Thursday, December 21, 2006

Part 2 of Pumped Up and Ready for Love: Sex and Fluid Physics

More ways that your experiences during arousal and sex result from changes in your blood flow, and how you can use fluid physics to make it even better.

Listen to the podcast with roboreaders Audrey and Paul.

Circulation's Chemical Connection

Last week, we talked about several ways to increase the volume of blood in your erectile tissues. Generally, the methods involved inhibiting the blood flow out of your groin through the veins - with selectively applied pressure or sexual aids such as cock rings and penis pumps. Another way to enhance your penile or clitoral erection is to improve the flow of blood in towards your genitals.

A simple way to do that is with the use of drugs that relax the muscles that constrict your genital arteries. The relaxed muscles allow the arteries to expand in diameter.

According to the laws of fluid physics, pressure in a tube carrying a flowing liquid will be higher where the tube diameter is larger, and lower where it is narrower.

(Incidentally, that's what happens in an arterial aneurysm. A defect in an artery leads to a bubble in the arterial wall. Because the diameter is larger in the expanded bubble, the pressure increases and leads to a growing aneurysm. It's a vicious cycle that can eventually result in a disastrous pop, or other, equally unpleasant complication.)

The expansion of the diameter of healthy genital arteries, raises the pressure in your erectile tissue, compressing the outgoing veins and increasing the volume of the erectile reservoir.

Urologist Giles Brindley famously demonstrated the approach by injecting his own penis with the muscle relaxant Phentolamine. He displayed the resulting erection, while making a presentation at a medical convention in 1983. Many men faced with erectile dysfunction began injecting their penis's with muscle relaxant, in the years following Brindley's display.

More recently, a growing number of men have opted for drugs such as Viagra and Cialis, which treat erectile dysfunction in a very different way.

The drugs don't affect arteries themselves, instead they change the chemical signals that control the arterial muscles. Although you rely on nerves for movement in most of your body, they don't directly control erectile tissue. Instead, nerves leading into your groin trigger the release of chemicals that cause the muscles in the walls of genital arteries to relax. This allows them to expand in diameter.

As your arousal subsides, enzymes in your erectile tissue break down the chemicals that relaxed the muscles in your erectile arteries. Viagra and related drugs block the enzymes. As a result, the drugs keep the levels of muscle dilating chemicals high, and the blood vessels stay open. In essence, the drugs expand the arterial vessels that lead to the genitals, rather than directly constricting the flow out through the veins, as cock rings do.

The reason that you may have heard that Viagra is not considered an aphrodisiac, even though many people think of it that way, is that it can't lead to erectile tissue engorgement unless your body produces the chemical to relax your genital arteries in the first place. Viagra can't cause arousal, as a true aphrodisiac would; it only works if you are already aroused and your body produces the initial muscle-relaxing chemical.

The enzymes in your genitals that break down the dilating chemicals are unique to your erectile tissue. They're slightly different from the enzymes in other parts of your body. Viagra is designed to block the enzyme that turns off your arousal response. But no drug is perfect. It also mildly blocks the enzymes in the tissue of your retina, which is why some people experience changes in their vision while on the drug. The various side effects of Viagra and other erectile drugs are mostly related to the fact that developing precisely targeted drugs is very difficult.

Viagra should work for women as well as men, leading to enhanced genital engorgement and the improved vaginal lubrication that comes with better blood flow. Unfortunately, recent studies have shown that because arousal in women is more complex than it is in men, most female test subjects found that Viagra did little to improve their sexual experience.

Circulatory Troubles

Anything that keeps the arteries from dilating can hamper sexual response. Arteries hardened by age or disease can't expand to allow the additional pressure and blood supply required for erectile tissues to swell. The nicotine in cigarettes, causes the muscles of the arteries to clamp down and reduce blood flow, particularly in the extremities and genitals. If you must smoke, try not to do it just before having sex, or else the nicotine will work against the artery-dilating chemicals accompanying arousal. Besides, you'll smell better when it comes time for that first kiss.

Even fatty foods can dampen sexual response. High concentrations of dissolved fat make blood more sticky and viscous. Thickened blood flows poorly through your arteries, which means that there's less pressure than there should be by the time blood makes it to your groin. Losing weight by cutting back on fatty foods helps blood to flow better and can increase erectile vigor, which means larger erections and more fully engorged clittorises and labia as well as smaller, sexier waistlines.

The combination of rich food and cigarettes is particularly devastating to blood flow, creating a double whammy on your sexual function. Thick, fatty blood squeezing through nicotine narrowed arteries puts an extra strain on the heart, which is why heart attacks may be more likely to strike as you enjoy a cigarette after a big rich meal.

Assume the Position

Circulatory fluid flow, of course, is responsible for more of the sexual experience than mere genital mechanics. Flushing in your cheeks comes with increased blood flow as small vessels in the skin expand in response to things like overheating, embarrassment, or arousal. In fact, rouge and lipstick may owe their sexiness in part to the fact that they mimic the facial flush that accompanies sexual excitement.

Changes in circulation can also affect the sensations you feel during some activities by modulating the amount of oxygen that gets to your brain. The lightheadedness that comes with riding a roller coaster is in part due to the forces you experience during the ride, which push blood up toward your brain or down to your feet.

People who practice erotic asphyxiation attempt to heighten their orgasmic sensations by reducing oxygen in their brain. Often they achieve the effect through partial strangulation to slow blood flow. It's a highly dangerous activity, and leads to many unfortunate deaths every year, particularly among young men.

There are, however, some much less dangerous sexual techniques that create similar sensations, because they involve positions that modify blood flow without strangulation or asphyxiation.

Your circulatory system is designed to operate best when you're standing up, lying down, or somewhere in between. Veins have tiny valves in them that work against gravity. When you stand, the valves prevent the blood in your veins from backing up into your legs, but the valves only work in one direction. If you stand on your head, you'll feel an increase in the blood pressure in your face and head as the valves in your veins become useless and gravity takes over. Although there's more blood in your upper body when you're upside down, it doesn't flow as well. Your heart will work harder to keep your blood moving, but you will probably experience some lightheadedness due to the reduction of oxygen in your brain.

There are some, rather athletic sexual positions that involve one partner essentially standing on their head to reduce blood flow in the brain, but you can get the same effect by simply hanging your head off the edge of the bed during sex. A slightly less effective method is to stand and bend deeply at the waist while your partner enters from behind or stimulates you, manually or orally. In either position, the longer you do it and the lower your head in comparison to your torso, the more intense the sensation. Of course, you may end up with a throbbing headache when it's all over.


Now you know the basic fluid physics important for sex. Whether or not you experiment with sex toys and methods for modifying the blood flow in your body, be sure to take a moment to appreciate the importance of fluid physics in your sexual activities.

The Physics of Sex will be on vacation for about two weeks, so expect our next episode, "Sex and Sensibility: The Nervous system," in the second week of January.

In the meantime, take a moment to leave your comments and questions about Physics and Sex. Or email us at If we use your comment in a future column, we'll send you a free Physics of Sex coffee cup from our Caf├ęPress store.

