Larger Penis Size

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Human Uniqueness Compared to "Great Apes": 
No Difference
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Humans are often claimed to have the largest penises among primates. 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). Unlike humans, chimpanzee and bonobo penises are filiform (tapered into a narrow tip) with no visible glans penis. Bonobos also have two small lobes at the tip of their filiform penis. 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. Orangutans have smaller penises (erect length ~ 8.5 cm) and gorilla penises are even smaller (erect length ~ 6 cm). Neither orangutan nor gorilla exhibit a chimpanzee/bonobo-like filiform penis, and both, like humans also posses a glans penis. 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. chimpanzees), tend to have longer and more conspicuous penises. 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. It has been claimed that the pronounced glans of the human penis may have evolved in order to displace semen from other males.

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References: 

Dixson, A.F., Mundy, N.I. 1994. Sexual behavior, sexual swelling, and penile evolution in chimpanzees (Pan troglodytes). Arch Sex Behav. 23:267–280