Memorialized in monuments, Grecian statues and bathroom graffiti everywhere, the penis may be the most famous and well-studied human organ on the planet.
But despite its outsized profile in the popular imagination, the male member still has a few secrets left to reveal. From penis spines to the ideal size and even penis shame, here are eight wild facts about the male sex organ.
Editor's note: This countdown was first published Aug. 21, 2013.
The average erect penis is about 5.56 inches (14 cm) long, according to a 2013 study detailed in the Journal of Sexual Medicine that surveyed 1,661 men. But variety is the spice of life, and men in that study had members that ranged from 1.6 inches (4cm) long to 10.2 inches (26 cm) long.
Not all erections were created equal. Those who measured their penises after oral sex or intercourse sported larger penises than those who relied on fantasy alone, the study found.
And because it reduces blood flow to the penis, smoking can shorten the average penis by up to 0.4 inches (1 cm), other studies have found.
When it comes to penises, size does matter — at least for some women. Women who are more likely to have vaginal orgasms say it is easier to orgasm with men who have longer penises, according to a 2012 study published in the Journal of Sexual Medicine. Though it's not clear exactly why, a longer penis may be better able to stimulate the vagina and the cervix, study co-author Stuart Brody, a psychologist at the University of the West of Scotland, told LiveScience at the time. In a 2013 study detailed in the journal Proceedings of the National Academy of Sciences, researchers reported women said the ideal penis size varied with a man's height, with a larger organ looking better on taller men.
Very rarely, a man may be born with two penises, a condition that affects every 5 million to 6 million males and is known as diphallus. Unfortunately, this condition doesn't mean double the fun: Both organs are rarely fully functional, and the condition often comes along with other anomalies in the genital area that require surgery to correct.
Men with another penis condition called priapism suffer from a persistent erection that won't go away after four hours. Usually the cause is a failure of blood to return from the penis to the rest of the body, although it is also occasionally found in those with sickle-cell anemia or leukemia. The situation is usually a medical emergency that requires a shot of pseudoepinephrine to constrict the smooth muscles in the penis.
The penis may have been a lot scarier in humans' evolutionary past. At one point in time, the male penis had spines, but human ancestors lost those prickly structures before Neanderthals and modern humans diverged some 700,000 years ago, according to a 2010 study published in the journal Nature. Scientists aren't clear on the function of those spines, but some propose they allowed for quickies because they can create an erection quickly, and are more common in promiscuous species, such as cats (tomcats have rather terrifying spines on their penis).
Another relic of times past is the penis bone, or baculum. Though most apes have a bone to keep their member erect, human males lost theirs at some point and now rely on blood pressure for stiffness. In other animals, the penis bone sits inside the body and is pushed out into the penis for an instant, reliable erection. It's still a mystery why males lost this trait, but in "The Selfish Gene" (Oxford University Press, 2006), biologist Richard Dawkins proposes the bone-free penis was selected for because it allows females to gauge potential partners' health — those who can't get an erection probably have poor blood flow.
Whether or not men are celibate during the day, their penis is working out at night. Most men have three to five erections a night during the rapid eye movement (REM) phase of sleep, whether they are dreaming about grandma or supermodel Elle Macpherson. This nighttime action apparently keeps the male member in shape — penises that don't experience regular erections risk losing their elasticity and shrinking.
Because this is such a basic physiological process, many doctors ask whether a man has nighttime erections to determine the cause of erectile dysfunction.
When baby boys are born, the foreskin tissue is fused to the glans of the penis. In the womb, the foreskin evolves from the same tissue as the clitoral hood.
The foreskin's inner surface is made up of mucous membranes similar to those found inside the eyelid or the mouth, making it a moist place. That unique environment could be responsible for the increased STD transmission rates associated with uncircumcised men in some studies. [5 Things You Didn't Know About Circumcision]
The foreskin also has an abundance of Langerhans cells, the immune cells infiltrated by HIV. That may explain why circumcised men in Africa have a 60 percent lower rate of HIV infection from heterosexual intercourse.
The American Academy of Pediatrics does not endorse or discourage circumcision, noting that circumcision carries both small risks and benefits. But "intactivists" disagree, citing studies that suggest circumcised men experience less sexual pleasure. Many doctors, however, are skeptical of this research, because the methodology has been problematic or biased.
The old adage is true: Some men are "showers" and others are "growers." There's no way to predict the size of a man's erect penis when it's flaccid, according to a 1996 article in the Journal of Urology. However, a stretched-out penis is a good predictor of its ultimate erect size, a 2000 study in the International Journal of Impotence Research found.