Sex is interesting. Why can women have multiple orgasms; why is their refactory period shorter? What are the functions of the female orgasm (“it feels nice” is a good one to encourage sex, but why did humans not evolve so that penetration would bring a woman to orgasm more easily)? Why can many (most?) women not orgasm without clitoral stimulation? Why is it a particularly pleasing compliment to a man if he can ejaculate more than once during penetretative sex, i.e. overcome the refractory period problem and ejaculate again – not just bring his partner to orgasm by oral stimulation or whatever.
My vague take on the thing is a gang-bang theory of sex. Basic idea: perhaps in times past it was common for multiple men to engage in penetrative sex with one woman within a short space of time. This kind of behaviour leads to sperm competition, which would have allowed natural selection to improve the reliability of fertilization. Multiple male on one female action happens in other animals (and still in humans, but I guess no longer for the purpose of fertilization!), and to stop it males of various species do things like impose copulation costs (for instance males of some species have spikey bits on their genitals (!) to make encounters with other males before fertilization has occured less likely), guard their partners, punish females for their promiscuity, and so on. Another method for doing this would be the volumous production of semen.
Found an interesting paper on the human penis as a semen displacement device (Gallup et al 2003) where they got a fake vagina (a sex toy for males) and lots of models of erect penises, then used them to see – in effect – how the different penises could act as a sort of semen plunger.
So natural selection occurs both at the level of female choice (e.g. discarding sperm, reducing offspring number, selecting sperm by selecting mates) and sperm competition (e.g. semen flushing, having crap sperm). (See Snook 2005.)
Anyway, does anyone have any ideas for further reading? I don’t know this literature, but it’s all fascinating stuff.
Gallup, G. G. Jr., Burch, R. L., Zappieri, M. L., Parvez, R., Stockwell, M., and Davis, J. A. (2003). The human penis as a semen displacement device. Evolution and Human Behavior, 24, 277-289.
Snook, Rhonda R (2005). Sperm in competition: not playing by the numbers, Trends in Ecology & Evolution, Volume 20, Issue 1, Pages 46-53.