The Notes on Why is Sex Fun

Notes of “Why is Sex fun?” by Jared Diamond:

Chapter 1: The animal with the weirdest sex life

1. The funny characteristics of human sexuality (Pg 8):  
1. Long-term sexual partnerships
2. Co-parenting
3. Proximity to the sexual partnerships of others
4. Private sex – not public like other animals
5. Concealed ovulation
6. Extended female receptivity
7. Sex for fun
8. Female menopause

2. (Pg 14) Natural selection is not the maximization of survival but the transmission of genes and survival is just one of the strategy that provides repeated opportunities to transmit genes.
3. (pg 14) Sexual strategies that have evolved actually depend on both the ecological parameters and the species’ biology:
1. for eg, sexual cannibalism in spiders is common because of low population densities and low encounter rates, and the female’s biological abilities to digest large meals and increase her egg output’s when well nourished.
2. For humans, men can to copulate many times in a lifetime and women can’t eat an entire man at one go, neither will it significantly help the nutritional needs of the pregnancy.
3. However, Sexual evolution is a result of long period of adjustment by the species and we cannot expect the species to change it characteristics when is moved to a new environment overnight as in the recent thousands of years whereby we started living in cities.

Chap 2: The Battle of the sexes

1. When there is fertilized egg copulation, many ‘choices’ occur:
1. Should there be co-parenting?
2. Should the young be left for themselves?
3. Should the male take care of the young?
4. Should the female take care of the young?

However, which ever party that chooses to take care of the young, it means

2. Factors that decides the ‘choice’ of parent to be caretaker:
1. Relative size of investment in the young – usually the egg is a bigger investment as compared to one sperm.   Usually bigger for the females, especially for internal fertilization.
2. Foregone opportunity - giving up resources and time that could have been used in reproducing again.  Often, when a female is pregnant, there is not genetic good to copulate again, not the case for males.
3. Confidence of parenthood – Will spend more time and effort if you are sure that the young is yours.  More apparent for internally fertilized

females but not so for the males then.

Therefore, men are more interested than women in sexual variety, including casual sex and brief relationships because it maximizes transmission of a man’s genes.  However, women participating in extramarital sex is more due to marital dissatisfaction and they tend to be looking for a new lasting relationship.

3. The 3 exceptions to general pattern of male post-copulatory desertion:
1. Eggs are fertilized externally hence the male are confident that the young are his, eg some fishes.  
2. Sex-role –reversal polyandry where big females fight over males and males do most of the parenting work.  Examples included shorebirds like the Wilson’s Phalaropes.   Favourable factors include:  Relatively independent young that only need one parent’s care to survive; clutches of few but large eggs such that if the females are free from parenting, they can fatten up again to reproduce and  severe losses of eggs due to predation hence it is good if the female can produce another clutch of egg soon.
3. Co-parenting is needed for the young to survive.

Chapter 3:  Why don’t men breastfeed their babies?

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

1. Trinity of bizarre human reproductive behaviours:
1. Concealed ovulation – why have sex when you don’t know whether it will result in fertilization or not?
2. Nearly constant female sexual receptivity – at the ‘wrong’ time of the cycle, during pregnancy and after menopause
3. Recreational sex – not efficient at all.

2. Hence we are not “efficient” when it comes to sex.  Why should all species try to be efficient in sex?
1. Sperm production is costly for males – worms with a mutation that reduces sperm production lives longer.
2. Sex occupies time that can be used to search for food.
3. Couples having sex risk being surprised by enemy or predator.
4. Fights between males for females lead to injury and death.
5. Extramarital sex is costly if caught for many animal species.

3. Two theories to explain the concealed ovulation in females:
1. “Daddy-at-home” – Man has no idea where females are fertile and they will stay at home and assiduously make love to her to fertilize her when she happens to be fertile.  It is also to guard her against other men since she may be fertile when he is away.  Basically, if the period which she is fertile is obvious, then he will only be around during that period to fertilze her.  
2. “Many-fathers” theory – concealed ovulation so that males cannot be sure if the child is his and will do his part to take care.   This is more relevant in traditional human societies where infanticide was common.  When a male wrestles over the control of the group and becomes the dominant male, he will kill the infants of the previous dominant male.  He knows that the infants do not have his genes and by so doing he terminates the mother’s lactation and allows her to be fertile again.  Females’ concealed ovulation makes the males unsure that the child is not his, and this ensures benevolent neutrality of almost all potentially murderous males in the immediate neighborhood.   

