1. Use It or Lose It
You need to have erections regularly to keep your penis in shape. "It has to be essentially exercised," says Tobias Kohler, MD, assistant professor of urology at Southern Illinois University School of Medicine.
If a guy is physically able to get erect, but never has erections during the day -- maybe he finds himself in very un-erotic circumstances for a long time -- he needn't worry. The brain has an automatic penis maintenance function built in.
Impulses from the brain cause erections during the dreaming phase of sleep, called the REM phase. It doesn't matter if you're having a hot sex dream or a zombie apocalypse nightmare -- your penis gets hard during that period of the sleep cycle.
"If they don't do anything to maintain normal erections, they will get shortening of the penis," Kohler says. Without regular erections, penile tissue can become less elastic and shrink, making the penis 1-2 centimeters shorter.
A device like a vacuum pump, which forces the penis to swell with blood, can help men with physical erection problems maintain a healthy penis, Kohler says.
2. Your Penis May Be a 'Grower' or a 'Show-er'
Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.
In one study of 80 men, researchers found that increases from flaccid to erect lengths ranged widely, from less than a quarter-inch to 3.5 inches longer.
Whatever the clinical significance of these data may be, the locker-room significance is considerable. You can't assume that a dude with a big, limp penis gets much bigger with an erection. And the guy whose penis looks tiny might get a surprisingly big erection.
An analysis of more than a thousand measurements taken by sex researcher Alfred Kinsey shows that shorter flaccid penises tend to gain about twice as much length as longer flaccid penises.
A penis that doesn't gain much length with an erection has become known as a "show-er," and a penis that gains a lot is said to be a "grower." These are not medical terms, and there aren't scientifically established thresholds for what's a show-er or a grower.
Kinsey's data suggest that most penises aren't extreme show-ers or growers. About 12% of penises gained one-third or less of their total length with an erection, and about 7% doubled in length when erect.
3. The Pleasure Zone
Many men consider the underside of the glans (head) of the penis and the underside of the shaft to be most sensitive to sexual pleasure.
The underside of the glans and underside of the shaft had the highest sensitivity rating for a significant majority of men, followed by the upper side of the glans, left and right sides of the glans, sides of the penis, upper side of the shaft, and foreskin (for the minority of men who were uncircumcised). The study findings were reported in the British Journal of Urology International in 2009.
4. Sensitivity Declines With Age
Studies show that the penis steadily loses sensitivity as men age though it's hard to say exactly by how much. That's because different researchers have used different ways to stimulate the penis and measure sensitivity.
In general, the sensitivity of the penis is gauged by the least amount of stimulation a man is able to feel. That is called the "sensory threshold."
Despite differences between studies, the data show a clear trend when taken together. From age 25 on, sensitivity starts to decline. The sharpest decline in sensitivity is seen between age 65 and 75.
What's less clear is whether men really notice a loss of sensitivity as they age.
Kohler says that if they are aware of it, his patients seldom mention it.
"It is a super-rare complaint," he says. "On the other hand, difficulty with erections and difficulty achieving ejaculation are much more common."
5. Vibrators Work on the Penis Too
Vibrators aren't only for women. They work on the penis, too. In fact, vibration is so effective on the penis that often men with spinal cord injuries can ejaculate with the aid of a special medical vibrator. For this kind of treatment, the vibrator is usually held against the underside of the head of the penis.
"Medical-grade vibrators aren't necessarily more powerful," Kohler says. The vibrators are tuned to stimulate parts of the nervous system involved in ejaculation. "They work at frequencies or amplitudes that are more specific to the [nerve] pathways."
Most men don't need a medical vibrator to trigger an orgasm. Kohler says when patients see him about delayed ejaculation -- difficulty reaching orgasm -- he suggests they try a store-bought vibrating personal massager.
Although vibrators often help men with ejaculation problems, you don't have to have any kind of medical condition to use one. You could do it just for fun.
6. There's More to the Penis Than Meets the Eye
"Most guys would be proud to know that their penis is twice as long as they think it is," Kohler says.
That's because half the length of the penis is inside your body. Just like you don't see all of a big oak tree above ground, you don't see the root of your penis tucked up inside your pelvis and attached to your pubic bone.
As seen in an MRI picture, an erect penis is shaped like a boomerang.
7. Your Penis Is a Habitat
The skin of your penis is home to a diverse community of bacteria.
Lance Price, PhD, and Cindy Liu, PhD, researchers at the Translational Genomics Research Institute, in Flagstaff, Ariz., used genetic tests to identify the bacteria found on men's penises. Their study showed there were a total of 42 unique kinds of bacteria inhabiting the skin of the penis.
"We see that the human body is essentially an ecosystem," Price says.
But uncircumcised and circumcised penises don't have the same variety and abundance of bacteria, the study showed. The researchers first analyzed samples from the penises of 12 men who were planning to get circumcised. Samples were taken and analyzed again after the men were circumcised.
After circumcision, there were fewer kinds of bacteria on the men's penises. Many of the kinds of bacteria found to be less common or absent after circumcision were anaerobic -- meaning that they don't need oxygen to grow.
The inner fold of the foreskin is a mucous membrane, like the inside of a person's eyelids. Price says that certain anaerobic bacteria thrive in that environment but not on dry skin.
"I liken it to clear-cutting a forest," Price says. "You're going to get a lot more sunlight, and you're going to drastically change the environment."
The study was done in Uganda, and all of the men studied were Ugandan.
Liu says that she would expect to see some variation in the kinds of bacteria found on men in other parts of the world. "I think there is certainly variety even among the Ugandan men themselves," she says.
But the researchers are less interested in surveying the penile bacteria of the world than in understanding changes brought about by circumcision.
Their research could help explain why circumcision has been linked to a lower risk of getting HIV. One theory is the anaerobic bacteria may prompt the immune system to respond in a way that makes cells more vulnerable to HIV infection.
8. Most Men Aren't Circumcised
Worldwide, approximately 30% of males aged 15 and older are circumcised, according to a 2007 report from the World Health Organization (WHO) and UNAIDS.
Rates vary greatly depending upon religion and nationality, the report states. Almost all Jewish and Muslim males in the world have circumcised penises, and together they account for almost 70% of all circumcised males globally.
Some research shows that there may be health benefits from circumcision. For instance, circumcised men may be less likely to pass sexually transmitted diseases to their female partners or to develop penile cancer.
A 2012 policy statement by the American Academy of Pediatrics (AAP) affirms, “Evaluation of current evidence indicates that the health benefits of newborn male circumcision outweigh the risks and that the procedure’s benefits justify access to this procedure for families who choose it. Specific benefits identified included prevention of urinary tract infections, penile cancer, and transmission of some sexually transmitted infections, including HIV.” This statement has also been endorsed by the American College of Obstetricians and Gynecologists.
In recent years, several studies have shown that circumcised men are less likely to be infected with HIV. The WHO and UNAIDS now recommend male circumcision as an HIV prevention measure. "There is compelling evidence that male circumcision reduces the risk of heterosexually acquired HIV infection in men by approximately 60%," the WHO states.