normal sperm can penetrate an egg and the majority of sperm are abnormally shaped.” “Abnormal” can mean two heads, no tails, or just no ability to move at all. According to Dr. Fisch, a man with 15 percent viable sperm is doing very well indeed.
But every once in a while some lucky sperm swimming upstream detect an egg meandering downstream through the Fallopian tubes. When they sense the egg’s chemical signature, they switch to what Australian sperm expert Dr. Moira O’Bryan calls a “crazed figure-eight motion.” A few of those actually get close enough to the egg to penetrate its outer shell and, sometimes, almost magically, one of them does.
Just often enough for all of us to be here.
BY THE WAY, there’s one interesting way for a man to tilt the odds toward reproductive success: pornography. But not just pornography involving women—it has to involve the competition, so to speak. Think back to our discussion of sperm competition relative to testicle size. Larger testicles mean more sperm, and this increases the odds of reproductive success.
But having more sperm isn’t the only way to improve your odds in the fertilization race—faster, stronger sperm can make a real difference, too. A 2005 Australian study showed that when men looked at pornographic images of two men and a woman together, they produced significantly better quality sperm than when they looked at images of just women. Evolutionary biologist Leigh Simmons, one of the researchers of the study, stated that “males ejaculate more sperm, or sperm of better quality, when the risk of sperm competition is high…. We found men viewing images containing both men and women had higher sperm motility in masturbatory ejaculate compared to men who were viewing images of just women alone.”
Although there is still a lot of confirmatory work that needs to be done, this research is really exciting because it suggests the very strange and hard to believe possibility that men can actually dictate the quality of sperm they ejaculate.
BY THE WAY, just because most of semen is a sophisticated support team for sperm success, it often has more of a starring role sexually, especially where oral sex is concerned. As far as semen is concerned, you are what you eat. Yes, it’s true: what you eat affects how your semen tastes. Foods and beverages with bitter flavors, like coffee and alcohol, can make your semen taste bitter. Foods with more delicate flavors, like pineapple, celery, and melon, can make your semen taste less “strong.” And sure enough, someone with a diet rich in meat is likely to produce semen that, according to some, is thicker and gummier. For the lightest semen of all, vegetarians are the connoisseur’s choice.
But don’t take my word for it. The BBC actually commissioned a taste test to check out the theory in the real world. Three couples participated. The men were put on one of three specific diets for three days and their partners were not told which one. One man ate all seafood, another ate hot and spicy food, and the third was put on fruits and vegetables. The women were asked to identify their partner’s diet after tasting a sample of his semen from a plastic test tube on camera. Sure enough, the woman whose partner ate all seafood identified a fishy quality, and the woman whose partner ate all fruits and vegetables found his semen to be positively “lighter” than it had been. Incidentally, the woman whose partner was on the seafood diet demanded an immediate return to her partner’s cheeseburger-eating ways. She hates fish.
By the way, we’re not the only animal that engages in oral sex. It seems that some types of macaques, cheetah, hyenas, gibbons, and even goats, perform oral sex and even swallow semen.
Semen can actually do far worse than leave a bad taste in your mouth. Some people are actually allergic to their partner’s semen, and when they are exposed to it through any type of sexual intimacy, it can cause itching, burning, and, in rare cases, difficulty breathing. A semen allergy can be a response to specific proteins in your partner’s semen alone or the result of a more general allergy to all semen. The good news is that
semen allergies can be treated, and you can usually be desensitized. How? By repeated exposure to the allergy-prompting semen. But take note: a serious semen allergy, like any other allergy, can be dangerous, even life-threatening, and desensitization should only be done in consultation with a doctor.
And there’s a little more good news. Once desensitized, you need regular maintenance procedures to preserve the reduced sensitivity. Otherwise known as regular sex.
CHAPTER 3
i’m so excited and i just can’t hide it
Millions of words—in books and on blogs, in magazine articles and advice columns—have been written about mastering the search for Ms. or Mr. Right. And millions of people have wondered why they keep dating the wrong man or woman when they think they know what’s really good for them. They think they know what they need; the problem is, what they need isn’t always what they want.
So why do we want what we want anyway? How much of what turns us on is hardwired?
Like everything else, attraction and arousal (and possibly love, for that matter) are the products of millions of years of finely tuned biological engineering. And there’s really only one goal behind the engineering—to get you to have sex.
As you’ll see, much of what we are preprogrammed to find attractive may be connected to what it tells us about the health, fertility, and compatibility of potential mates. Genetic compatibility, that is. Nature isn’t really concerned about similar political views or favorite movies, although it does place stock in appearances. From the standpoint of survival, in some sense it really cares that we have strong offspring, and that they get what they need to grow up and give us grandkids in turn. But genetic compatibility only gets us halfway there in terms of successful offspring—it can give us healthy babies, but those babies need parents to protect and nurture them into maturity. And that’s where love comes in. Falling and staying in love—the pair bonding that keeps a couple together long enough to have, raise, and care for children—almost certainly involves chemical processes that are a product of millions of years of evolution.