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Tuesday, December 12, 2006

Part 1 of Pumped Up and Ready for Love: Sex and Fluid Physics

Much of what you experience during arousal and sex results from changes in the way your circulatory system moves your blood around. Your heart races and your cheeks flush as excitement builds. Sooner or later, if all goes well, swelling of the erectile tissue in your groin will lead to the erection of your clitoris or your penis, as the case may be. Generally, your body handles all the blood flow issues automatically. Still, a look at the physics of fluids reveals that there are many things you can do to take control of your blood flow during arousal.

Listen to the podcast with roboreaders Audrey and Paul.

Despite the fact that male and female genitalia look very different from the outside, they operate in essentially the same way, from the point of view of fluid physics. Both the penis and the clitoris are built of erectile tissue that becomes engorged with blood during sexual arousal. The chief difference between the two is that while most of the engorged portion of the penis is visible on the exterior of the male body, the exposed clitoris is only a small part of the erectile structure in a woman's genitals. In fact, women have roughly the same amount of erectile tissue as men. It's just hidden from view behind the clitoris and extends down on either side of the vaginal opening.

In either case, a physicist could describe genitals in terms of a simplified model consisting of a reservoir connected to an incoming tube, which supplies fluid, and an outgoing tube, which drains the fluid away. Your body adjusts the amount of fluid in the reservoir by regulating the relative flow, in through the source and out through the drain. It's similar to the way you can control the amount of water in your kitchen basin by turning the faucet handle.

Arteries are the blood vessel analogue of the kitchen tap. Instead of water, of course, they deliver blood provided by the pumping of your heart. They are stretchy tubes wrapped in muscle tissue. The muscles control blood flow through arteries by contracting and closing them down somewhat to slow the flow, or relaxing to let the vessels dilate and pass lots of blood.

Veins, like arteries, are also stretchy tubes. Although they don't have the sort of muscle layers that surround arteries. Instead, veins passively expand or contract as blood pressure rises or falls.

Most of the time, the muscles wrapping the genital arteries squeeze down to restrict blood flow, and the erectile tissue is relatively empty. During arousal, the arterial muscles relax to open up the tap and let lots of blood through to begin the process that leads to an erection. If there were enough blood flow, this would be sufficient to do the job without making any changes in the outward flow of blood through your veins.

In your genitals, however, the situation is a little more complicated. The erectile tissue reservoir is really more like a sponge than a kitchen basin. The large arteries that supply blood to your groin branch off into smaller and smaller vessels that are embedded in the spongy erectile tissue. Veins also fan out through the tissue, collecting blood and returning it to the large veins that lead back out.

When the arteries dilate during arousal, the swelling erectile tissue puts pressure on the veins. This forces them to collapse slightly and reduces the rate of the blood flowing out. The whole process amounts to opening the taps while simultaneously closing off the drain somewhat.

If things were to continue in this way, you would eventually have a problem as increasing pressure shut off the veins altogether and the blood flow stopped. Fortunately, there's a natural safety mechanism. Erectile tissue can only expand so far before it's fully engorged. At that point the blood still has room to force its way out through the veins.

In physics terms, the change in your erectile tissue as you become aroused is called a transient state. When you are not aroused, your genitals are in an unchanging steady state. Your genitals are also in a steady state when they are fully engorged. During a transient period, the flow of blood into your groin is different from the flow out as the erectile tissue becomes engorged. During a steady state, fluid physics requires the flow and out to be the same. In fact, the total flow of blood through your groin in either the aroused or relaxed state is just about the same, only the amount of blood loitering in the erectile reservoir is different between the two states.

It's a bit like using a damn to turn a valley into a lake. Diverting a river into the valley begins filling the reservoir. Closing the flood gates in the dam stops the flow of water out of the valley and downstream. Once the valley is filled up, the total amount of water flowing into the lake and spilling over the dam at the other end is the same that always flowed down the river. However, a lot more water is stored in the dammed up reservoir, just as more blood is stored in your erectile tissue when it's engorged.

You could rely solely on your body's natural processes to achieve a penile or clitoral erection, but many people enhance their experience with various sexual devices and techniques. In terms of the lake metaphor, most methods either widen the river flowing into the reservoir or raise the dam higher at the reservoir's outlet.

One of the most common sexual aids is the cock ring. It is a constriction placed at the base of the penis that effectively raises the dam holding more blood in your groin reservoir.

Cock rings squeeze down on the outside of a penis. They compress both arteries and veins, which causes them to collapse a little, restricting blood flow through the penis. This might lead you to wonder why they should enhance an erection. After all, if they squeeze down on both the arteries and veins you could imagine that a ring would reduce the blood flowing in as much as it reduces blood flowing out.

Fluid physics provides the solution to the cock ring puzzle. Here's how -

Whenever a liquid flows through a tube, it experiences some friction that opposes its motion. The amount of friction depends in part on how thick the fluid is. Physicists call this the viscosity. Water flows easily because its viscosity is low. Slow moving molasses is highly viscous, and blood has a viscosity somewhere between water and molasses.

When your heart pumps blood into your arteries, the resistance to the flowing blood causes the pressure to decrease the farther it moves away from your heart. (Other things affect the pressure as well, but we will worry about those later.) Narrow tubes, such as capillaries, resist flow more than wide arteries and veins.

The blood that flows through your groin passes through many tiny capillaries. If you were to measure the blood pressure on its travels from the arteries to the veins , you would find a significant pressure drop. Because the pressure in the out-going veins is so much lower than in the in-coming arteries, they are easier to pinch closed and they collapse more than arteries do under the same force.

You don't need a cock ring to test the effect. If you wrap a piece of string around one of your fingers, you will find that the finger tip swells and turns purplish-red as the string squeezes the veins down but the arteries remain more open.

Of course, a ring that's too tight could potentially shut down your veins entirely, leading to a stagnant pool of blood in the penis. This can cause permanent tissue damage as the cells run out of the oxygen that your blood provides. Most cock rings, however, include snaps or other fasteners that allow you to release them if things get out of hand.

You can also enhance an erection by pinching the base of the penis - with your thumb on the upper side, and your fingers pressing under the testicles just in front of the perineum. The pressure compresses the veins leading out of the penis just as a cock ring does. Although your hand will be in the way for some activities, and the effect will subside immediately upon releasing pressure, it can temporarily enhance girth and pleasure for a man, particularly during oral sex.

Unfortunately, the structure of a woman's genitals means that there are no feminine equivalents of cock rings, although the clamps that some people apply to their clittoris alters blood flow and often causes engorgement in a small region.

The penile pinch, however, can be adapted to women. If you spread your fingers in a V shape and place them on either side of the clitoris, you can restrict venous blood flow, and enhance clitoral size and sensation by pressing and gently squeezing your fingers together. Just as is the case with the penile pinch, this can be particularly pleasant for your female partner when you are performing oral sex.

Tensed muscles put pressure on blood vessels running through muscle tissue, much as a ring squeezes veins in a penis. When weightlifters and other athletes strain to perform an exercise, the veins near the surface of their skin stand out as the muscles squeeze the blood out of deep muscle veins. The extra engorgement of the labia, clitoris and penis just before orgasm comes in part from similar muscular tension, primarily due to straining in the abdomen, buttocks, thighs, and calves.

