The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (35 page)

adaptations for courtship: we don't have much fossil evidence about their antiquity, but we can infer a lot from their modern human form and their absence in closely related ape species. If sexual selection drove our bodily divergence from other apes, it may have driven our mental divergence as well. Rene Descartes saw a dichotomy between body and mind, but sexual choice judges them as a package. As Walt Whitman put it in his 1855 poem "One's-Self I Sing:"

Of physiology from top to toe I sing,

Not physiognomy alone nor brain alone is worthy for the Muse, I say the Form complete is worthier far, The Female equally with the Male I sing.

Penises, clitorises, breasts, and beards are fascinating not only in their own right, but also for what they reveal about sexual selection among our ancestors.

Which Body Traits Evolved as Sexual Ornaments?

Many of our body traits such as penises, breasts, buttocks, beards, head hair, and full lips show the hallmarks of sexual selection through mate choice. They are uniquely amplified in our species. Many of them show large sex differences. Mostly, they appear or enlarge only after puberty, and become more engorged with blood during sexual arousal. All around the world they are clearly valued as sexual signals, and are made more conspicuous through embellishment and make-up. They probably evolved partly as fitness indicators and partly as ornaments through runaway or sensory preferences. A body trait does not have to fulfill all of these criteria to qualify as a sexually selected ornament or indicator, but the more the better. Many of these criteria work for mental traits as well as body traits, so we'll be using them often throughout the rest of this book.

As we have seen, sex differences are highly diagnostic of sexual selection. Traits found in one sex but not the other usually result from sexual selection. Yet sexual selection does not always

produce sex differences. Where we find sex differences it is likely that sexual selection has affected at least one sex, but even if we do not find sex differences in an ornament, sexual selection may still have affected both sexes.

A common fallacy is to argue that sex hormones are sufficient to explain sex differences. This is not an alternative to sexual selection, it just identifies a mechanism that sexually selected genes use to produce sex differences. For example, the hormone testosterone is a simple molecule that cannot by itself carry instructions for growing a complex trait such as a penis or a beard. Rather, the genes for growing penises and beards have evolved the ability to be switched on in response to testosterone, because testosterone tells these genes that they happen to be in a male body in this generation. (Most genes underlying distinctive male and female traits are present in both sexes, but are activated only by the cascade of sex hormones during fetal development.) A trait's sensitivity to sex hormones is itself a product of sexual selection.

Active display in courtship is a good sign that sexual selection has shaped a trait. Since courtship is restricted to sexual maturity, any trait that grows only after puberty is likely to be a result of sexual selection. Prepubescent girls don't grow breasts because they would be physiologically expensive and encumbering. Only when attracting a mate becomes a potentially adaptive thing to do, do the breasts sprout. Likewise for male beards and other body hair, male penises, male upper-body musculature, and many other traits. On a shorter time-scale, some bodily ornaments change their state during sexual arousal, the most intense phase of courtship. The penis grows erect and larger. A sexual flush spreads over a woman's neck, chest, and breasts. The breasts, lips, and labia engorge with blood. Traits that attain their full form only during sexual maturity and sexual arousal probably evolved through sexual choice.

Is the trait still viewed as sexually attractive today, across human cultures? Traits shaped by prehistoric sexual choice should still be considered sexually attractive today, insofar as our

sexual choice mechanisms remain similar. If a bodily trait is considered sexually attractive across a wide range of cultures and historical epochs, the trait was probably viewed that way during human evolution. The manifest sexual appeal of female breasts and buttocks, for example, seems subjectively obvious to all heterosexual male humans, and that obviousness is good evidence for these traits having arisen through male mate choice. Around the world, the same bodily traits tend to be emphasized with special clothing and ornamentation when individuals wish to appear attractive, the same traits are covered when they wish to avoid sexual harassment, and the same traits are mutilated as punishment for sexual offenses.

When anthropologists claim that standards of beauty vary capriciously from one culture to another, they are usually studying the wrong traits in the wrong ways. Individuals of different cultures may like skin of different shades, but they all prefer clean, smooth, unwrinkled skin. Women differ in the exact male height they prefer, but almost always prefer a man taller than themselves. Different ethnic groups may prefer different facial features, but all prefer faces that are symmetrical and averagely shaped for their population. If you don't look for the universals of human beauty at the right level of description, you will not find them.

There is another test we have seen throughout this book: traits that are unique to one species are often the outcome of sexual selection. This is because traits shaped by natural selection that prove useful for survival tend to make a species successful, and successful species tend to split apart into daughter species. The species turns into a genus (a group of closely related species), and the useful trait is shared by all members of the genus. Sexual ornaments do not usually increase survival success, however, so each particular ornament tends to stay restricted to one species.

Even within a species, sexual selection produces diversity between populations. In humans, the runaway effect can take different populations ("ethnicities," "races") off in different evolutionary directions, ornamenting them with different face shapes and body traits. Where the divergence has no apparent

relationship to different climates or ecological challenges, it probably arose through sexual selection. Human populations differ markedly in skin color, eye color, hair length, facial features, breast size, and penis size. Darwin took such differences as evidence for such traits having diverged rapidly and recently through sexual selection, but he may have overstated his case. Natural selection can account for some latitude trends, explaining why skins got lighter, noses got larger, and bodies got shorter and thicker as human populations migrated from equatorial zones to colder climates. However, latitude and climate cannot account for most of the subtler differences between populations. Most differences in eyes, hair, facial features, and the sizes of breasts, buttocks, and penis are more likely to be consequences of sexual choice focusing on different traits in different populations.

Because sexual choice often shapes traits to work as fitness indicators, it can also produce traits that show large differences between individuals within the same population. If male choice selected female buttocks as reliable indicators of fertility, health, and youth, we should not expect all females to have identical buttocks, for that would make the trait useless as an indicator. Evolutionary psychologists are discovering that many human body traits advertise a particular aspect of fitness called "developmental stability." This refers to an individual's ability to grow a trait in a normal form despite the mutations they may be carrying, and despite the environmental challenges (poor nutrition, parasites, injuries) that they may encounter during development inside and outside the womb. For traits that normally grow symmetrically, like faces and breasts, the exact degree of symmetry can be a powerful indicator of developmental stability, which in turn is a major component of fitness. (Symmetry is just one way to measure developmental stability—it could also be measured by comparing the similarity of identical twins who have grown from the same genes, for example.) Bodily symmetry is biologically important because it is one of the easiest components of fitness for biologists to measure, and for animals to assess when choosing mates. The symmetry of sexual ornaments is an important determinant of

sexual attractiveness in many species, including our own. Many of our bodily ornaments, not least faces and breasts, probably evolved in part as symmetry indicators.

We can use these criteria to identify parts of the human body that probably evolved through female choice, male choice, or both. The more evidence we find for mutual choice having shaped the body, the more reasonable it becomes to suggest that mutual choice shaped our minds as well, without creating large sex differences in mental abilities.

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.