A new study published in March 2008 in the journal Evolution and Human Behaviour shows that being in love with someone actually works to dampen the sexual appeal of people of the opposite sex that we might otherwise find attractive. In another study, Florida State University researcher Jon Maner took two groups of heterosexual college students who “were currently in a committed romantic relationship” and showed them rapid-fire bursts of images depicting very attractive and average-looking men and women. Before watching the images, each group wrote essays. The first group of students watched the images after writing essays about extreme happiness. The second group wrote essays about moments when they felt extreme love toward their partners. In the group that was primed by writing about happiness, the participants in the study seemed to pay about the same amount of attention to attractive people and average people. But in the group that wrote essays about moments of extreme love toward their partners, their attention, in the words of the researchers, “was captured substantially less by attractive alternatives than by other targets.” The researchers’ theory is that concentrating on the love you have for your partner may block the normal reflexes that might otherwise cause you to consider other potential “attractive” partners. This may be a mechanism that evolved to keep couples together. If you keep seeking alternatives to your partner, the likelihood of building a lasting and successful relationship, especially one that leaves you with children, diminishes. There will always seem to be a more attractive prospect than the person you love, so love may work to stop you from looking. Maner further described his findings:
We found that when people just thought about being in love with their current partner, their visual attention got repelled, rather than grabbed, by an attractive member of the opposite sex. [That] happens at the very initial stages of visual processing, at the very first moment they are aware of the photo.
Love was actually working to limit individual receptivity to potential sexual partners that posed a threat to existing mates. Joseph Forgas, a psychologist from Australia’s University of New South Wales, explained the potential significance of this research:
Psychologists have long had a problem explaining the functions of romantic love, a very strong emotion that sometimes seems to take over our lives and lead to what appear to be irrational feelings and actions. What these studies suggest is that romantic love serves a very important function, tempering our natural desire to pay attention to, and to continuously seek out, the best available mate.
BEFORE WE PLUNGE into exactly what turns us on and why, let’s pause for one further twist. A lot of what we find sexually attractive may be hardwired, but I need to mention a little wrinkle. What we find attractive can change. For some people, who they find sexually attractive can actually be different depending on the time of the month.
Numerous studies have shown that women’s preferences shift with their menstrual cycles. When they’re at peak fertility, the few days before, during, and after ovulation, they lean heavily toward supermasculine types: think tall, deep-voiced, darker-skinned, and muscular. Many of these traits can act as a sort of well-groomed genetic résumé, telegraphing the health and fitness of a potential partner and suggesting his suitability as a source for future children.
“Women know they have attractions that come and go, but they probably don’t realize that these urges are tied to their cycle—as well as our evolutionary past,” writes Martie Haselton, a UCLA researcher who has studied the connection between attraction and fertility. “They just know that suddenly one day they’re attracted to their hunky neighbor or handsome co-worker…. Ancestral women who were attracted to these features produced offspring who were more successful in attracting mates and producing progeny. The legacy of the past is desire in the present.” In other words, at ovulation time, good traits mean good mate.
Of course, good traits aren’t all that high levels of testosterone can mean. Higher levels of testosterone correlate with higher levels of aggression, a trend toward dominance, and a lack of fidelity. Which is possibly why when women are not ovulating, they tend to be attracted to a different set of characteristics, like softer features and larger eyes, which some link with stability, nurturing, and other qualities that suggest someone would be a good partner and parent.
A recent study led by Dr. Haselton seems to indicate that women actively work to make themselves look more attractive when their fertility is highest. Researchers recruited a group of adult women of childbearing age, between eighteen and thirty-seven, and photographed them twice: first, when they were close to ovulating, when fertility is highest, and second, when they were close to menstruating, when fertility is lowest. Dr. Haselton and her team then showed the photos to a group of volunteers, asking them, “In which photo is the person trying to look more attractive?” The researchers wanted to understand if the women actually changed the style of their clothes and accessories in a way that related to their fertility levels. The women’s faces were blocked out so variations in facial expressions wouldn’t distract the volunteers from the women’s clothes, jewelry, and so forth. Sure enough, overall, the volunteers thought that the women were trying to look more attractive in the photos that were taken around the time the women were ovulating. It’s worth noting that this fertility effect appeared even though all the women photographed in the study described themselves as being in committed relationships with men.
It’s almost as if women are looking to mate when they’re ovulating, but for a mate when they’re not. And by the way, when women are looking for the right set of traits in a potential father, they’re willing to look a little farther and a little wider than usual. Two recent studies have shown that women are more likely to cheat when they’re ovulating.
“We found that women were most attracted to men other than their primary partner when they were in the high fertility phase of the menstrual cycle,” Haselton has said. “That’s the day of ovula
tion and several days beforehand.”
A related UCLA study published in Evolution and Human Behavior found that women were more likely to fantasize about men other than their partners when their fertility was highest. Both studies, however, found an exception—women with highly attractive partners did not experience the heightened desire to stray or fantasize about it.
By the way, you undoubtedly recognize that the effects of cheating can be quite complicated, but if you think you’ve heard all the ways cheating can change things, think about this: cheating can lead to twins with different dads. That’s right. It’s called heteropaternal superfecundation, and here’s how it works. As in the normal conception of fraternal twins, a woman releases two eggs when she ovulates. But instead of having sex with only one man during her ovulatory period, this future mom of half-sibling twins has sex with two (or more) men and each egg is fertilized by a different man’s sperm. So they share the same mother but have different fathers. If you’re curious as to how common bipaternal twins may be, one paper suggested that one in four hundred pairs of fraternal twins born to white married women in the United States may actually have different fathers.
Before any woman considers using ovulation as an excuse for her promiscuity or any man starts tracking his partner’s menstrual cycle out of paranoia, they would do well to consider the view of Elizabeth Pillsworth, a UCLA assistant professor of journalism and psychology who coauthored one of the studies. “Whether they [these desires] translate into unfaithful behaviors is a matter of their own choosing. Cheating is a choice,” observes Pillsworth. “I hope the message women get is that they can use this information to realize their biology is toying with their desires and to ask themselves, ‘Am I going to let that run my life, my sexual decision-making?’ For the men I would say not to be too fearful of these findings. While women may notice other men during this part of their cycle, unfaithful behavior is relatively rare.”
How Sex Works Page 7