The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (36 page)

Size Mattered

Male scientists have traditionally viewed the penis as a sperm-delivery device or a symbol of dominance in male competition. They neglected to consider the possibility that the penis evolved through female choice as a tactile stimulator. One popular theory,

developed in the 1960s, was that human penile displays evolved to intimidate rival males rather than to attract females. This is an odd idea, given that in most ritualized threat displays males advertise features related to fighting ability. Dominant gorillas intimidate subordinates with their awesome muscles and sharp teeth, not their one-inch penises. I suspect that heterosexual male scientists find it difficult to think of the penis as something that evolved through sexual choice because it felt good inside one's body.

Most female scientists have been equally reluctant to suggest that penis size or shape was important to the sexual satisfaction of ancestral females. In her book
Mystery Dance,
biologist Lynn Margulis argued that "penis dimension is neither the major determinant of female sexual pleasure nor is a big penis a guarantee of female pleasure." Other women who wish to avoid perpetuating the myth that penis size is all-important go to the opposite extreme and claim that modern women do not use penis size at all as a mate selection criterion, so neither did our ancestors. Nonetheless, I suspect that few modern women would be happy with a sexual partner who had a penis of chimpanzee design—less than three inches long, half an inch thick, and rigid with bone. Of course, no single sexually selected trait is a guarantee of satisfaction. Sexual selection works on the principle of all else being equal. Given two otherwise identical hominid males, if female hominids consistently preferred the one with the longer, thicker, more flexible penis to the one with the shorter, thinner, less flexible one, then the genes for large penises would have spread. Given the relatively large size of the modern human penis, it is clear that size mattered. If it had not, modern males would have chimp-sized sexual organs.

So, why did picky female hominids start selecting for larger penises? Perhaps upright walking gave females a better view of male genitals. Anthropologist Maxine Sheets-Johnstone has argued that bipedalism may have evolved in part because it makes penile display more effective. She observed that in other primates, bipedal standing and walking are most often done by males displaying their penises to potential mates. Bipedal genital

displays to strangers are now considered a criminal offense rather than a legacy of primate courtship. Likewise, the male open-legged sitting position, still universal across cultures, resembles open-legged penile displays by chimpanzees. If Sheets-Johnstone is right that bipedalism originated as a form of male sexual display, then here is another example of an evolutionary innovation originating through sexual selection and later proving useful for survival.

Against the visual display idea, however, is the fact that human penises are a rather sorry spectacle. We have not evolved a bright purplish-pink scrotum and a bright red penis with a yellow tip, as one species of mandrill has. Male vervet monkeys have a blue scrotum and a red penis set off against white hair. When primate penises are selected for visual appearance, they evolve much more color, and females seem to consider them much more attractive. The male human penis does not appear to be especially well adapted for producing auditory, olfactory, or gustatory stimulation. That leaves the sense of touch as the medium for female choice.

Female Choice Continued After Copulation Began

The role of female choice in penis evolution is revealed in the way the penis is used during copulation. Biologist William Eberhard has argued that copulation is not the end of courtship, but rather its most intense phase. In most species, female choice does not end when a male penis first enters, but can continue until sperm actually reach a fertile egg. Eberhard calls this "copulatory courtship." Some female insects can store the sperm of several males for weeks and use it when they want to fertilize their eggs. Many female mammals (unconsciously) squeeze the ejaculate of some males back out after copulation—a process called "flowback"—as if rejecting sperm from males whose copulation is not up to their standard. In a human female with concealed ovulation, a male's sexual ability may influence whether she keeps copulating with him, and that will determine his likelihood of producing offspring with her. If she rejected him after one or two unexciting

encounters, he is very unlikely to father her children.

The duration and intensity of copulatory courtship in a species is a clue to the power of female choice. If efficient sperm delivery were the only point of copulation, a single thrust would be sufficient. Tomcats use this hit-and-run strategy. Copulation in most birds is very brief, and this absence of copulatory courtship is probably why birds have not evolved penises. Most primates make several separate "mounts" and several thrusts per mount before ejaculating. Copulatory thrusting seems designed to maximize the intensity, duration, and rhythmicity of tactile stimulation delivered to the female genitals. Delivering stimulation in addition to delivering sperm suggests that female choice has been important.

Copulatory courtship was probably especially important among hominids. Continuous sexual receptivity and concealed ovulation gave our female ancestors an unprecedented opportunity for testing males as sexual partners, while running a lower risk per copulation of unwanted pregnancy than any other primate did. Sex during menstruation, pregnancy, and breast-feeding would also have given ample opportunity for judging potential long-term lovers by their copulatory skills.

In species that do not use copulatory thrusting, especially insects, penises evolve more obvious tactile stimulators: nubs, spikes, ridges, curls, barbs, hooks, and flagella. Male insects often try to push each other off during copulation, so copulatory thrusting would risk disengagement. Better to lock the genitals together and have internal flagella to excite the female. With primates, it is not so common for male rivals to swarm over females knocking each other off. This allows couples a bit more copulatory leisure, with more complex movements favoring

simpler penis designs. The human penis is especially streamlined because ancestral females apparently favored whole-body

copulatory movement over the flagellar vibrations favored by female insects. Perhaps whole-body copulatory movements, requiring much more energy than waving a couple of vibrators on the end of the glans, were better indicators of physical fitness. It is

not clear whether many middle-aged men do actually have heart attacks during vigorous sex with mistresses, but this plausible risk reveals the energetic costs of human copulation, and one way that female demands for tactile stimulation separate the healthy from the unhealthy. The loss of the baculum (penis bone) also reveals female choice for tactile stimulation. Since male human penises become erect with blood rather than muscle and bone, this gives them more flexibility, and permits a greater range of copulatory positions. Although bonobos also enjoy face-to-face copulation, their positional variety pales in comparison to the
Kama sutra.
Human penises evolved as tactile stimulators for use in copulatory courtship. Further research may clarify whether penises and copulatory courtship evolved mostly as fitness indicators or just as sexually selected entertainment.

