The Evolution of the Penis

Sexual reproduction does not really require many sex differences. Males must make sperm, and females must make eggs. But males do not have to grow penises, and females do not have to grow clitorises. Male frogs and birds do not have penises. Genitalia are products of sexual choice, not requirements for sexual reproduction. The traditional distinction between "primary" sexual traits (such as penises) and "secondary" sexual traits (such as beards) is misleading. Perhaps for reasons of Victorian propriety, Darwin wrote as if female choice applied only to the secondary sexual traits. But modern biologists view penises themselves as targets of sexual choice. Biologist William Eberhard has argued convincingly that male genitals in a wide range of species are shaped as much by female choice as by the demands of sperm delivery

Adult male humans have the longest, thickest, and most flexible penises of any living primate. The penises of gorillas and orangutans average less than two inches when fully erect, and those of chimpanzees average only 3 inches. By contrast, the average human penis is over 5 inches when erect. The longest medically verified human penis was about 13 inches when erect, more than twice the average length.

Even more unusual than the length of the human penis is its thickness. Other primate penises are pencil-thin, whereas the erect human penis averages over one inch in diameter. Also, most other primates have a penis bone called the "baculum," and achieve erections mostly through muscular control, like a winch raising a rigid strut. The penis bone is typical of most mammals. By contrast, the male human relies on an unusual system of vasocongestion. The penis fills with blood before copulation, like a blimp inflating before flight.

Although it is larger than any other primate's, the human penis has plenty of rivals in more distantly related animals. Blue whales and humpback whales have penises eight feet long and one foot in diameter. Bull elephants have penises around five feet long. Boars have 18-inch penises that ejaculate a pint of semen. Hermaphroditic snails have penises about as long as their entire bodies. Stallions, like men, use blood rather than muscular contraction to fill their much larger penises. Dolphins have voluntary control over the tips of their man-sized penises, which can swivel independently of the shaft. Male genitals are even stranger among the invertebrates, sporting a dizzying variety of sizes, flagella, lobes, bifurcations, and other ornaments, apparently designed to stimulate invertebrate female genitalia in as many different ways as there are species.

Didn't penises evolve just to deliver sperm? Sperm competition is certainly one of the most important forms of reproductive competition. If two males copulate with a female when she is fertile, their sperm are in competition. Only one, at best, will fertilize her egg. The male with the fastest, longest-lasting, most numerous sperm is more likely to pass on his good-sperm genes to his sons. Heritable differences in sperm quality and sperm delivery equipment will be under intense selection. Male humans show many adaptations for sperm competition, both physical and mental. For example, some studies have shown that when a woman returns home from a long trip, her partner tends to produce a much larger ejaculate than normal, as if to overwhelm any competitor's sperm that may have found its way into his unwatched partner's vagina.

However, comparisons of male testicles across species reveal that penises did not evolve purely for spermatic firepower. Among primates, the intensity of sperm competition correlates much more strongly with testicle size than with penis size. For example, male chimpanzees face much greater sperm competition than humans. When female chimps ovulate, they copulate up to fifty times a day with a dozen different males. In response, male chimps have evolved huge, 4-ounce testicles to produce sperm, but only small, thin penises to deliver it. At the other extreme, male silverback gorillas guard their harems vigilantly and violently, and tolerate no sperm competition, so they have evolved very small testicles. Humans have moderately sized testicles by primate standards, indicating that ancestral females copulated with more than one male in a month fairly often. Sequential fidelity to different men in different months would not produce any sperm competition, because each egg would be exposed only to one man's sperm. The fact that male human testicles are larger than those of gorillas is one of the strongest pieces of evidence that ancestral females were not strictly monogamous.

For sperm competition, sperm count and ejaculate volume are more important than penis length or thickness. A thick penis might tend to keep a competitor's sperm inside a female rather than allowing it to wash out. A long penis tends to overshoot the cervical opening rather than meet it accurately. Many species adapted for heavy sperm competition evolve penises with scoopers, scrapers, suckers, and flagella for removing rival sperm. If sperm competition were the driving force behind penis evolution, males might have evolved scary-looking flagellated genitals. Men would copulate by inserting their equipment, instantly flooding the cervix with half a pint of semen, and then lying on top of the woman for the next three days to make sure no rivals have the chance to introduce competing sperm. I understand that such behavior is quite rare.

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