In some primate groups where sexual selection hinges on female choice, males tend to have larger penises. In primate species whose females do not have so much choice in mate selection, not so much. For example Silverback male gorillas [ . . .] monopolise a harem of females and are poorly endowed.
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The origins of the primate sex drive go back more than 60m years to the late Mesozoic era when the first primate evolved. A lot of sex has taken place since then, and a significant proportion has been motivated by female choice between rival males. Female primates can experience multiple orgasms, and it has been theorised that ancestral hominid females sought out males who would sexually satisfy them. Through the mechanism of sexual selection, this will have increased penis size and altered structure.
Today, the average erect gorilla penis is 3cm (1.25 inches) long, the average chimp or bonobo penis comes in at around 8cm and the average human penis stands at around 13cm. Most primates, including chimpanzees, have a penis bone and achieve erections through muscle contraction. The human penis has evolved the unusual system of vasocongestion to achieve erection, making the erect organ far more flexible than that of other primate species.
This unique adaptation is thought to have been selected through female mate choice, and by the time Homo erectus arrived on the scene, the hominid penis was significantly longer, fatter and more bendy than our ape cousins'. It has even been theorised that bipedalism evolved in humans to allow the fashionably new, larger, flexible penis to be displayed to discerning females.
Carole Jahme