Female hominids may not have preferred thicker, longer, more flexible penises per se. They may simply have liked orgasms, and larger penises led to better orgasms by permitting more varied, exciting, and intimate copulatory positions. This rather contradicts the view of the penis as a symbol of male domination. If we were a species in which males dominated the sexual system, we would have one-inch penises like dominant gorillas. The large male penis is a product of female choice in evolution. If it were not, males would never have bothered to evolve such a large, floppy, blood-hungry organ. Ancestral females made males evolve such penises because they liked them.

The Penis and the Brain

Why have I paid so much attention to the evolution of the penis? One reason is its importance as a genetic conduit. Every gene in every human body has passed through thousands of penises over thousands of generations of human evolution. Equally, every gene has passed down through thousands of eggs inside female ancestors who chose to copulate with particular males. In sexually reproducing species, copulation is the genetic gateway from one generation to the next, which is what makes it so important evolutionarily, physically, and psychologically.

The penis is an easy trait to study because it is visible, measurable, and directly comparable to the corresponding organs of other species. Yet even for such a simple trait we have seen how the biases of male and female scientists may have influenced their views on penis evolution. We have considered both the sperm competition model and the "symbol of dominance" model for penis evolution. I could have mechanically run through the checklist of criteria for identifying sexually selected traits, but that would get rather tedious for every adaptation I shall be assessing in the rest of the book. The penis's fit to the criteria is rather obvious anyway: the penis shows distinct sex differences (it is much larger than the homologous female organ, the clitoris), grows mainly after puberty, is used during copulatory courtship, is considered sexually attractive by internal touch if not by sight, and differs markedly between species.

Physical organs shaped by sexual choice can also be seen as metaphors for mental organs shaped by sexual choice. Just as the human penis has been misunderstood as nothing more than plumbing for delivering sperm, the human mind has been misunderstood as wiring for processing information. In both cases, I argue that the organ evolved for the stimulation it can deliver, not to solve some straightforward physical problem of insemination or toolmaking. The sexual choice that mattered did not focus directly on the physical form of the organ, but on the shared experiences it could generate. Ancestral females did not apparently favor penises directly as visual ornaments, but favored them indirectly for the copulatory pleasure that they afforded, so they came back for more. Perhaps our ancestors did not favor intelligence and creativity directly, but indirectly: for how they contributed to having a great time with someone. If

the penis really did evolve through female choice as a copulatory stimulator, then it should be considered not just a physical organ

that reaches inside the body, but a psychological organ designed to reach inside the pleasure systems of another individual. It happens to have a physical form only because the other individual's pleasure systems happen to be connected to tactile sensors.

The Clitoris and the Orgasm

In most species in which males have a penis, females have a homologous organ called the clitoris. "Homologous" means that both organs grow from the same kinds of cells in the fetus. Anatomically, the human clitoris has the same three-part columnar structure as the penis: a glans, a shaft, and bifurcating roots. The main differences are that the penis is much larger overall, its shaft protrudes much more from the pelvis, it keeps blood from flowing back out when aroused, and it has a tube down the axis for urine and semen.

The human clitoris shows no apparent signs of having evolved directly through male mate choice. It is not especially large, brightly colored, specially shaped, or selectively displayed during courtship. By contrast, in spider monkeys the clitoris is almost as large as the penis, protruding nearly an inch. In hyenas, the female clitoris is larger than the male penis, and seems to play a role in female competition. The human clitoris could easily have evolved to be much more conspicuous if males had preferred sexual partners with larger, brighter clitorises. Its inconspicuous design combined with its exquisite sensitivity suggests that the clitoris is important not as an object of male mate choice, but as a mechanism of female choice. It helps to select for males who provide pleasurable foreplay, copulation, and orgasms, and such discriminative power is just what we should expect from an organ of female choice. Yet this has led to all sorts of confusion among evolutionists.

Some male scientists, such as Stephen Jay Gould and Donald Symons, have viewed the female clitoral orgasm as an evolutionary side-effect of the male capacity for penile orgasm. They suggested that clitoral orgasm cannot be an adaptation because it is too hard to achieve. Sigmund Freud suggested that clitoral orgasm was a sign of mental disorder, and counseled his female clients to learn how to have purely vaginal orgasms. Other male scientists such as Irenaus Eibl-Eibesfelt and Desmond Morris have viewed female orgasm as a reinforcement mechanism for promoting long-term pair-bonding that keeps a

female faithful to her mate. They also wondered why clitorises have such trouble provoking orgasm. They assumed that if clitorises worked properly like penises, they should just do their job of promoting marital satisfaction without so much copulatory effort.

These men seem to have overlooked the possibility that clitoral orgasm is a mechanism for female choice rather than pair-bonding. Mechanisms for choice have to be discriminating: they must fire off excitedly when given the right stimulation, and emphatically must not fire off when given inferior input: As a mechanism for female choice, we would not expect female clitoral orgasm to respond to every male copulation attempt, however inept, lazy, inattentive, brief, and selfish. It is possible for a woman's vagina to become lubricated during unwanted sex to avoid injury, but women under such conditions practically never have orgasms. This is strong evidence of clitoral orgasm's role in female choice.