4. Hence “Daddy-at-home” views that woman’s conceal ovulations and constant receptivity evolved to promote monogamy, paternal care and fathers’ confidence in their paternity, while “Many-fathers” says conceal ovulation of females minimizes the threat of their offspring by adult males, confuses paternity and effectively undoes monogamy.

5.  Which is correct?  Also, why can’t females be aware of their ovulation period when actually only the males need to be unaware?   Could it be  because it is difficult for a female to fake interest in sex when she knows she is unfertile and turned off, hence it would be hard for her to deceive her husband unless she herself is deceived.

6.  From the following 3 studies among higher primates:

1. Signs of ovulation
2. Mating system
3. Any tendency for more or less conspicuous ovulations to be associated with a particular mating system?  
4. Study of family tree among primates using genes.

Conclusion – The evolution of concealed ovulation

Mating system	Harem	Harem	Monogamy
Ovulatory signals	Slight	Concealed	Concealed
Function of ovulatory signals, or lack of	Efficient sex	Confuse paternity, prevent infanticide	Keep daddy at home

Hence, concealed ovulation arose in a promiscuous or harem-holding species.  Then with concealed ovulation already present, the species switched to monogamy.

Chapter 5: What are men good for?

1. Anthropologies view that there is a job division among men and women, where men spend more time hunting big game while women spend more time gathering plant food and small animals and caring for children.  Hence, theoretically men should pursue the hunting strategy that yields the most meat and he should share it preferentially with his wife and kids then with non relatives.

2. However, it is found women through gathering collect more in terms of calories.  Though men may bring in a lot when they are successful in a big game, but the days that are unsuccessful greatly outnumber the days that they are.  Also, the men share most of their meat with non-relatives.  

3. Hence, it is suggested the reason for men’s interest in hunting is that it is serves him well in attracting more sex partners.  It is better to be a “show-off” who hunts and occasionally have lots of food to share with others but brings less food to his family than to be a “provider” who gathers/hunts food with a moderately high returns with high predictability.  In conclusion, men are not so noble in “bringing home the bacon”.

Chapter 6: Making more by making less

1. Women experience menopause which is a sharp decline in fertility around 40s, this is unique among animal world where many species either remain fertile till death or experience a gradual decline in fertility like men.  It appears to be self defeating cause if women are fertile till old age, they are then able to bear more children and hence propagate more of their genes.
2. Why do laboratory mice with abundant food and no risks, receiving better medical care than any wild turtle hardly live longer than 3 years, while wild turtles can live up to a century and their organs like eyes remain in very good condition till the end of their lives?
3. Answer:  Our genes face a trade-off between repairing the old body that contain the genes and making new containers (babies) for the genes.  Hence the evolutionary determinant of biological investment in repair (lifespan under the best possible conditions) is the risk of death from accidents and bad conditions.  Therefore, wild mice which are exposed to many predators to the wild are like less likely to live long as individual and rather invest resources in creating new bodies. Conversely, porcupines, protected by quills, age more slowly ( more resources into repair) than mammals of similar size.  
4. Therefore, we find that bodies of animals wears out at about the same time.  Wild animals which are subjected to predation, show few signs of age-related deterioration cause they die when the body is significantly impaired.  However, captive animals display gradual age-related deterioration like us humans, even for the male reproductive system.  The only glaring example is the female reproductive system which shuts down way before the rest of her organs shut down.
5. Thus, a woman’s apparently counterproductive evolutionary strategy of making fewer babies must actually result in her making more babies.  Reasons:
1. Human children have a long period of parental dependence.  The human child relies on the parents not only for food, but also not the parents to impart the use of tools which depends on language and can take up to a decade.  Tools are important cause humans acquire most of their food with tools (nets, spades) as well as to defend themselves (we lack sharp teeth).  Hence a mother must see her child goes into teenage years to be sure that her genes actually succeed.
2. Hunter-gather mothers suffers from high risk of child-birth deaths as well as delayed risk of death from exhaustion due to lactation, carrying a young child and working harder to feed more mouths.  Therefore the birth of a new child threatens the lives of her previous children whom are not independent yet.
3. Infants of older mothers suffer from more health defects.

6. In conclusion, the older a woman, she accumulates more children and have taken care of them longer.  With each successive pregnancy, she is putting a bigger investment at risk.  Also her risk of childbirth death and the risk of an unhealthy child increases with age.  In effect, the older mother is taking on more risk for less potential gains.
7. There is also another extended role of postmenopausal women. Without children to take care, therefore more hours at hand and with the experience, old women can gather an impressive amount of food that can go into feeding her children and grandchildren – all which bear her genes.