Penis Evolution: Intersexual and Intrasexual Selection

Why Human Penises Are Different from Other Primate Species, and How They May Have Gotten that Way

One of the biggest questions in the scientific world is why the human penis is so different compared with its closest relatives, the great apes: chimpanzees, gorillas and orangutans. The human penis is unusual in many different respects. It is abnormally large, compared to other primate species. The average length of a human penis is five to seven inches(1). The closest genetic relatives to humans are chimpanzees. The average chimpanzee penis is about half of the size of human penises, or about three inches(1). The gorilla’s penis is the smallest, at only an inch long. Along with size, the human penis is also unique because of its structure. The human penis lacks an erectile bone, and has a distinctive coronal ridge. Also, the human penis is alone in having a foreskin. 

Penises evolved mainly as an intromission device. It has since been modified by many organisms using intrasexual selection. Some have hooks at the end to scoop out sperm while others have bottlebrush features to them. Although human penises have been modified, they still are not the oddest penises in the world. There have been suggestions that all unusual penises are due to female selection. However, it has been shown that there are other evolutional reasons for unusual penis shapes. There is some evolutionary evidence that the human penis evolved as a mechanism to displace sperm(1). By using the coronal ridge to create a suction mechanism, the penis is able to remove sperm already deposited in the female’s reproductive tract. This is significant because it would imply that females are not monogamous, otherwise sperm displacement would be useless, because only one male’s sperm would be present. Humans are culturally socialized to believe that women do not seek out sex. If the penis did evolve as an instrument to displace sperm, which there is much evidence to suggest, Western culture may have to reassess the way they categorize women sexually.