Vacuum pumps, including both penis pumps and clitoral pumps, also aid in erectile tissue engorgement, but they operate on an entirely different principle from cock rings. They reduce the air pressure outside of the organ rather than compressing veins inside. Normally, the atmosphere pushes on us from all directions with a force of about fifteen pounds per square inch. You don't notice the pressure because the fluids inside your body press outward and balance the air pressure on your skin. If you could selectively reduce the pressure around part of your body it would bulge outward because the pressure inside of you pushing out would no longer be balanced by the atmospheric pressure pushing back in.

A vacuum pump allows you to reduce the air pressure surrounding your penis or clitoris by surrounding it with a chamber that seals to the skin of your groin. You can then reduce the pressure by sucking the air out of the chamber with a pump.

Because the air pressure on the rest of your body remains unchanged, it's effectively the atmosphere pushing on the rest of you that forces blood into your genitals.

Unlike cock rings, which require least some arousal to function, pumps can inflate a penis or clitoris even in the absence of any natural erectile response. Once a penis is engorged with a pump, a cock ring can help maintain the erection.

Pumps can cause injuries if things are taken too far. The high pressure difference between the inside of the blood vessels and the vacuum in the pump may lead to ruptured blood vessels and permanent vein damage. They are generally safe, however, if you follow the manufacturer's instructions.

There are many other things that can affect blood flow in your genitals, including drugs, diet, and even your choice of sexual positions. We'll address several of them in next week's episode - Part two of Pumped Up and Ready for Love: Sex and Fluid Physics.

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Saturday, December 02, 2006

Slip, Slide, or Stick: Friction and Lubrication

Your hand slipping over your lover's back, metal pieces sliding past each other in a car engine, and even blood flowing in your blood vessels all involve friction. A little friction is a good thing in the bedroom, but a whole industry has developed to provide products to keep sexual friction under control.

Listen to the podcast with roboreaders Audrey and Paul.

The amount of friction between two surfaces depends on the roughness of the materials, their chemical composition, and the force pressing the materials together. A lubricant is any substance that reduces friction. Some lubricants are powders, such as graphite, but the lubricants involved in sex are generally liquids, gels, and creams.

Lubricants for sex, which are often called personal lubricants or simply lubes, come in a dizzying array of varieties. There are water-based, silicone-based, and petroleum-based lubes. Some lubes include fragrances; others are edible and offer an array of succulent flavors. Many lubes feature additives such as pigments and dyes, anesthetics, moisturizers, preservatives, and chemicals that warm or cool skin.

How do you know which to choose? The Physics of friction and lubrication can help you figure out the best lube for you and your partner, no matter what activity you have in mind.

Natural Lubricants

The human body is a complex machine, with hundreds of moving parts. Like all machines, it needs lubricants. Elbows, knees, ankles and the rest of your joints would eventually grind themselves to dust if they lacked lubrication. Tears lubricate your eyeballs, saliva lubricates your mouth and throat, and your skin is constantly moistened with sweat and oils to keep it supple as millions of skin cells jostle against each other.

The natural lubricants most important for sex are saliva, vaginal fluids, and male pre-ejaculate.

Saliva consists primarily of water and mucus. Mucus is made of long proteins called mucins, which are coated in compounds related to sugars. The mucins bind to water to make saliva slippery.

The slipperiness of saliva mucus makes it a convenient lubricant for oral sex, hand jobs, and anal sex. Unfortunately the high proportion of water in saliva makes it dry out quickly as the water evaporates.

Vaginal fluids are also packed with mucins to ease penetration and protect vaginal linings from germs. In addition, vaginal fluids include various acids to provide the right chemical environment for sperm, and sugars that nourish sperm swimming through the vaginal fluid toward the ovaries.

Women produce vaginal fluids when they become sexually aroused. The amount varies depending on their age, health, and the timing of their menstrual cycle. Smoking just before sex can reduce fluid production by diminishing blood flow to the vaginal lining. Antihistamines and other drugs can reduce natural lubrication as well. All women need a little lubrication help from time to time, and even the most abundantly lubricating women may need to supplement their vaginal fluid with saliva or artificial lubricants - for extended love sessions.

Men produce some lubricant as well. The Cowper's gland near the prostate secretes a small amount of slippery fluid commonly called pre-ejaculate or pre-cum. Some sex experts believe it helps to lubricate the head of the penis before penetrating a woman's vagina, but most men only produce a few drops and it is often ill timed for insertion. It's more likely that the Cowper's fluid prepares the urethra for the passage of sperm by adjusting acidity, clearing out any traces of urine, and lining the urethra with a slippery, sugar-rich energy source to get the sperm swimming.

There is some lubrication in the anus, but nowhere near enough for most anal sex activities. Generally, you're going to need to bring along some artificial lube for anal play.

Water-Based Lubes

Pure water is an excellent lubricant - sometimes. If you've ever slipped on a wet tile floor, or nearly broken your neck stepping into a tub, you know how slick water can be. Many lubes consist primarily of water.

Unfortunately, water can sometimes dramatically increase friction instead. For example, people may lick a finger to add friction before turning a page in a magazine. Slightly moistening your hands will give you a better grip when trying to take the lid off of a jar, provided you don't make your hands too wet.

When you step into a tub, you usually experience a little of both water's lubrication and its tackiness. After initially sliding over the bottom of a filled tub, your feet will suddenly gain traction, giving you much more grip than you would have had if the tub were totally dry.

The two radically different properties of water stem from the attraction that water molecules have for each other and for some other materials. Materials that attract water are called hydrophilic, or water loving, and materials that are not attracted to water are hydrophobic, or water hating. Water beads up on a freshly polished car because the polish is hydrophobic and repels water. Rain droplets spread out on a car that needs waxing because the old oxidized polish is hydrophilic and attracts water.

Water is a liquid because the attraction between the molecules is too weak to turn it into a solid and they slip and slide over each other. It's strong enough, however, to bind water into small droplets when it falls as rain or runs down a window pane.

When your foot slips as you're stepping into a tub, it's because there's a thick layer of water between your foot and the porcelain. The mild attraction between the water molecules makes them act a bit like tiny marbles, with very low friction.

As you put weight on your foot, you squeeze most of the water out of the way, until there is a very thin layer between your skin and the tub. In some places, the layer is only a few molecules thick. Because your skin and the surface of the tub are slightly water-loving, the molecules are attracted to both. The slight attraction the water molecules have for the tub and your foot combine to give you traction.

It's because of the dual lubricating and adhesive properties of water that making love in a pool tub may seem like a good idea, but rarely turns out well. Your skin slides easily over your lover's skin, as long as the contact is light and there is a lot of water between the two of you. Once your skin presses together, you lose water's lubricating properties and the adhesion takes over, which can make vaginal and anal penetration particularly rough experiences.

But it's possible to exploit the forces between molecules to ensure that water stays slippery. That's what's going on in water-based lubes. These types of lubricants work in one of two ways; either by ensuring that the water molecules clump together so that you are less likely to get a thin adhesive layer, or by reducing the water molecules' attraction to each other and other hydrophilic materials. Some lubes have ingredients that do both.

Mixing in glycerin is one way to make water molecules clump together and form a good liquid lubricant. Glycerin is a small molecule that's hydrophilic in two places. As a result, water can attach to each side of a glycerin molecule. Another glycerin then attaches to the water, and so on. Eventually long molecular strings will form. If you could see the mixture through a powerful enough microscope, you would see that the strings tangle up like spaghetti. They slip and slide, like a plate of heavily buttered pasta noodles. To the naked eye, the result is a clear liquid that is much thicker and slicker than water.

