![]() ![]() Female chimps or bonobos in oestrus often mate with several different individuals, so males must reproductively compete in this way and larger testes will therefore confer greater reproductive fitness.īy contrast, female gorillas live in harems and don't often get a chance to exercise a choice between mates, though occasionally a female and a male from outside the group may risk it. The more sperm a male produces and ejaculates inside a female the greater the probability that one of his will fertilise the ovum. 4 In multi-male/multi-female groups, males must compete to reproduce and frequently the competition takes place inside the female reproductive tract. Why?Ĭorrelations can be found between primate mating systems and male genital anatomy. 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. ![]() It has been claimed that the pronounced glans of the human penis may have evolved in order to displace semen from other males.The origins of the primate sex drive go back more than 60m years to the late Mesozoic era when the first primate evolved. Chimpanzees and bonobos also use their penises to display to females, which might also affect penis length and color, as a larger penis would provide a better display. chimpanzees), tend to have longer and more conspicuous penises. Penis size may also be related to group composition and sexual competition in that primate males living in groups with many females and many males (e.g. Neither orangutan nor gorilla exhibit a chimpanzee/bonobo-like filiform penis, and both, like humans also posses a glans penis. Orangutans have smaller penises (erect length ~ 8.5 cm) and gorilla penises are even smaller (erect length ~ 6 cm). It has been argued that the long and narrow penises of chimpanzees and bonobos have been shaped by coevolution with the large sexual swelling in the females of these species, which greatly increase the distance between vaginal opening and cervix. Bonobos also have two small lobes at the tip of their filiform penis. Unlike humans, chimpanzee and bonobo penises are filiform (tapered into a narrow tip) with no visible glans penis. This is true for penile girth, but not for length given that chimpanzee and bonobo erect penis length is comparable to that of humans (erect length 14 - 17 cm). Humans are often claimed to have the largest penises among primates. ![]()
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