Table of Contents
Why is Sex Fun? by Jared Diamond tries to explain a seemingly obvious question contained in the title. To most people, the answer is obvious and needs no further explanation. People have sex all the time, because sex is fun, there’s nothing else to explain. But to a scientist like Diamond, this answer is not satisfying. By comparing human habits to those of other animals, particularly those closest to humans, we get a more interesting and nuanced answer – this is what this book is out.
Humans and the Great Apes
Sexuality, posture, and brain size comprise the trinity of ways human ancestors and great apes diverged.
Orangutans are usually solitary, males and females only associate to copulate, and males provide no paternal care. A gorilla male gathers a harem of a few females and has sex with each of them at intervals over several years. Chimps and bonobos live in troops with no enduring male-female pair bonds of specific father offspring bonds.
Child Care
Humans are exceptional animals in that our fathers and mothers remain together after copulating and both are involved in raising the child. No one could say that these parental contributions are equal, but most father make some contribution to their children, even if it’s just food, defense, or land rights. We take these contributions for granted that they are written into law: divorced fathers pay child support, and an unmarried mother can sue a man for child support if genetic testing shows that he is the biological father.
Child care for men is less rewarding than it is for women, genetically speaking. A man who fertilizes one woman and devotes himself to child care may foreclose enormous alternative opportunities – such Morocco’s Emperor, Ismail the Bloodthirsty, father of seven hundred sons and presumably a comparable number of daughters.
Another factor that makes child care genetically less rewarding for men than for women is the justified paranoia about paternity that men experience. A man who cares for a child risks supporting the genes of a rival. This fact is the underlying cause for many repulsive practices that restrict a woman’s opportunity to have sex with other men. Among these are female “circumcision” (clitoridectomy) to reduce a woman’s interest in having sex, whether marital or extramarital, and infibulation (suturing a woman’s labia majora nearly shut).
But a man is not like a male hummingbird or male tiger, he can’t walk away immediately after copulation, content in knowing that his deserted female sex partner will take care of the work required to promote the survival of his genes. Human infants need two parents to care for them.
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Why is Sex Fun?
Many animals engage in sex, and they look like they’re having a great time, probably more than humans, judging by their intense involvement. Marsupial mice copulate for up to 12 hours. Another interesting question is, why do animals consider sex fun only when the female can be fertilized?
Behavior evolves through natural selection, as does anatomy. If sex is enjoyable, natural selection must have been responsible for this. Natural selection favors individuals whose behavior lets them pass their genes to the most babies. But how does it help you make more babies if you crazy enough to enjoy sex at a time when you couldn’t possibly make a baby?
Humans are exceptional in concealed ovulations, unceasing receptivity, and recreational sex, and it can only be so because we evolved this way. But it’s paradoxical that in humans, a species unique in self-consciousness, women are unconscious of their ovulation. Even female animals as dumb as cows are aware of it. There must be something special that conceals ovulation from a female as smart and aware as a woman.
It has been difficult for scientists to discover what that special thing was. There is a simple reason why most animals are stingy about copulation. Sex is costly in energy, time, and risk of injury or even death.
We would gain a big advantage if we were as sexually efficient as other animals. So what compensation do we get from our apparent inefficiency?
One explanation is that female sexual receptivity keeps the male from leaving her. By giving him sex, he won’t leave her in search for someone else. This enhances the chances that she and her offspring will survive.
3 Theories of Body Sexual Signals
A body sexual signal is a structure on the body of one sex but the opposite sex of the same species. They are used as a signal to attract potential mates of the same opposite sex or impress rivals of the same sex. There are three competing theories that try to account for such sexual signals. The first tis Fisher’s runaway selection model. Females have a hard time figuring out which male to mate with, it’s not obvious for a female to assess the quality of a male’s genes. But suppose a female was genetically programmed to be attracted to males with a certain structure that gave the males a slight advantage at surviving compared to other males. Those males would gain an extra advantage, they would attract more females and create more offspring. Females who preferred those males would gain an advantage too. They would pass on these genes to their sons, who be preferred by other females.