The binding between the glycerin and water is weak enough that water molecules can break free of the mixture. They may then evaporate or get absorbed into your skin, which means that lubes relying on glycerin to hold water molecules together will slowly dry out. The glycerin molecules that have lost their water will be mildly attracted to your skin, which makes the lube get sticky as it dries. Adding a little water will restore the glycerin lube's slipperiness.

Glycerin is related to the sugar glucose. If you taste some, you will see that glycerin lubes are sweet. Like sugar, glycerin is a good energy source and sometimes serves as a nutrient for microbes. Women may find that glycerin lubes foster yeast infections. If you or your lover suffers from frequent vaginal infections, look for glycerin-free water-based lubes. There are several other molecules that can hold water together as glycerin does, without feeding populations of vaginal bacteria and fungi.

Another way to make water stay slippery is by mixing it in a gel. Gel lubricants get their jelly-like consistency from long molecules of protein rather than short glycerin molecules. The proteins in gels have many places along their lengths that attract water. Chemical treatments or heat cause the long molecules to crosslink, which means that they connect to each other in some places.

It's like tying pieces of string together at random places to make a loose, three-dimensional web. Water molecules get trapped in the web at the hydrophilic points along the protein molecules. Food gelatins, like Jello, trap water the same way.

The more places that the molecules are connected in a crosslinked gel, the more rigid and jelly-like the gel will be. Like glycerin lubes, water-based gels may dry out in time. Because they're more complex than liquids, with water trapped in a net of crosslinked proteins, you cannot rejuvenate them as well by simply adding water. It's better to add fresh gel if it gets too dry.

Some lubes rely on chemicals called surfactants that reduce the attraction between water molecules. Instead of ensuring that there is a thick layer of water between your skin and your lover's skin, surfactants make thin layers of water less adhesive. They're generally medium length molecules, longer than glycerin and shorter than most proteins.

Surfactant molecules each have a hydrophilic connection at one end. Surfactant molecules link up with water, effectively making the molecules larger and more bulky. This keeps them farther apart. The attraction that water molecules feel for each other gets much weaker if they are even slightly separated. The reduced attraction also reduces the adhesion of the surfactant-water mix.

Many gel and liquid lubes include surfactants to make the water in them ultra slippery.

Water-based lubes are safe for use with latex condoms and diaphragms, as well as all sex toys. They wash off easily with nothing more than warm water. Of course, that means they rinse away too readily for making love in the bath.

Oil-based Lubes

Some lubes don't include any water at all. Lubricants based on vegetable oils and petroleum products are often very slick and long lasting. Vegetable oils common in lubes include olive, sesame, and palm oils, to name just a few of the many possible varieties. Most petroleum-based lubes are varying grades of petroleum jelly, with Vaseline being the best known brand.

Both plant oils and petroleum lubricants are made of hydrocarbon chains, long strings of carbon atoms with hydrogen atoms attached to the sides. The texture of petroleum-based lubes is determined primarily by the lengths of the carbon chains.

Molecules made of chains ten to fifteen carbon atoms long form mineral oils and light watery lubricants. Longer chains are heavier and clump together to form jellies like Vaseline. Still longer chains result in paraffin wax.

Vegetable oils are a little more complicated. They're also made of hydrocarbon chains. However, they typically consist of multiple chains linked together by a glycerin molecule. Like petroleum products, heavier oils usually are made of longer chains. In addition, light, watery oils can be made to solidify by changing the number of hydrogen atoms attached to the chains, through a process known as hydrogenation. Margarine and shortening are made from light vegetable oils that have been hydrogenated.

Hydrocarbon chains that make up oils are highly hydrophobic. If you've ever made salad dressing with oil and vinegar (which is mostly water) you've seen how hard it is to mix the two.

The molecules in oils and petroleum products don't attract each other or your skin very strongly, which is why they're slippery.

You might think oils and petroleum jellies would be easier to clean off of your body if they're not as strongly attracted to your skin as is water, but that's clearly not the case. One reason it's harder to remove hydrocarbon lubes is that their large molecules don't evaporate very well. If you get water on your skin, just wait a while and it'll dry all by itself. Oils and petroleum products will stick around for ages with little or no sign of evaporation, which is good for long lovemaking sessions, but not so great for the post-coital clean up.

The fact that the lubes are hydrophobic also means that you cannot simply rinse them off with water. To remove the lubes you'll need to wash with soap.

Many men prefer oils and petroleum products over water-based lubes for masturbation. Some people feel that they are better suited for anal sex because the petroleum jellies in particular are heavier and last longer. Hydrophobic vegetable oils and petroleum lubes work well in the tub or pool because they won't rinse away.

Unfortunately, all oils and petroleum products dissolve latex, and should never be used in combination with latex condoms, diaphragms, and latex sex toys. It's also generally a bad idea to use them for vaginal sex because the soap necessary to clean the lubes away removes the protective vaginal mucous as well. This leaves the delicate membranes open to infection.

Silicone-based Lubes

Some of the newest lubes on the market are silicones. Silicone molecules have essentially the same structure as petroleum except that the long carbon chains are replaced by chains of alternating silicon and oxygen atoms. Like oils and petroleum products they are hydrophobic, long lasting lubes. They're just as slippery as oils but will not dissolve latex. They wash off with soapy water, making them less than ideal as vaginal lubricants but much better alternatives for anal sex when latex condoms are involved.

There are also water-based lubes that replace glycerin with dimethicone, a silicone molecule that can link water molecules into long slippery just as glycerin does. They're good lubrication alternatives for vaginal intercourse if you want to avoid glycerin. Like all water-based lubes, those that include dimethicone wash off easily with water and are no good in the tub or pool. They're safe to use in conjunction with latex products, but will still damage silicone sex toys.

Emulsions and Creams

Although water and oil don't mix as a rule, there's a way to almost make them get together, which leads to another type of lubricant - emulsions. Surfactants do the trick. You've already seen that surfactant molecules that are hydrophilic on one end can make water more slippery. If the other end of the surfactant molecule is hydrophobic, then it can help get water and oil molecules close together, even if they don't actually mix.

Creams and many creamy lotions are emulsions. If you mix a surfactant in water and then add oil, the surfactant molecules will surround oil droplets with their water-hating ends pointed in, toward the oil, and their water-loving ends pointed outward. The surfactants create tiny balloons of oil in the water called micelles. The oil and water still don't actually mix, but the micelles act like large dissolved particles in the water. Mayonnaise is a common emulsion of vegetable oil and water, with a bit of egg white mixed in to act as an emulsifying surfactant.

All emulsions, including most lubes advertised as creams or lotions, have either oil or silicone mixed with water. Be sure to check the ingredients before using an emulsion in combination with latex or silicone prophylactics and toys; all the same precautions apply for emulsions as do for simple oils, petroleum jellies, and silicones.

To summarize. . .

- Water-based lubes are usually safe with latex condoms as well as silicone and rubber, provided they don't have any oils or silicone surfactants. (Check the label to be certain.) They're also easy to clean up.

