![]() As a proportion of overall body size, chimp and bonobo testes are twice as large as human testes, whereas gorilla testes are half the size. Interestingly, while the human penis is the biggest of all the ape species in length and girth, both in absolute terms and as a proportion of body size, the human testes are not. ![]() It has even been theorised that bipedalism evolved in humans to allow the fashionably new, larger, flexible penis to be displayed to discerning females. 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'. 2 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. Most primates, including chimpanzees, have a penis bone and achieve erections through muscle contraction. 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. ![]() Through the mechanism of sexual selection, this will have increased penis size and altered structure. 1 Female primates can experience multiple orgasms, and it has been theorised that ancestral hominid females sought out males who would sexually satisfy them. A lot of sex has taken place since then, and a significant proportion has been motivated by female choice between rival males. Carole replies: The origins of the primate sex drive go back more than 60m years to the late Mesozoic era when the first primate evolved.
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