A runaway process of selection would unfold, favoring males with genes for the structure in an exaggerated size and favoring females with genes for an exaggerated preference for the structure. Over time, the structure would grow until it lost its original beneficial effect on survival. A slightly longer tail might be good for flying, but a huge peacock tail is no use for flying.
The process would stop when further exaggeration of the trait becomes detrimental for survival.
A second theory, by Israeli zoologist Amotz Zahavi, states that many structures that function as body signals are clearly detrimental to the owner’s survival. A peacock’s tail is not only useless for survival, but makes survival more difficult. A long, broad tail makes it hard to move through dense vegetation, fly, and escape predators.
Zahavi argues that any male that manages to survive despite such a costly handicap is advertising to females that he must have remarkable genes in other respects. A female can be certain that when she sees a male with that handicap, that he is not cheating by carrying the gene for a big tail but being inferior in other things. He could not have afforded to make that structure, and he wouldn’t be alive unless he were truly superior.
Consider some human behaviors that conform to Zahavi’s handicap theory. While a man can boast to a woman that he is rich, and that she should go to bed with him in the hopes of enticing him to marriage, he could be lying. But when she sees him splurging money on useless expensive jewelry and sports cars, she can believe him.
The final theory of sexual signals was formulated by American zoologists Astrid Kodric-Brown and James Brown, and is termed “truth in advertising.” Like the previous two theories, the Browns emphasize that costly body structures must represent honest advertisements of quality, because an inferior animal could not afford the cost.
But unlike Zahavi, who saw the costly structures as a handicap to survival, the Browns saw them as either favoring survival or linked to traits that favored survival. The costly structure is a doubly honest advertisement. The big antlers of male deer require a big investment of calcium, phosphate, and calories, yet they are grown and discarded every year. This tells the female deer that he is healthy and parasite free. This is like the rich person with Porsche. But unlike the Porsche, the antlers help the deer get access to the best pastures by helping him defeat rival mates and predators.
Examples of Body Signals
Men’s body muscle tends to impress women as well as other men. While extreme muscle development is grotesque, most women find a well-proportioned muscular man more attractive than a scrawny man. Men also use muscle development of other men as a signal, a way of quickly assessing whether to get into a fight or retreat. And in traditional societies that were based on human muscle power rather than machine power, muscles are a truthful signal of male quality, just like deer’s antlers.
On one hand, muscles help men gather resources like food, build houses, and defeat rival men. On the other hand, men with other good qualities are more capable of acquiring all the protein necessary to grow and maintain big muscles.
You can fake your age by dyeing your hair, but you can’t fake big muscles. Naturally, muscles weren’t evolved merely to impress others, but were evolved to perform useful functions. So, both men and women have evolved to respond to muscles as a truthful signal.
A beautiful face is another truthful signal. The face is the most sensitive to age, disease, and injury. Particularly in traditional societies, individuals with scarred or badly shaped faces may be advertising their proneness to infections, or the inability to take care of themselves. A beautiful face was
a truthful signal of good health that could not be faked until twentieth-century plastic surgeons came around.
Female sexual ornamentation can also have sexual significance, because while it is possible for any female to attract a mate, it is difficult for a female to attract a high-quality mate. A male human signal that exemplifies the Fisher’s runaway selection model or Zahavi’s handicap principle may be the male penis. Some could argue that I serves a nonsignaling function, and is nothing more than a well designed reproductive machinery.
But that is not a serious objection to Diamond’s hypothesis. Women’s breasts are both signals and reproductive machinery, for example. In the same way, excess size can serve as a signal. Gorillas and orangutans have erect penises of 1.5 inches compared to 5 inches in humans, even though humans have much smaller bodies. Those extra inches may be a functionally unnecessary luxury. One counterinterpretation is that a large penis might be useful in a wider number of copulatory positions compared to other animals. But the 1.5 inch penis of the male orangutan allow it to perform multiple positions that rival humans, while hanging from a tree.
As for the possible utility of a large penis in sustaining longer intercourse, orangutans beat humans here too (mean duration 15 minutes vs 5 minutes for the average American man).