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8 Things You Didn't Know About Your Penis

Sensitivity, pleasure, size, and other surprising facts.
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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.

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