Why are over 250 million sperm cells released from the penis during sex?

The average male will produce roughly 525 billion sperm cells over a lifetime and shed at least one billion of them per month.

A healthy adult male can release between 40 million and 1.2 billion sperm cells in a single ejaculation.

In contrast, women are born with an average two million egg follicles, the reproductive structures that give rise to eggs. By puberty, a majority of those follicles close up and only about 450 will ever release mature eggs for fertilization.

But if it only takes one sperm and one egg to meet and create a baby, then why do men produce such a whopping number of sperm? Wouldn’t it be less wasteful for a man to release a single sperm, or at least fewer, to meet one egg?

The reason for this predicament boils down to two words: sperm competition. Since the dawn of the sexes, males have vied with each other to get as many of their own sperm near a fertile egg as possible. Getting more of your sperm closer to an egg means there is a greater probability that it will be you and not your neighbor fertilizing it.

This kind of competition is an evolutionary imperative for males of any species. If a rival’s sperm fertilizes an egg, then an opportunity to pass on your genes is lost. Through many generations, as the reproductive spoils continually go to the highest sperm producers, their genes are passed on. The genes of the smaller sperm producers are eventually weeded out of the population and become a footnote to evolutionary history.

But if it was just a matter of ‘more is better,’ then animals of all species would have evolved ridiculously large testicles in a bid to overwhelm the competition. But it’s not quite that simple—numbers are important, but so is proximity. Fertilizing an egg is not just about how much sperm you can produce. It is also about how close you get your sperm to it.

In the early 1980s, researchers in the United Kingdom and the United States realized that both proximity and number were important factors in the physiology of primates, including humans. In primate societies with rigid social structures and one dominant male who mates with all the females, testes trend towards the small. In gorillas, for example, they are very small relative to body weight. (Don’t tell them that.) In gorilla society, one male defends a harem of females to ensure only his sperm gets anywhere near their eggs. In this case, making a lot of sperm doesn’t really help the male gorilla get the job done.

For chimpanzees, on the other hand, sperm competition is a serious issue. In chimpanzee society, many males and females live together in large troops, and females have sex with many males in a short span of time. This is why male chimpanzees possess the largest testes of all the great apes, weighing in roughly 15 times larger than gorillas, relative to their body weight. This gives them a better shot at swamping out the competition.

Human males fall somewhere in between gorillas and chimps. The average man’s testes are roughly two and a half times as big as a gorilla’s but six times smaller than a chimp’s, relative to body weight. This has led some researchers to question whether sperm competition was ever at work in human societies, or whether our relatively large testes are just a hold over from an earlier period in our evolutionary history.

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Why do we get bags under our eyes?

If the eyes are the window to the soul, then what do those hefty bags beneath your eyes say about you?

Co-workers, loved ones and even your coffee shop barista might be quick to point out that they make you look like a sleepy soul.

While people often associate under-eye bags with lack of sleep, one main cause may actually be much more fundamental: gravity. The gravitational pull weighs down on all Earthly objects, including your skin. The longer you’re exposed to gravity (i.e., the older you get) the more your facial tissues sink toward the floor.

But prolonged exposure to gravity is not the only bag-forming effect that the aging process bestows on us. As we get older, the tissues around our peepers change.

The upper and lower eyelids are composed of skin, muscle and fat. With age, the muscles weaken and can’t hold up the skin as tightly. Skin also changes because the collagen inside it degrades. Collagen is a protein that gives structure to our cells. In skin, it provides elasticity. With less collagen, the skin starts to wrinkle and sag.

Beneath the skin and muscle, the main culprit for under-eye puffiness is fat. “As you get older, your fat, like everything else, starts drooping,” says Dr. Melanie Grossman, a dermatologist in New York.

Fat deposits around our eyes help protect them. But in our 40s and 50s, these cushiony fat pockets can escape from the membrane that normally contains them. As the membrane weakens with age, the fat slips out and occupies new spaces under the skin. “When people have puffiness it may be misplacement of the fat,” says Grossman.

A new study by plastic surgery researchers at the University of California, Los Angeles shows that this long-accepted theory may be off the mark. Rather than the membrane weakening with age, the scientists report that the amount of fat beneath the eyes actually increases to cause baggy lower eyelids. More research is needed to pin down the exact mechanism to explain droopy lower eyelids, but scientists agree that as the calendar pages turn, the bag-forming process naturally progresses.

So is it fair when friends point fingers at your puffy eyes and tease that you’re not sleeping enough? Dr. David McDaniel, a dermatologist in Virginia Beach, says that while there is no proof of a relationship between snoozing and under-eye bags, it does seem that a lack of sleep affects the severity of the condition.

