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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.
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. 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. Why?