Brief summary of the book “Why is sex fun? The evolution of human sexuality” (Jared Diamond)

Preface:

Give the main purpose of this book: try to analyze why human sexuality appears as it is now from a scientific aspect.

1: The animal with the weirdest sex life

In this chapter, the author turned the conception of the sex life of the normal people. He pointed out that we humans are the very bizarre species even among the mammals, not to mention the other animals.

A very important notion he gave is that the animal society system also evolves and adapts – however it seems not to evolve or adapt directly in my opinion.

Here are the unique points of human sexuality: (copied from the book)
a. Most men and women in most human societies end up in a long-term pair relationship (‘marriage’) that other members of the society recognize as a contract involving mutual obligations. The couple has sex repeatedly, and mainly or exclusively with each other.
b. In addition to being a sexual union, marriage is a partnership for joint rearing of the resulting babies. In particular, human males as well as females commonly provide parental care.
c. Despite forming a couple (or occasionally a harem), a husband and wife (or wives) do not live (like gibbons) as a solitary couple in an exclusive territory that they defend against other couples, but instead they live embedded in a society of other couples with whom they cooperate economically and share access to communal territory.
d. Marriage partners usually have sex in private, rather than being indifferent to the presence of other humans.
e. Human ovulation is concealed rather than advertised. That is, women’s brief period of fertility around the time of ovulation is difficult to detect for their potential sex partners as well as for most women themselves. A woman’s sexual receptivity extends beyond the time of fertility to encompass most or all of the menstrual cycle. Hence, most human copulations occur at a time unsuitable for conception. That is, human sex is mostly for fun, not for insemination.
f. All women who live past the age of forty or fifty undergo menopause, a complete shutdown of fertility. Men in general do not undergo menopause: while individual men many develop fertility problems at any age, there is no age-clumping of infertility or universal shutdown.

2: The battle of the sexes

Here the basis of the evolutionary opinions on the sex is elucidated. Examples covering from fishes to apes are given to prove the idea that all the sexual appearances of the animals are aiming at producing more offspring carrying the producer’s gene. Note that this “aim at” phrase here is passive – it is not originated from the animals themselves, but by the pressure of the nature.
The roles of the two sexes play in the continuance of their species are not “fair” in our view. The three factors given by the author: sex differences in obligate parental investment, alternative opportunities foreclosed by child care and confidence in parenthood lead to the different strategies adopted by the two sexes (which we can observe among the animals and ourselves). This is the meaning of the title “the battle of the sexes”.

3: Why don’t men breast-feed their babies? – The non-evolution of male lactation

This question seems strange, however the author explained that human in default are women, not men. If there is no additional direction by a series of chemicals (hormones, etc.), a fertilized egg will develop to female. And men have the potential to lactate – there are lots of examples.
The fact here is that in the majority group of mammals (90%) the male is not necessary for the care of babies, thus the lactation work is abandoned by the male since it costs lots of energy. The birds, as the only warm blooded animals excluding mammals, are the contradiction examples – most of them take care of the offspring together. The reason may lay here: the eggs of the birds are externally developed and only have a very short “pregnancy” time whilst the mammals have a long pregnancy time. In this way the investment on offspring of female mammals is much greater than the female birds, and thus caused the female mammal to invest more on the baby care (the initial investment on pregnancy makes it a loss too big for the death of the baby). And this feedback plus the low confidence on the paternity make the male mammals chose to take less baby care work whilst the birds tend to share the parental care (the less investment on egg/baby care by female birds makes it impossible for female alone to rise up the offspring).
Though human is in fact the kind that needs biparental care, human just merely inherited the basic structure from the mammal ancestors who do not do this. And the history of modern human is so short that it is far from enough for the structure, which is deeply committed by the human ancestors, to change.

4: Wrong time for love – the evolution of recreational sex

The concealed ovulation time, unceasing receptivity and recreational sex for human are big problems since they cause waste of time and energy, increase the death risk and do not increase the birth rate, thus seems oppose to the evolution – human have sex even after the female has lost its ability of fertility.
Lots of hypothesises had been risen for this problem, like the “daddy-at-home” and “many-fathers” theory (check on internet for the detail). However after the investigation, these two hypothesises which seems contrary are in fact all correct, because they should be applied on different evolution period of mammals. The “many-fathers” theory reduces the infanticide and starts the parental care from males, the mammal society turns from promiscuity to harem. Then the “daddy-at-home” theory continued the work to further strengthen harem and even induced monogamy (in human). The concealed ovulation is the reason for the first step, and is alternatively used in the second step. Moreover, it prevents the females from escaping from being pregnancy since to give birth is dangerous and painful for human (different to other mammals).

5: What are men good for? – the evolution of men’s roles

This chapter is not so inspiring… It just give the evidence of the remained tradition of our ancestor mammals which related to harem and careless of female by male.

6: Making more by making less – the evolution of female menopause

The reason for menopause which is rare and not inherited from the mammal ancestors is explained in this chapter. The whole explanation is based on the idea that natural selection adjusts the relative investment on repairing the bodies (which makes life longer) and on reproduction (which improves the birth rate) to maximize the transition of genes to offspring.
The higher death risk leads to the second choice, and every part in the body should meet its end at approximately the same time (if one part is repaired better and last longer than the other parts, it makes no sense). The menopause is the result of this choice, and it is more serious in human – the span available for fertility is much shorter than the life span even in ancient time. This is because that human child has a much longer time than the other species during which they are not independent, the new baby will take away the time spent on the elder ones, and along with the increase in age the risk of death on giving birth to new babies increases as well (this will leave the earlier babies homeless). Moreover, experiences from old people really help the society to survive.

7: Truth in advertising – the evolution of body signals
The content of this chapter is fairly common. Introduced reasons for our appearance related to sex which may be originated from the monogamy/harem and the lock of couples in society (big pennies, big breast, etc. in comparison to the other mammals) – the choice of mate becomes more important than ever before.