- Oil and petroleum lubes are super slick and long lasting, but destroy latex condoms and toys, and are hard to wash off.

- And finally, silicone lubes are also very slick and long lasting, but don't harm latex products. Although they can be as tough to clean as oils, and will damage silicone toys

The options for sexual lubes are vast. So which is best for you? It's hard to say, but I can't think of a better way to answer the question than buying several types and spending a night trying them out. It'll be a slippery, sloppy session of sexual fun, and in the end I hope you'll have a new appreciation for the physics of friction and lubrication.

Next time . . . Pumped Up and Ready for Love: Fluid Physics and Sex

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Saturday, November 25, 2006

The Physics of Sex podcast listings

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Sexual Rhythms

The swaying of breasts, the menstrual cycle, and hip thrusts are just a few of the important oscillations in our sex lives. Physics provides a simple and powerful description of rhythmic motion and cycles, and can help you get the most out of oscillations, from choosing the best bed for your sexual pleasure, to enjoying and exploiting the natural rhythms of your body parts during sex.

Listen to the podcast with roboreaders Audrey and Paul.

Seasons shift through the course of a year, the ocean tides ebb and flow every day, and your mood may swing with the periodic changes in the chemicals in your brain, but the most common types of oscillators are mechanical - a tree bending back and forth in a breeze, a string bowed on a violin, or a couple making love.

Mechanical oscillators work by transferring energy between two forms - kinetic and potential. Anything that moves has kinetic energy. Stored energy is potential, as in the case of a ball poised to roll down a mountain. The energy in oscillators is sometimes kinetic, sometimes potential, and usually a bit of each.

When a playground swing moves back and forth it's briefly stationary at the highest point in its arc. For an instant it has no kinetic energy. Because it is higher above the ground than at other times in its motion, it has a maximum amount of potential energy, just like a ball on the verge of rolling down a hill. When the swing descends, it speeds up as its potential energy is converted to kinetic. The swing is moving fastest, with the most kinetic and least potential energy, at the lowest point of the arc. As it swings up in the other direction, the kinetic energy is converted back into potential.

Similarly, when you make love on your bed - at least when one partner is bouncing up and down on top - you rhythmically compress the mattress springs. The springs have lots of potential energy when they are compressed, but as the springs extend and push you upward, the potential energy is converted into the kinetic energy of your moving bodies.

The rate that the energy flows back and forth in an oscillator is its resonance frequency. The frequency of an oscillator is measured in hertz, which is the number of oscillations in a second. A clock ticks at one hertz , or once per second; your heart can beat at two hertz or more during heavy exercise and sex; and middle C on a piano is a 440 262 hertz vibration in air, which we hear as a musical note.

All oscillators have at least one resonance frequency. Many, such as violin strings, have several resonances. The distinctive sound of an instrument has a lot to do with the many resonances that are produced along with every note; it's the combination of resonances that ensures that a violin and a piano produce rich and distinct sounds even when they are playing the same note.

One resonance, however, is usually more important than the rest - it's the fundamental resonance. In the case of a musical instrument, the fundamental is the note a musician is playing. When a violinist chooses to play middle C, for example, he or she presses on a string to ensure that its fundamental resonance occurs at the 440 hertz frequency we identify with the note. Other, lesser resonances are called harmonics.

Lovers on a bed form an oscillator with a fundamental resonance and a spectrum of harmonics. Just as a gifted musician can make sweet music by adroitly manipulating an instrument's resonances, lovers can add to their resonant bliss in the bedroom by understanding and controlling their bed's oscillations.

When you sit on a bed, you'll sink into the mattress until you reach an equilibrium position. At that point, the force of gravity pulling you down is balanced by the force of the bed springs pushing you up. If you start to bounce up and down, you'll find that there's a certain frequency that allows you to get a big steady bounce. That's the bed's fundamental resonance. Most beds resonate at frequencies of a few hertz.

By rhythmically bouncing on the bed, you're doing what physicists call driving the oscillator. It's easiest to drive an oscillator at its resonance frequency. At frequencies just below or above resonance, it takes much more force to get a big bounce. If you start out very slowly, you'll probably end up oscillating well below your bed's fundamental resonance and won't bounce on the bed much at all.

Increasing your speed can bring you into resonance, allowing you to achieve large bounces with seemingly little effort. Once you're moving too quickly , you may pass the resonance. As a result, you'll end up working against the bed's rhythmic sweet spot. If that happens you will have to exert much more force to get a good bounce at the same time that you're trying to move quickly. The chances are, you'll be rapidly exhausted. If you stick close to the resonance, on the other hand, you get maximum motion for minimal energy input, which helps you keep going longer before you wear out.

Many things can affect a bed's dynamics, but it's the mattress' firmness that plays the greatest part in determining the resonance frequency. Firmer mattresses have higher resonances than soft mattresses. If you visit a mattress store, you can check this for yourself. Sit on several beds with different firmness and bounce on each. You'll find that the frequency varies from a very firm to very soft mattress.

Beds get their bounce from springs, and springs have resonance frequencies that depend both on their firmness, which physicists call the spring constant, and the mass on top of the bed. That means the resonance frequency will be different for you than it will be if you have someone in bed with you. In fact, if you and your partner are about the same weight, the resonance frequency will be roughly half two-thirds as fast with the two of you close together on the bed as it will be with just one of you. (To be more precise, it will be 1/(the square root of 2) or about 0.7 times slower.)

Determining a bed's resonant frequency is only part of the issue. After all, you may not be content within the confines of one rhythm. Fortunately, there's another factor that affects bed motions. Harmonic oscillators, and beds in particular, often include a certain amount of damping, which gives you a little more leeway in choosing your own rhythms.

Shock absorbers in cars are a good example of damping. Car suspensions consist primarily of simple springs. When a car hits a bump, the springs allow the wheels to travel up or down relative to the car, maintaining contact with the road. If it weren't for shock absorbers, a car would continue to oscillate on its springs after hitting a speed bump or a pothole, leading to a nauseatingly bouncy ride. Shocks settle a car down quickly by dissipating the energy of the bounce. As a result, they reduce the resonance frequency and make the resonance less pronounced. If you push down on the bumper of a car with bad or missing shocks, you can easily get it to resonate and bounce dramatically. It is much more difficult to find the resonance frequency of a car with good shocks.

Damping has the same effect on a bed that shock absorbers have on a car - it will be harder to drive a resonance on a very damped bed, but at least you won't suffer the frustration that comes from trying to move at rates higher than resonance.

Although no commercial beds currently come with automotive-type shocks, padding in the mattress adds damping. Alternatively, you can add your own damping by spreading a thick comforter on the bed and making love on top of it. A few well-placed pillows under you can increase damping too.

The bottom line is this: if you want to use the resonance to your advantage and you like it fast, choose a firm bed; if you like it slow, go with a soft bed; and for maximum flexibility, buy a firm bed but keep a few soft comforters and pillows around to dampen the resonance to suit your mood.

If you have the soul of an experimental physicist, try making love on a trampoline, which has almost no damping and a powerful resonance. Then try it on a water bed, which also has little damping and strong resonance, as the water sloshes from one place to another, but at a much lower frequency than a trampoline. To round things out, make love on a squishy foam bed, like the Tempurpedic mattress, to experience lots of damping with very little resonance. It can be an exhausting and frustrating challenge.

