I read an interesting theory yesterday regarding penis size amonst primates. As it seems, humans are unusual in the primate world in that we have a penis size out of proportion to our body size. There seems to be a close relationship in terms of promiscuity and penis size. The female gorilla and orangutan tend to be faithful and mate with only one male while the chimpanzee and human females then to mate with multiple males. Interestingly, the mighty 200kg gorilla and orangutan only have a 2 inch penis while that of their smaller, but more promiscuious cousin, chimps is twice that size; the human penis is larger than that of any other primate, both in proportion to body size and in absolute terms. Amongst the more monogamous members of the primate world the smaller penis seems sufficient to assure fertility because competition with other male's sperm is rare. Among more promiscuious primates, humans included, sperm competition from multiple males greatly diminishes the success of passing on one's genes; so any evolution that would favor delivering sperm closer to the uterus or in greater volume would be advantageous.