Some other behaviors that appear to affect puffiness are eating salty foods, which causes your body to retain water, and rubbing the eyes because of allergies. Irritants in the air such as pollutants and mold also seem to exacerbate the bags.

But changing your behavior won’t obliterate sagging lower lids from your face. There is a genetic factor at play as well. If your parents puffed up, then you probably will too at around the same age.

Envy those lucky individuals whose genes predict that their eye fat will remain at bay and the skin below their eyes will stay nice and tight. For the rest of us, there are remedies that people claim can at least minimize the bags.

Plastic surgery called blepharoplasty can remove or reposition the fat that creates under-eye bags. Sometimes surgeons pair this with Botox or facelifts to revitalize the face. Some specialty eye creams found on drugstore shelves claim to reduce puffiness.

Cheaper alternatives include folk remedies like cucumber slices and tea bags laid on top of closed eyes. People use them because they think the cooling from the cucumbers or the natural anti-diuretic in caffeinated tea might help. But there’s no proof that these techniques work, says Grossman. It’s not clear whether the caffeine can even penetrate the skin and, if it can, whether it has any effect.

If you’re genetically predisposed to get under-eye bags, there is not much chance of avoiding them. But maintaining the health of your skin can play down their appearance. “Overwhelmingly, good diet, exercise and sleep are probably the things you can do to help yourself,” Grossman says.

While there’s no definitive link between healthy behavior and smooth skin surrounding the eyes, that advice seems to echo the common refrain from doctors. “Your eyes reflect the health of your skin and your body,” says McDaniel.

And one more thing: dark circles under the eyes often coincide with bags, but these two ugly features occur separately. Aging is partially responsible for both. So what we see in others seems to be older (not sleepy) souls.

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The Trust Hormone

Oxytocin may make you more trusting, but is that a good thing?
Between emails from Nigeria promising millions of dollars in exchange for your bank account number and advertisements proclaiming the deal of a lifetime, someone is always after your money.

While no one is immune from the occasional bad investment, most of us pride ourselves on being too smart to be duped out of our hard-earned cash.

Recently though, researchers have discovered that the naturally occurring hormone oxytocin might make us more trusting with our money—even after someone betrays us. A Swiss study detailed in the May issue of the journal Neuron showed that volunteers who were given the hormone oxytocin through a nasal spray were more trusting than those given a placebo.

Using functional magnetic resonance imaging, a scanning technology that measures neural activity, the researchers found that the amygdala, the brain’s fear center, was less active in the group that received oxytocin.

Reducing amygdala activity lowers social fear and anxiety, says Markus Heinrichs, author of the study and psychology professor at the University of Zurich. Eventually, he thinks oxytocin could be used to treat disorders characterized by an overactive amygdala, like social phobia.

However, the prospect of inducing trust with a hormone—particularly if administered without consent—raises a few eyebrows. Ethicists worry about the potential for misuse, and one company is already marketing a “trust perfume” made with oxytocin.

“If it turns out that oxytocin makes someone more pliable and receptive, one can think of nefarious uses,” says Paul Root Wolpe, a neuroethicist at the University of Pennsylvania. For instance, he says it would be inappropriate to use oxytocin on prisoners or criminal suspects to elicit a confession because it is unethical to give drugs without consent, not to mention a violation of the Geneva Conventions.

Other researchers assert that oxytocin research is much more likely to lead to useful therapies than abuses, and they say the Neuron study was an important step. They think oxytocin could be used as a treatment for disorders such as social phobia and even autism. Those disorders are characterized by fear, so some think oxytocin could help make people more trusting and less fearful in social situations.

“People are very excited about oxytocin’s use,” says Adam Guastella, a senior clinical research fellow at the Brain and Mind Research Institute at the University of Sydney, Australia, who is not connected with the study. “There are many potential applications.”

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Oxytocin is a mammalian hormone produced in the hypothalamus, an almond-sized area at the base of the brain. The hormone is released by the adjacent pituitary gland, particularly during labor and breastfeeding. It has also been associated with sexual arousal, giving it the nickname the “love hormone.” But oxytocin’s function extends well beyond love, and recent studies have examined its role in trust and social interactions.

A 2005 study in Nature, for example, found that participants who received oxytocin were more likely to trust other people than participants who received a placebo. The recent Neuron study expands on the earlier experiment, showing that not only are people more trusting, but they are more trusting even when they know they are likely to be betrayed. The study also combines brain imaging data with the behavioral data to see which regions of the brain are affected by oxytocin.

“This was the first study where we combined brain imaging, social behavior and substance administration,” says Heinrichs.

Heinrichs and his colleagues performed the experiment on 49 male volunteers. Half of them received a nasal spray of oxytocin and half did not. Then each group played a trust game. The men had to decide whether or not to invest money with another volunteer in hopes of gaining more money. The amount they chose to invest would automatically be doubled. Then the volunteer receiving the money could decide how much, if any, to give back to the investor. The more money the volunteer invested, the more he stood to gain or lose.