Some people find that their favorite lover is a machine - specifically, their vibrator. Vibrators get their buzz from an electrically powered oscillator.

In battery-powered models, vibrations generally come from an electric motor attached to a rotating disk, with its weight placed off-center. The principle is the same thing that causes unbalanced washing machines - which are some, other, people's favorite lovers - > to buck violently when more of the laundry is on one side of the washer drum than the other. The faster the motor turns, the higher the vibrator frequency.

Many of the vibrators that plug into the wall generate oscillations with a different type of electric motor called a solenoid. Instead of spinning an unbalanced weight, electricity passing through a coil of wire forces a metal slug to vibrate rapidly back and forth. The speed of vibration in a plug-in vibrator is related to the 60 hertz oscillations of the electricity in wall sockets. They are usually more powerful than battery vibes. Unfortunately, their power comes at a price - they can only vibrate at the frequency of the electricity in the wall, at a multiple of the electrical frequency, or certain fractions of the wall frequency. Unlike battery powered vibes, which can run at a spectrum of speeds, most plug in vibrators have only one, two or three speed settings.

Ideally, vibrators would also come with adjustments to increase the strength of the vibrations independently of the speed, but that is not the case with any vibrators currently on the market. This is likely due to the fact that adding a power setting would complicate vibrator design, but you can always adjust the power you feel by changing how hard you press the vibrator against your body.

Vibrators have resonances just as beds and cars do. That means that increasing the speed of a continuously variable model can either increase the power of the vibrations or decrease them, depending on whether you are approaching or passing the resonance. As a rule, the power of the vibrations will slowly increase as you turn up the speed, then reach a maximum at the resonance frequency, and slowly fall as you continue to turn it up.

Fortunately, you can gain a bit more control over your vibe with damping, just as you can use damping to adjust a bed's dynamics. Changing how tightly you grip a vibrator and where you hold it will change the amount of damping, allowing you to modulate the speed and intensity. Inserting it into your vagina or anus will also change the speed as the soft tissue touching the vibrator absorbs energy. If you listen to the pitch as the vibrator moves in and out, you can hear the speed change. Alternatively, pressing the vibrator against a soft rubber or gel can also slow it down.

Some dildos have a cavity that allows you to insert vibrators into them. The softer and heavier the dildo, the more it will dampen the vibrations and slow the resonance. Many manufacturers of plug-in vibrators offer soft sleeves and attachments to allow you to dampen oscillations in the same way.

Damping is the reason that vibrators are more comfortable when used in the anus or vagina than on a hard penis or clitoris. A rigid clitoris or penis has much less damping than softer and more enveloping anal and vaginal tissue. The vibrational motion, and resulting energy, is transmitted at full intensity to sensitive nerves in a small area where the vibrator makes contact with the rigid tissue. In the vagina and anus, the energy is distributed to more tissue and nerves, which feels less intense.

Springs, like those in a mattress, are just one classic type of oscillator. Pendulums are another. A pendulum consists of a weight at the end of a rod or string. It will swing at a rate determined by its length, regardless of the amount of weight on the end. A long pendulum swings slowly, and a short pendulum swings quickly. It's easy to adjust a pendulum's frequency by changing its length. Old fashioned clock pendulums included adjustments for fine tuning the length of a pendulum depending on whether the clock ran fast or slow; if it ran too fast you could turn a screw to lengthen the pendulum, or turn it the other way to shorten it if the clock ran slow.

Much of sex involves motions that have the characteristics of a little of both pendulums and springs. From a physics point of view, a woman's breasts are a complicated combination of springs and pendulums. Depending on whether she is lying on her back, standing, or on her hands and knees, her breasts are more like one or the other, which can have a huge effect on how they move.

Take a woman on her hands and knees, for example. Her breasts will hang down and sway as a result of her motions. Because hanging breasts are similar to pendulums, they have resonances determined primarily by the length they extend from a woman's chest. The resilience of her skin and breast tissue will also have an effect, but for gentle motion, the pendulum-like aspects are most important. Breasts will naturally swing slower if they hang farther from the chest, and swing faster if they are more compact.

It's easy to tell when hanging breasts have reached their resonance frequency, because they will swing dramatically back and forth. Increasing the frequency of the forces driving her breasts will reduce the amplitude of their motion, as they pass the resonance frequency, until they stop moving altogether.

When a woman is on her back, her breasts will tend to resemble springs more than pendulums, and will resonate at a frequency that has more to do with their resilience and mass. Heavier breasts have lower resonance frequencies, and tauter breasts (with higher spring constants) have higher resonances.

Resonant frequencies of breasts vary dramatically from one woman to another, and will vary even in a specific woman as her breast size and tissue resilience changes over time, or as she changes position from standing to lying on her back to getting up on her hands and knees.

Resonances are also the reason some women need sports bras when they exercise. It can be painful if a woman moves with rhythms close to her breasts' resonance frequency because that is when they are moving the most. Sports bras solve the problem by compressing breasts and making them, in effect, more taut. This raises the resonance frequency, hopefully beyond the frequency of jogging and other repetitive motions. Depending on a woman's cup size and the design of the bra, it may just move the resonance frequency up enough to make one exercise comfortable while making another painful.

For instance, if a woman with large breasts finds that her resonance frequency comes at a slow jog, it's possible that she will experience less motion if she sprints at a rate that drives the breasts at frequencies above resonance. Essentially, speeding up changes the bounce to a jiggle. Potentially, if she were to wear a sports bra that makes jogging comfortable it could move the frequency up to the point that things get bouncing and painful when she sprints.

Penises too have natural frequencies, which can change depending on arousal. Although the first sports bra (according to one story of their origin) was made from a pair of modified jock straps, men do not usually need extra support to prevent the sorts of resonances that plague women's breasts during exercise. A hanging, flaccid penis and testicles form short pendulums with resonance frequencies well above the frequencies of nearly any athletic activity. All jock straps do is lift the testicles up and forward to keep them from being squashed between the thighs - resonances aren't usually an issue during exercise.

An erect penis, however is like a large mass on a spring attached at the pelvis - the larger and heavier the penis, the lower the resonance frequency - and it may well resonate at rates that would interfere with many sports, but a man with an erection probably isn't in the right state of mind for jogging, basketball or soccer anyway.

Some types of penis enlargement involve snipping the tendon that supports the penis, allowing it to extend farther from the pelvis. This can radically reduce the spring constant of the penis attachment, and lower the resonance frequency of the erect penis a great deal. It's hardly iron-clad evidence, but if you watch a porn movie, you may notice that two men with similar sized erections seem to have very different penile resonance frequencies. (Look for the motion when their penises are not being touched directly, but are being driven by some indirect motion, say shifting position on the bed or walking across a room in the nude.) It's possible that a fellow with a low frequency oscillation has had his penile tendon snipped.


Bouncing beds, humming vibrators, and oscillating body parts are only a few of the ways that simple harmonic motion is important in the boudoir. In fact, your entire body is a kind of a simple harmonic oscillator. Moving from one position to another changes the portions of your body that come into play and affects the resulting resonances. The rhythm that feels natural with one partner on top is likely to be different from the rhythm when the other partner is on top, particularly if they are significantly different sizes.

