Why do humans have big penises

Most male mammals wield a penis covered with spines made of keratin, the same material that forms fingernails, to sweep out competitors' sperm and irritate a female into ovulating. You can add humans' lack of penile spines to the list of ways we are misfits among primates, along with our absence of tails and fur. Even chimpanzees, our closest relatives, have penile spines. A new study suggests that this feature disappeared due to a chunk of DNA that went missing after our evolutionary divergence from chimps. The researchers have identified another DNA deletion that may have contributed to humans' bigger brains.

The question of what makes us distinctly human is hardly a new one, of course, but developmental genomicist Gill Bejerano and developmental geneticist David Kingsley, both of Stanford University in Palo Alto, California, decided to look at the issue from another angle. Maybe humans don't have an advantage over chimps genetically, as we often like to think we do—maybe we've actually lost something. Bejerano and Kingsley compared the chimp genome with the human genome, looking for DNA regions that chimps had but humans did not. And rather than looking at genes, as most research in the past has done, they examined DNA regions that don't code for genes but instead regulate how nearby genes are expressed.

They found 583 deletions in the human genome, and Bejerano says choosing which to study first was a tough decision. "Each region could be its own adventure," he says. They ended up choosing two: a deleted region near a gene for male hormone response and a region close to a gene involved in brain development. The Neandertal genome also lacks these regions, indicating that these deletions occurred more than half a million years ago.

To discover what these lost DNA regions did, the researchers cloned the chimp DNA into mice, engineering it so that places where the DNA was active would turn blue. The first stretch of DNA showed up in the penile spines of developing mouse embryos, as well as in spots on the face where sensory whiskers would develop, indicating that it caused these structures to develop in response to male hormones. The second piece of DNA appeared in an area of the brain called the subventricular zone, a hotbed of neuronal growth and expansion. Deleting this DNA may have removed a brake on neuronal growth, allowing humans' brains to expand, the researchers hypothesize this week in Nature.

"There's a tendency to think of genetic loss as a loss of things," says genomicist Maynard Olson of the University of Washington, Seattle, who was not involved in the research. But sometimes less is more, he says. In terms of evolution, he points out, deleting a stretch of DNA is a much faster process than evolving a whole new genetic system would be.

Human geneticist James Noonan of Yale University said that although the approach was "very clever," speculation about the physiological changes these two deletions caused and the role they played in shaping human evolution is premature.

With well over 500 deletions left to analyze, Bejerano and Kingsley expect many other researchers to jump in. "This study brings together genetics and development in a nice way; it's a pleasure to see these fields converge," says Bejerano. "Evolutionary stories play out best when you can wield both."

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If you've ever wondered whether humans, do in fact, have a bigger dick than most apes - and you don't work in a zoo - then you probably have issues. But nonetheless you would be on to something.

It is true. We are better hung than similar species.

Humans have a much longer and wider penis than the other great apes. Even the largest of gorillas, that are twice our weight, have a penis just two-and-a-half inches long when erect.

But it's swings and roundabouts as our testicles are much smaller than theirs.

Why you ask? Well, the relative size of our penis and testes is all down to our mating strategies.

Male chimps have sex all the time with any female. A female therefore may contain sperm from multiple partners at any one time, which puts the sperm itself into direct competition. Because of this, chimpanzees evolved massive testicles in order to produce copious amounts of sperm.

On the other hand, male gorillas are also much larger than females, but they have a polygynous mating system where many females live with a single male. With little or no competition actually inside the uterus, gorillas have had no need for the production of loads of sperm, so their testes are relatively small.

This is similar to modern humans, whose testes are also of very modest size and produce a relatively small amount of sperm.

The human penis is large when compared with those of our closest relatives: chimpanzees, gorillas, and orangutans.

The human penis, however, does not have lumps, ridges, or any other interested feature that other primates have. In primates, this lack of penis complexity is usually found in monogamous species.

