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Why Is Sex Fun?

The Evolution of Human Sexuality

Jared Diamond

Sexual cannibalism seems weird - male spider eaten by the female - but male spiders rarely meet a female ready to mate, so makes sense to maximise sole reproductive chance by a)providing extra protein b) distract her long enough to transfer more sperm and fertilise more eggs.

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The 'battle of the sexes' is actually 'behaviour that is in the male's genetic interest may not coincide with the female's best interest. If the baby will not survive without care, it becomes a game of chicken to see who can get away with abandoning it. Makes sense to leave if a) you can rely on yr partner raising the child b) you can find another receptive partner

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A man ejaculates up to 200 million sperm. If he did that every day for a month, he produces enough sperm to fertilise every one of the approximately 2 billion reproductively mature women on the planet (if only he cd arrange for each of them to receive one of his sperm). Whereas the man who devotes himself to child care forecloses almost all of those alternative reproductive possibilities

Reverse polygamy - some birds big females acquire a harem of up to 10 males, each of whom looks after a nest and who does most of the work of incubating and protecting the eggs. All traces back to a gene controlling egg size. Spotted Sandpiper, for example, produces really big eggs, 20% of mother's weight. But chicks are born able to run and to find food. Just need to be protected, not fed, and one mate can do that. Need to be like that because nest on exposed and vulnerable beaches. So makes sense for male to do all work while the female recovers, so that if first clutch gets wiped out, can lay a replacement.

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Then there's the Pied Flycatcher. Male finds a good nest hole, woos female and stays with her until she starts laying eggs. Then finds another hole (not next door) and sets up house with mistress. But the primary mate gets twice as much food as secondary, and raises more, bigger chicks with better chance of survival. But that's not enough: flycatchers also enjoy one-night-stands: if a resident male is away from the nest for more than 35 minutes, another male will have copulated with his wife. And of course the guys with mistresses were ones most vulnerable, since they had to spend lot of time providing for both families. Testing showed that 24% births were from intruders.

Some human societies have multiple husbands - eg Tre-ba of Tibet where brothers marry the same woman to avoid subdividing a small landholding.

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Humans just about the only species that enjoys sex even when female not capable of being fertilised. (Bonobos being the most spectacular exception). Pied Flycatcher female loses interest in sex as soon as eggs laid. But if female 'widowed' by mate disappearing, females will immediately start to solicit mating and copulate with new males, who will then start bringing food to the hatchlings, apparently believing they are the fathers.

If your wife worked like that, you would stay home on the days she was receptive and assidiously fertilise her. Then for the rest of the month you could leave her alone, safe in the knowledge that she wasn't interested in sex, and go chasing other females who were receptive. But this would leave women and children unprovided for, and they wd die off. So today's women are descendants of women who didn't telegraph their fertility, and so kept the uncertain males close by and attentive all the time.

Hunter-gatherer societies the women on average supply more calories with the boring gathering and pounding grains etc. But the hunters produce more conspicuous extra food, and good hunters get higher status and so more sex. But essentially it's showing off more than actually producing - the village wd be better off if they helped with the gathering.

Sex is a negative for survival:
1. sperm production is costly (worms with a mutation which reduces sperm count live longer
2. sex eats into yr food gathering time
3. shagging couple are vulnerable
4. older individuals vulnerable (Nelson Rockefeller had a fatal stroke shagging the upstairs maid)
5. fights over females often injure either or both
6. being caught at extra-marital sex can be hazardous (especially for humans)

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Why do women have menopause? Male reproductive capacity continues right up to death. Elephants and whales keep reproducing until their 60's, and turtles much longer than that, so it can't be that eggs are decaying. Author suggests it helps women get old - if they were continually having babies the risks of dying in childbirth wd eventually catch up with them, and effect of weakening with childbirth also eventual hazard. And societies where the wisdom of old people is available can get thru danger times easier.

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