Some interesting stuff about sex

During the Olympic ceremony (which was infinitely better with twitter), I saw PZ Myers retweet some comments that were so wrong , they caused me psychic pain, and led me to this article, which appeared the source of the confusion.

Here is the main body of the article, in all its wrongness:

Men sow their seed hither and yon; women guard their vaginas like Fort Knox.

But is this true?

Let’s think about it logically. With each act of sex a male ejaculates semen into the female. This is an investment of physical resources that takes time for the male to replenish. So male animals are limited in how often they can have reproductively viable sex. Female animals, on the other hand, are not. The sex act has zero cost to them.

Now, the reason this obvious truth is invisible to us is because we lump in the cost of carrying young with the cost of sex. Yes, carrying young is high female investment but the sex act itself is higher male investment.

For humans who are often trying to avoid the hassle of pregnancy, this makes some sense. For female animals, it makes no sense. The whole purpose for a female animal, in going into heat, is to get pregnant. Because that’s her instinctive imperative, the hassle of pregnancy is irrelevant to her cost of sex.

In fact, for female mammals, the real risk is investing resources in sub-par offspring. Because of this it’s in their best interest to have sex with as many males in as short a time as possible. Multiple male partners improve both her chance of conception and the quality of the conception via sperm competition. This is why the majority of female mammals are polyandrous—they mate with multiple males.

It is not, however, in the best interest of the male animal to mate with as many females as possible.

Why? Because, quite simply, his sperm is in limited supply. He wants to prioritize delivery of sperm to a.) high quality females and b.) less mated females. 

Our ‘common knowledge’ about so-called male promiscuity is better termed common stupidity.

Further, the relative costs of sex and the priorities of both males and females explain why a male mammal might evolve to pair bond.

A pair-bonding male has maximized his access to a less mated female. He does trade off in terms of access to a higher quality female than himself—higher quality in terms of status and fertility—but he is less likely to obtain this higher quality female and, overall, he’s better off as a pair bonder then a promiscuous breeder. 

It's got it all the wrong. But that's ok, because it's interesting stuff, and gives me an excuse to write about it.

Firstly, sperm is not really in a limited supply for a male. We do, after all, produce 1,500 sperm a second. In fact, we can joyfully masturbate and waste entire ejaculations just for fun and still not worry about our sperm count dropping. I couldn't (quickly) find decent figures on how often men can ejaculate a day, but it is certainly in double figures. Sperm is not a limited resource about which men need to worry. We make loads of the stuff (fertility problems not withstanding).

Secondly, it's seems daft to not count the cost of carrying young in with the cost of sex (not to mention months of lactation, which isn't easy). It's akin to saying that eating at Alain Ducasse au Plaza Athénée (where, if you order certain types of tea, they bring the plant to you, and cut off leaves with silver shears) is cheap, because when you're eating, no money is passing hands. Of course you should include the menstruation, pregnancy, lactation and child rearing as costs of sex! Even if you discount those, the costs of coitus itself are fairly similar between the sexes. Granted, men do have a cost in the sperm (though sperm is cheap), but vaginas need lubricating too - which is an expense for the woman.

Consequently, for men, it can be a good evolutionary strategy is to sleep around and pass on your genes, without worrying about looking after the child. Sure, this lack of paternal investment may not be good for the child, but ultimately, it's a better use for your sperm than masturbation, or not doing anything with it at all, as you do stand a chance of passing on your genes. Of course, it can also pay to stick with a partner though - human children take years before they are ready to face the world on their own, and paternal support can help them achieve adulthood and hopefully pass (some of) the father's genes on to their own children.

Women, of course, are more choosey - they want to make sure that they get a good set of genes for their child, as they don't get many chances to produce offspring. This doesn't exclude cuckoldry of course - they may well cheat on their partner to get the best of both worlds - a partner who will invest time in the child, whilst the sperm comes from a "better" mate. Though in the long run, of course, this doesn't help propagate genes for men who stick around.

Ultimately though, it is men who have to compete with each other to have sex with women. This is why men are more up for casual sex with strangers; why victims of rape are usually women at the hands of men; why the sex industry is generally females providing services for men; why women are punished more severely for cheating than men are; why we have the term "mistress" but no real equivalent for a man; and much more besides. In fact, marriage probably evolved as a way to ensure the fidelity of a female partner.

It's also why this sentence: "In fact, for female mammals, the real risk is investing resources in sub-par offspring. Because of this it’s in their best interest to have sex with as many males in as short a time as possible." makes no sense. It only takes one sperm to fertilize an egg, and whilst getting more sperm in your vagina might increase your chances of insemination, it implies that good father candidates are all over the place - indeed, the women who sleeps around in such a fashion is unlikely to have a father that sticks around, as their is no point in looking after a child that's unrelated to you (unless your a cassowary).

This of course doesn't give carte blanche for men or women to cheat on their partners. Whatever the reasons for infidelity are, evolutionary biology is not a good excuse.

Sexual behaviour and sperm production in other primates is also interesting. Gorillas have harems - one male has almost exclusive access to many females (who generally aren't promiscuous). Consequently, they have small testicles in relation to their body size. Chimpanzees on the other hand are far more promiscuous, they have massive testicles in relation to their body size as they need to make lots of sperm to increase their chances of passing on their genes.

On top of this, there is sexual dimorphism (one of the sexes is larger than the other) - where dimorphism is high, male–male competition is commonplace; conversely, where dimorphism is low, competition among males is less frequent. Gorillas and orangutans for example have high levels of sexual dimorphism. Chimpanzees are more moderate.

Humans fall almost in the middle of these range, both in testicle size to body weight; and sexual dimorphism - which is why there is probably a bit of variety with monogamy, infidelity and polygyny (and more rarely polyandry which is, interestingly, more commonly fraternal polyandry - brothers sharing a wife. This is a good strategy where resources are scarce - even if your genes aren't passed on, your brothers might be, which is the next best thing. Which makes sense, given the beginning of this blog post).

If you want to find out more about this How the Mind Works by Steven Pinker, and (if I recall correctly) the opening chapters of The Ancestor's Tale by Richard Dawkins, are good places to start.

Of course, why stick with humans? There are lots of interesting ways that animals have sex too, and you could do worse than checking out Green Porno.

As for the promotion of "pickup culture" in the essay, I'll leave it to XKCD: