Sexual Selection in Penises and Clitori

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 contra- dicts 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.

[...]

Mechanisms for choice have to be discrim- inating: 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.

From a sexual selection viewpoint, clitorises should respond only to men who demonstrate high fitness, including the physical fitness necessary for long, energetic sex, and the mental fitness necessary to understand what women want and how to deliver it. The choosy clitoris should produce orgasm only when the woman feels genuinely attracted to a man's body, mind, and personality, and when the man proves his attentiveness and fitness through the right stimulation.

[...]

The sex difference between penis and clitoris can be viewed as a physical manifestation of Fisher's runaway process: a highly developed male trait (the penis) designed to stimulate, and a highly discerning female preference (the clitoral orgasm) designed to respond selectively to skillful stimulation. If this runaway model is right, then there was a sort of stimulatory arms race between the human penis and the human clitoris. The penis evolved to deliver more and more stimulation, while the clitoris evolved to demand more and more.

This tension explains why women and men are not well adapted to giving each other easy, simultaneous, repeated orgasms. If the function of orgasm were simply to reinforce monogamous pair-bonds, why should evolution make female orgasm so difficult and male orgasm so easy during vaginal intercourse? If female orgasm is a side-effect of male orgasm, why does it just happen to work when an attractive man provides a lot of foreplay and deep, slow copulatory thrusting, but not so well when sex is hurried or the partner is undesirable? Surely, sexual selection theory offers insight into this ancient human mystery. Female orgasm seems poorly designed as a pair-bonding mechanism, but it is perfectly designed as a discriminatory system that separates the men from the boys.

Yet the image of an evolutionary arms race between penis and clitoris is not quite accurate. The female mechanism for assessing penis size is not the clitoris itself, but the ring of nerves around the entrance to the vagina, which sense circumference. The clitoris does something more sophisticated, assessing the male's ability to move in pleasurable, rhythmic ways during copulation. Also, clitoral stimulation usually leads to orgasm only when the female mind is feeling erotic about the man and the situation. Human female orgasm depends on an interaction between the clitoris, the hypothalamus (the brain's emotional center), and the cerebral cortex (the brain's cognitive center). The clitoris is only the tip of the psychological iceberg in female choice. Having a mate with a large penis is not enough. To be fair, the penis is not just an insensate stimulator either. It is also a mechanism for male mate choice. If it is happy, its owner may be more likely to stay in a long- term relationship with a woman.

[...]

Just as the penis can be seen as a metaphor for the mind's sexually selected entertainment abilities, the clitoris can be seen as a metaphor for the mind's judgment and discrimination abilities. When we see a human perceptual or cognitive ability that looks curiously sensitive to stimulation yet resistant to satisfaction, we should not assume that it is a poorly designed information pro- cessing system. It may be part of a system for sexual or social discrimination. Consider humor. Some theories of humor have proposed that laughter evolved to promote group bonding, dis- charge nervous tension, or keep us healthy. The more laughter the better. Such theories predict that we should laugh at any joke, however stupid, however many times we have heard it before, yet we do not. A good sense of humor means a discriminating sense of humor, not a hyena-like shriek at every repetitive pratfall. Such discrimination is easy to understand if our sense of humor evolved in the service of sexual choice, to assess the joke-telling ability of others.

Female sexual preferences may have guided the evolution of these sexual organs.