The two groups played a round of six games and afterwards were told how often their investments paid off. About half the time, the trustee betrayed the investor. Each group then played another round of trust games with different trustees, and in this round the investors in the placebo group invested less often. But the oxytocin group continued to invest the same amount of money with the trustees.

After learning that their trust had been betrayed, the placebo group’s average investment dropped from about 7.7 to 6.5 out of a possible 12, while the oxytocin group’s average stayed about the same.

All participants were hooked up to a scanner while playing the trust games so their brain activity could be monitored. Researchers found that the amygdala was much more active in the placebo group than in the oxytocin group.

“The amygdala is very clearly identified with fear and anxiety,” says Ron Stoop of the Center for Psychiatric Neuroscience at the University of Lausanne, Switzerland. “When it is activated you get a fear response; when inactivated, people get very calm.”

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Oxytocin binds to proteins embedded in the cell membrane of the amygdala. This binding activates neurotransmitters that inhibit cells in the amygdala, reducing neural activity. The exact length of oxytocin’s inhibitory effect is unknown, but Stoop says it appears to only be short-lived.

Researchers are now trying to figure out whether oxytocin affects other areas of the brain, says Stoop. Oxytocin could be specific to the amygdala or it could have a general effect on the entire brain, he adds, noting that there are receptors in other parts of the brain like the hippocampus, where memories are formed, and the stria terminalis, which is a band of fibers that connects the amygdala to the hippocampus.

Oxytocin could eventually be used to treat social disorders characterized by excessive fear, such as social phobia, autism and post-traumatic stress disorder. Heinrichs says that the feeling of fear in these disorders is caused by a hyperactive amygdala. He thinks that oxytocin could help reduce that hyperactivity.

But Paul Zak, a neuroeconomist at Claremont University who studies how neural activity influences economic decisions, says it is premature to speculate on clinical uses of oxytocin. The study only evaluated its effects on men, and since oxytocin has a role in breastfeeding and childbirth, it would be important to determine whether the hormone has a different effect on women, he says.

Zak was also critical of the neural imaging used in the Neuron study. One effect of oxytocin is that it lowers a person’s heart rate. Since imaging shows brain activity based on the amount of blood flow, the activity in the amygdala might be lower not because it is being inhibited by oxytocin, but simply because there was less blood flow to the brain in general, according to Zak.

Sydney’s Guastella says it is also important to understand whether or not oxytocin increases pleasure or feelings of reward from social interactions. He says that for oxytocin to effectively treat social disorders, it will have to increase the person’s desire to interact with others, not just increase their likelihood to trust.

So far, giving people extra oxytocin has not produced any harmful side effects, but this needs to be researched better before being used as treatment.

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Still, researchers are taking oxytocin seriously, and not just as a potential clinical treatment. They also acknowledge that its ability to shape behavior raises certain ethical questions. “You can imagine a wide range of scenarios,” Stoop says. He points to politicians potentially using oxytocin to gain voters’ trust or executives who might use it to garner trust in a business deal.

“The danger would be if you could administer oxytocin in a way that someone would not realize it,” Heinrichs says. But, he says, that would be difficult to do since oxytocin has to be administered through a nasal spray to be effective.

However, the Florida-based company Verolabs is already marketing a “liquid trust” perfume. Company officials would not return phone calls, but their website boasts that the perfume will help wearers gain the trust of others in both their professional and romantic lives.

Heinrichs and others dismiss this idea, saying that aerosolized oxytocin would not affect the brain because it quickly degrades. Oxytocin in a perfume spray would diffuse rapidly in the air and it wouldn’t reach the brain in high enough concentrations to have any effect, researchers say. Companies are just looking to make a quick buck, Heinrichs says. Even so, the University of Pennsylvania’s Wolpe still has concerns. “It does raise a number of questions on how it is we form relationships with other people and why some people are more trusting and others are more suspicious and wary,” he says.

But, he also points out that the effects shown in the Swiss study were relatively mild. Oxytocin didn’t cause a dramatic change in behavior, so it would be premature to jump to conclusions on how it could be used, he says. “This isn’t a magic bullet to put something over on someone,” he says.

It is important to figure out the context in which oxytocin could or should be used, says Eric Racine, director of the neuroethics research unit at the Institute of Clinical Research in Montreal. “This study really reflects the exciting advances that have been made in neuroscience, but also the potential ethical implications of knowing how we think, feel and behave,” says Racine. While other fields of science such as genetics and stem cell research have long spurred ethical debates, advances in neuroscience are just starting to raise similar ethical questions, Racine says. Just because we can shape behavior and feelings, he adds, doesn’t mean we should.