Clearly, we've only touched on a fraction of the ways rhythms are important in sex. We hope it's enough to convince you to keep oscillations and resonance frequencies in mind, in order to help you enhance your sexual experience - whether you're choosing a bed, buying a vibrator, or searching for a new position.

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Why do we have sex? Part 3

In the competitive game of natural selection, the winning organism is the one whose DNA is most prevalent and longest lasting.

Listen to the podcast with roboreader Kate.

When a bacterium divides, the offspring carry the same DNA as the parent, as do the offspring in the third and fourth generations, and every generation after.

Sexually reproducing organisms lack this sort of genetic continuity. Your children will each carry genes that come in part from you and in part from their other parent. When your children breed, the portion of their DNA that you contributed will be further diluted. In the course of a few generations, there's not likely to be much of you left in your descendents. On the bright side, genetically speaking, at least your descendents and your species as a whole, can survive in the face of extreme stresses, while an asexually reproducing species could be wiped out easily. In fact, the scarcity of asexual species suggests that they hardly ever survive for long in nature.

I know what you're thinking. If two sexes are better than one in dealing with stress and mutations, wouldn't three be better still? The fact that there don't seem to be any suggests the answer is no.

Nevertheless, physicists have developed mathematical models of hypothetical creatures that breed in sets of three. (The researchers who wrote the papers examining the three-sex models, unfortunately, didn't bother to explain how these creatures would get their three-way groove on.)

Three-sex creatures and their offspring are triploid, with three complete sets of genes rather than the two of diploids like us, or the single genes of haploid amoebas and other simple animals and plants.

The numbers work out poorly for three-sexed creatures. For one thing, it's much more complex to get a reproductive trio together. The simple fact that it takes more parents to produce the next generation means that the population will grow slower than that of competing diploids just as diploids are out-bred by asexual haploids. Triploids, however, have one thing going for them - they are less susceptible to random mutations than haploids or diploids, thanks to even greater genetic redundancy than two gene animals like us. Unfortunately, they lag behind when it comes to adapting to other sorts of stress. Like so many cases of competition in nature, too much of a good thing turns out to be bad.

If triploid genes work in the same way ours do (which seems like a good guess) then each of the three genetic sequences has a genes for every trait, but it's the dominant one that wins out, or the trait ends up being a blend of all three. If the blue eye gene is dominant in a triploid population, the fact that each member of the population has three shots at getting a blue eye gene means that it is much more likely that everyone will have blue eyes.

It's easier to grasp the problem if we take it a few steps further. Imagine a population with ten sexes (and ten sets of genes per cell), and blue eye genetic dominance. Even if nine out of ten genes code for non-blue eyes, the one remaining blue eye gene wins. In cases of incomplete dominance and co-dominant genes, blending more and more versions of a certain trait leads to genetic uniformity instead of diversity, just as mixing more and more colors from a painter's palette results not in brilliant new colors, but to ever muddier shades of brown. While single-gene haploids have essentially no genetic diversity, increasing the numbers of complete genes in an organism beyond the two of diploids also leads to steadily decreasing diversity, which means less and less opportunity for evolutionary adaptation.

For most organisms, at least the larger and more structurally complex ones like humans, two-sex genetic diploids have an optimal combination of diversity, adaptability, reproductive efficiency, and resistance to genetic errors.

When humans rely solely on the tools nature provides us, we reproduce as most two-sex creatures do - a male provides semen, a female provides an ovum, and another generation begins. With a little medical and scientific intervention, however, we have lots more options.

Artificial insemination of course is one of the oldest and simplest alternatives to actual intercourse. Infertility treatments involving insemination in a Petrie dish are much like the external insemination practiced by fish and other aquatic and amphibious creatures.

While it is not triploid sex, when a woman serves as a surrogate mother for a fertilized ovum she is part of an interaction much like the three part male-female-female mating model.

Human cloning, however, is perhaps the most controversial method that may soon be among our potential reproductive options. Setting ethics aside, humans who reproduce via cloning would gain many of the asexual benefits that bacteria enjoy. Presuming that people who choose to clone are women who carry their own fetal clones in their wombs, and tend to have the same numbers of children over their breeding lifetime as other women, they would be able to increase their numbers much faster than sexual human couples could.

Men who opt to clone themselves need to seek out a woman to host the fetus in her womb, which means the process still takes a man and a woman. So that's really not an advantage numerically. In addition, the woman host would have to agree to waste precious reproductive time and effort to bear a child of no genetic relation to her. Surrogate mothers do that today for couples who can't, or choose not to, carry their own children to term, but surrogates usually require fee in exchange for the rented womb.

In a community where male-female couples and cloning women each choose to raise two children, the numbers of mating couples stays constant with each passing generation, but the numbers of clones double from one generation to the next. The clones' numbers could grow exponentially. Again, it's the males that are the reproductive liability in mating couples.

Other asexually reproducing creatures are highly susceptible to changing stresses in their environment, which favors sexual populations molded by natural selection. Humans, at least those living in more highly developed nations, tend to deal with stresses through controlling the environment and counteracting the stresses rather than through evolution.

If it's too cold, there's no need to evolve fur; we turn up the heat or put on a coat. If it's too hot, we turn up the air conditioning. We no longer adapt natural immunity to diseases; instead we develop vaccines, antibiotics, and prevention methods. As a result, people who are reasonably well suited to surviving in modern society, and who reproduce via cloning, would face few, if any, drawbacks from their asexual reproduction while gaining all the benefits.

Scientists have managed to clone many types of animals including cattle, cats, sheep, and monkeys. As of this writing, there have been no confirmed human clones produced from adult DNA. The complexity of cloning and the risks of severe birth defects mean it may be years before human cloning is perfected. But it will happen eventually.

If cloning ever becomes accepted practice, society could rapidly become populated primarily with women who reproduce by strolling down to the corner pharmacy to pick up the Acme Home Clone kit, rather than wasting time and energy looking for a male mate.

Clones won't evolve, so there is no reason for them to lose their sexual urges. Although they will have to learn to rely on lesbian sex to fulfill their needs, because men will eventually die out as sexually reproducing people lose ground to the clones.

When clones come to rule the world, sexual intercourse will be nothing more than a source of recreation, relaxation, and social bonding. Sex will be useless for procreation.

Of course, that's already true 99.99% of the time anyway.

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Thursday, November 16, 2006

Why Do We Have Sex? Part 2

Although there are countless exceptions and variations when it comes to reproduction, there is one fundamental characteristic that typically distinguishes between sexual and asexual organisms - the structure of their genes.

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Most asexually reproducing plants and animals carry a single complete set of genes in their cells. In biological terms their genes are haploid, which is just a Latin word for simple.

Sexually reproducing organisms, on the other hand, are generally die-ploid, meaning they carry two complete sets of genes. Every one of your cells effectively has twice the genetic information it would take to make a single person. Your mother and father each contributed genes that encode the color of your hair and eyes, the size of your nose, the proportions of your limbs, and so on. Your mother's genes determine some of your traits, your father's genes determine others, and some of your traits are determined by both your mother's and father's genes.

Asexual reproduction has some distinct benefits. For one thing, it's fast. If a typical bacterium takes twenty minutes to split in two, it will have eight descendants in an hour, and sixty-four in two hours.