Anthropologists Clellan Ford and Frank Beach in their book, 'Patterns of Sexual Behaviour', suggested that 84 percent of the 185 human cultures they had data on engaged in polygany. However, even in these societies most people remain monogamous - with polygamy practiced primarily by the wealthiest and privileged.

As to the size of our dicks, it is thought the human male penis can be partially explained by our upright posture.

Walking on two feet leaves the penis out in the open with the possibility of attracting females. Since human females don't make themselves readily available like female chimpanzees, the human male penis must advertise itself.

So there we have it, I finally know why I'm hung like a donkey.

Bigger is better—at least when it comes to a woman's penis preferences, a new study says. The findings suggest that female choice may have driven the evolution of larger penises in humans.

Past psychology studies that have asked women about their preferences for penis size have produced mixed opinions, with some saying they don't care about the size, while others say they prefer larger penises. (Also see "Why Human Penises Lost Their Spines.")

But those women may have felt pressured to say the politically correct thing: That size doesn't matter, said study leader Brian Mautz, a biologist at the University of Ottawa, Canada.

So his team set up an experiment in which 105 Australian women—averaging 26 years old—each looked at 53 life-size images of various computer-generated male silhouettes projected onto a screen.

The gray, homogenous-looking men had different heights, shoulder-to-hip ratios (or body shapes), and flaccid-penis lengths. In past studies, women have generally perceived tall men with greater shoulder-to-hip ratios as more alluring, making these factors important in how a woman judges attractiveness.

The women in the current study were then asked to rate the men on an attractiveness scale of 1-7, with 1 being least attractive and 7 being most attractive.

Although body shape was the most important factor in whether or not women rated a male shape as attractive or not, penis size also significantly influenced attraction.

However, the relationship wasn't necessarily linear—i.e., "as you get a larger and larger penis, it doesn't necessarily mean you're getting more and more attractive," Mautz noted. (Read about the barnacle, which has the longest penis relative to body size in the animal kingdom.)

For example, the rate at which women rated the men as attractive fell when flaccid penises were longer than 2.99 inches (7.6 centimeters).

What's more, height was also a factor: "Tall guys with really large penises ended up being most attractive relative to other figures," Mautz said.

It's not totally clear why women prefer bigger penises, but studies have shown that women prefer larger sizes because they can increase sexual satisfaction, Mautz said.

The Evolving Penis

Since early humans didn't wear clothes, male penises were obvious to women. So if women chose their mates based on the size of their genitalia, it's possible that these decisions influenced the evolution of bigger penises, according to the study, which was published in the journal Proceedings of the National Academy of Sciences. (Explore an interactive of the human body.)

"We can't say for sure that female choice is what drove the evolution of penis size," but "at this point in time, penis size has an effect on attractiveness," said Mautz.

But evolutionary psychologist Gordon Gallup, Jr. said in an email that there are some limitations to the team's theory that female choice played a role in penis evolution.

For one, previous studies have shown that "the flaccid human penis is an imperfect indicator of the size of the erect penis," said Gallup, of the State University of New York at Albany, who was not involved in the study.

Theoretically, women in the early days of human evolution would have seen most penises in a flaccid state.

What's more, the results show that women paid more attention to shoulder-and-hip ratios than penis size when rating an image as attractive, he said.

Gallup has done his own research on penis evolution, which he said has been shaped by a man's competition with other males to fertilize a woman's egg.

Gallup's research has found that the penis's arrow-like shape is able to scoop rival male semen out of the female's reproductive tract. (Also see "Sperm Works Best in the Winter.")

What's more, studies using models of human genitalia have shown that deeper thrusting displaces more semen from a woman's reproductive tract than shallow thrusting. Therefore, "long penises may have also evolved to deposit semen in the deepest and more remote parts of the vagina to promote more effective sperm retention," he said.

Whatever the reason for large penises, this field of study addresses a question that some will find humorous, study leader Mautz acknowledged.

"It's important that you are serious in science, and our results are solid—but it doesn't necessarily mean you can't have fun with it."