“Oh man, what a dumb question. Everybody knows it is for fun.”

Hey, I hear your laughing. Cut it out! My topic is hard science, OK? If you are not comfortable with this topic or anti-evolution, please feel free to entertain yourself with something else in my blog such as diet and physical activity.

A cursory reflection on human sexuality tells us that our human beings live mostly in monogamy (even among most traditional tribes in Africa), and can enjoy sex at almost anytime anywhere (what a great pleasure). In addition, most women are willing to engage in sexual activity even during infertile periods, for example during pregnancy, lactation, or after menopause. The purpose of sex seems more likely for recreation rather than for reproduction.

It is instructive to compare human sexual behaviors with those of our closest relatives—gorillas and chimpanzees whose ancestors were the same as ours about 9 million years ago. In African jungles, gorillas live in small groups in which one male fertilizes several females (called harem), while chimpanzees live in big groups in which both male and female chimpanzees can freely copulate with the opposite sex (called promiscuity).Furthermore, both female gorillas and female chimpanzees accept sex only during ovulation periods. It seems that gorillas and chimpanzees copulate solely for reproduction. Additional analyses in other primates yield a more confusing picture: some live like gorillas while others live like chimpanzees.

There is another not so obvious phenomenon in primate sexuality. Without modern knowledge and techniques, human beings–both men and women– lack the instinct to detect the exact time of woman’s ovulation, while many female primates can show some visible signs to attract males during their ovulation periods, such as swollen or red derrieres.

Critical readers may wonder why I target woman’s ovulation and sexual receptiveness instead of something about man. Well, we must hold the assumption that men are “cheaper” in dispersing their genes at anytime. In fact, almost all male animals tend to philander with other females after discovering their successful fertilization. On the other hand, women invest much more than men do in this gene transmission business. Pregnancy, lactation, and daily caring for her kids consume too much time and energy in women.

Anyway, in human beings, the concealed ovulation and continuous sexual receptiveness in women, together with monogamy must have something to do with kids because it takes many years of growth for a kid to be independent. Mothers are unlikely able to raise children without some aids. Since men are such irresponsible creatures, it falls on women to figure out ways to retain their men. One naive theory postulates that if a woman can offer sex to his man whenever he wants, the man is likely to stay with her to protect kids. However, this sex-for-family theory is overtly superficial because primates like gibbons don’t need continuous sex to maintain monogamy, while others such as chimpanzees and bonobos have more sex than human beings do, but they live in promiscuity.

Furthermore, human species were evolving towards large groups in which all couples have to cooperate to survive. Adultery is always a challenge in large groups (even in modern society, 20% Americans acknowledged this type of sin). The concealed ovulation in women must have some merits in evolution.

Therefore, some theorists started with the concealed ovulation in women to examine human sexuality. One theory (proposed by Alexander and Noonan) suggests that without knowing a woman’s exact ovulating time, a man has to have sex continuously with her until she gets pregnant. He also has to stay home to guard his mate because any intrusion may yield fertilization. On the other hand, if he himself intrudes other couples, his effort is likely wasted because the chance that the other women ovulate at the time of copulation is low, while he left his mate unguarded. Therefore, the best strategy is to stay at home and focus on one woman.This facilitates the development of coparenting which greatly increases the survival of their offspring.

Keen researchers also notice that infanticide is common in animals. A non-biological father may abandon or kill his stepchildren because they don’t bear his genes. For example, imagining a human tribe in which infanticide is a social norm, if a man died of accidents, diseases, or fights, his children would be killed or abandoned by his successors. It is a big waste for both the tribe as a whole and the mother’s heavy investment on the kids. However, if every man may be the kid’s father, the kid is likely to be protected by the whole tribe. To accomplish this, a woman needs to have sex with as many men as possible before pregnancy. Therefore, by hiding her ovulatory signs, she can copulate with many men. Every copulation may lead to fertilization, and thus every involved man may think the kid has his genes. This theory (suggested by Sarah Hardy) explains the concealed ovulation and promiscuity very well. Unfortunately, our human beings live in monogamy.

Is it possible both theories have some elements of truth? We need data to test them.

Let’s take a look at our relatives again. Among 68 high primate species, ten of eleven monogamous species have concealed ovulation and the rest one only exhibits slightly visible ovulatory signs. Does concealed ovulation lead to monogamy? No. Out of thirty-two species with concealed ovulation, twenty-two species don’t live in monogamy but instead in promiscuity or harems. Further cross-sectional analyses suggest that ovulatory visibility may have nothing to do with mating system, and vice versa. Now we stuck.

A better analysis suggested by Sillen-Tullberg and Moller is to construct a family tree for these primates. It turns out that both mating system and ovulatory visibility switched back and forth several times over the evolution process in primates. By timing these switches, we can conclude that concealed ovulation evolved from promiscuity or harems, while monogamy never evolved in species with boldly visible ovulation. It is possible that the pathway to monogamy is from promiscuity/harems to concealed ovulation and then to monogamy.

Inferred from the family tree, the common ancestor of human beings, gorillas, and chimpanzees might have harem mating system and slightly visible ovulation. Because human species were evolving towards big society, women evolved concealed ovulation, thus permitting them to mate with many men to ensure the survival of their babies. This trait might lead to the ability of continuous sexual receptiveness in women. Armed with this weapon, women might then entice and lock strong men to protect their children. Continuously copulating with the strong men also assured them that they were endorsing their own genes. Families were then born. Together with fast intelligence development, monogamy was on the way.

On the other hand, chimpanzees might reinvent the promiscuity with visible ovulatory signs, while gorillas might keep the tradition of our ancestors. It serves them well.

Natural selection has no specific goal except for preserving genes. As the flip-flops of mating system and ovulatory visibility have demonstrated, a trait developed in one situation may be used for other purposes or may be dropped in other situations. Evolution is never a straight line.

Thank you for reading thus far, I hope you appreciate the origin of recreational sex. Needless to say, there is more to learn why sex is fun. Why don’t men nurture babies? Why do women live for a long time after menopause? I recommend the little book “why is sex fun? — the evolution of human sexuality” by Jared Diamond. For a general idea of genes and evolution, the book “the selfish gene” by Richard Dawkins may be of your interest.