If there's sufficient food around to support a population explosion, a single germ dividing at this rate could boast more than a million offspring in seven hours, and over a billion in ten hours. It's what physicists call exponential growth.

Sexual creatures can grow their numbers exponentially as well, although not as fast as asexuals. If two populations of organisms, identical in every way except for their mode of reproduction, were to squeeze into the same ecological niche, the asexual population wins, at least in the short term.

Rabbits are the iconic example of animals that breed like, well, rabbits. Imagine a fertile valley that's settled by two colonies of ten rabbits each. One colony consists of five male and five female rabbits that reproduce once a year in the usual way. The other is a colony of ten asexual females that also reproduce once a year, but have no need of males (fortunately this is a purely hypothetical type of rabbit).

If every female rabbit can bear a litter of ten babies each season, then in one year the sexual rabbits increase their numbers by five litters - one litter from each female - resulting in fifty babies plus the ten original colony members, for a total of sixty sexual rabbits. The asexual colony has ten litters, one hundred babies, plus the original ten for a total of a hundred and ten rabbits.

Presumably, the amazons give birth only to females, while the sexual rabbits produce half male and half female babies. After a single breeding season, the females in the colony of asexual rabbits outnumber the sexual females by nearly four to one.

The all-female asexuals will swamp the sexual rabbits in a few generations. Even if there are limitations of food and water in the valley that keep the total number of rabbits in check, the asexuals efficient breeding scheme allows them to overrun the sexual rabbits in short order.

As you can see, males are the true liability in breeding populations - they eat food that could go to the girls, produce waste, and take up precious space in the colony. But they are of little help in increasing population numbers other than donating sperm, which asexuals can live without.

Why don't we see asexually reproducing rabbits, squirrels, rats, elephants, or humans in the real world? The answer lies in adaptation to stress. And we can thank males for that.

Asexual populations consist essentially of clones, with each child carrying exactly the same genetic material as its parent. If a new disease, parasite, or predator were to come along with a particular talent for attacking our asexual rabbits, the whole population could be rapidly decimated.

Sexual rabbits have a better chance of surviving in the face of stress thanks to the presence of the boys. In mating, the male and female of a species each contribute a portion of the offspring's genetic material, which means babies are always at least slightly different from their parents. Sex stirs the genetic pot, leading to combinations that may occasionally handle stresses better.

When a fox finds a valley full of bunnies, you might imagine that it eats the slowest ones first. All the asexual rabbits are equally swift because they're identical. If the fox can catch one it can catch them all.

Some of the sexual bunnies however, will be faster than others as a result of the variability that comes from male-female breeding. Pretty soon, the pressure of having a fox hanging around might lead to natural selection of fleet-footed bunnies. Of course, rabbits could deal with foxes in other ways - developing better camouflage, enhancing their wariness, or growing wickedly sharp claws. But in any case it's the sexual ones that have the potential of finding solutions, while the asexuals are doomed.

Once sexual rabbits have developed an adaptation to deal with a specific stress, you might wonder what is to prevent them from spontaneously changing reproductive tactics to become a new asexual super rabbit that can fend off a given type of threat.

Based on some physicists' models, the primary reason is that foxes, germs and parasites evolve as well. A rabbit that adapts to the stress of a certain fox causes stress for the fox by denying him food, which in turn leads to the evolution of better hunters, forcing rabbits to evolve further, and so forth. Populations of rabbits and foxes ebb and flow as each adapt to changes in the other, leading to long-term stability of predators and prey that is maintained by sexual mixing of each species' genetics.

As organisms evolve, they face lots of shifting stresses, which firmly establishes sexual reproduction as the procreation method among just about everything larger than an amoeba.

Even if there were no threat of predators, parasites or diseases, all life faces the risk of random genetic mutations. Mutations are changes that arise from errors that occur when DNA replicates, or from exposure to things like radiation and chemicals. Asexual organisms that have a single precious copy of their DNA are in deep trouble as errors accumulate. Sexual organisms gain protection through their genetic redundancy - if an error develops in a gene contributed from one of your parents, repair mechanisms in your DNA can use the genes from your other parent as a map for repairing the problem. Or the problem may be moot if a healthy gene is dominant over the flawed copy.

In short, sex provides multiple levels of genetic protection. It offers a route to adaptation through gene shuffling, ensures backup copies of genes are available, and keeps genes in good shape with DNA repair mechanisms.

Tune in next time for part three of, “Why Do We Have Sex.”

I'm Kate. Thanks for subscribing to The Physics of Sex podcast.

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Why Do We Have Sex? Part 1

If your answer is "to make babies," you're wrong. Sexual activity among humans has nothing to do with fertilization more than 99.99% of the time.

Listen to the podcast by roboreader Kate

Typical heterosexual couples make love an average of a hundred times a year. Assuming they keep up this pace most of their adult lives, they will end up having had sex as many as four thousand times.

In addition to sex with a partner, most people seek lots of sexual relief when they're all alone. Men typically learn to masturbate in adolescence and keep up the practice daily until their twenties. The pace usually slows down as men age and often dips when they enter sexual relationships, but most men will probably masturbate ten thousand times in fifty to sixty years of sexual activity. Altogether, the average man can expect to experience fifteen thousand or more orgasms over the course of his life.

Women start off masturbating at a similar age and frequency as men. Most masturbate daily until their late teens, but slow down when they reach their early twenties to about a third the rate of men. Still, they typically enjoy sexual stimulation, either alone or with a partner, for a lifetime total of five to ten thousand sexual experiences.

Despite all that sexual activity, the population in the US and most other highly industrialized nations is fairly stable. That is, there is roughly a single child born for each person in the country, which means that there is one successful pregnancy for every ten thousand sexual experiences.

Humans are unusual creatures in this regard, though hardly unique. Certain apes, dolphins and wolves are among the animals that use sexual interactions for things like pleasure, bonding, and establishing social structure. But intercourse for most other organisms is all about making babies.

Even though humans rarely have sex in order to get pregnant, it's primarily our genetic mandate to mate and bear young that is reflected in our sexual desire. Evolution ensures things that are good for the propagation of our genes bring us pleasure. For most people, and apparently many animals as well, the orgasm is the benchmark of pleasure. The fact that it produces the most enjoyable sensations and the strongest desires that we are likely to ever experience suggests that sex is effectively the most important thing we do in life, from an evolutionary point of view. Although we have found ways to separate intercourse from procreation, sexual ecstasy is nature's reward to us for continuing the species.

Sex between a male and a female, however, is not the only way for organisms to reproduce. For millions of years, when life was new on Earth, plants and animals got on just fine without intercourse. Most of them were simple single-celled organisms. At times when there were plentiful resources to keep some organism alive, rather than letting anything go to waste, or go to a competitor, the organism would simply duplicate its DNA and split into two identical copies of the original. Modern bacteria, amoebas, and many types of algae and molds continue the practice.

A look at the pros and cons of various types of reproduction can help explain why we mate the way we do instead of opting for asexual reproduction, or some other scheme altogether. For physicists who ponder reproductive strategies, the subject is similar to many other sorts of problems in physics where systems, in this case populations, naturally find the optimal solution to complex and competing demands.

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