Male gorilla are also much larger than females, but they have a polygynous or harem-style mating system where many females live with a single male. With little or no competition actually inside the uterus, gorillas have had no need for a testicular arms race to facilitate the production of more and more sperm. Their testes, therefore, are relatively small. This is similar to modern humans, whose testes are also of very modest size and produce a relatively small amount of sperm. In fact human sperm count reduces by more than 80% if men ejaculate more than about two times a day.
The human penis is large when compared with those of our closest relatives: chimpanzees, gorillas, and orangutans. However, primatologist Alan Dixson in his wonderfully detailed book, Primate Sexuality, suggests that if we look at all primates, including monkeys, this is just wishful thinking.
Comparative measurements show the human penis is not exceptionally long. The Hamadryas baboon, for instance, a native of the Horn of Africa, has an erect penis that is five and half inches long – slightly shorter than an average human male, but they weigh only a third of our weight.
The human penis is in fact extremely dull – it does not have lumps, ridges, flanges, kinks or any other exciting feature that other primates have. In primates, this lack of penis complexity is usually found in monogamous species.
This observation clashes with the fact that men are significantly larger than women. This suggests our evolutionary background involved a significant degree of polygynous, rather than exclusively monogamous, mating. This is supported by anthropological data showing that most modern human populations engage in polygynous marriage. Anthropologists Clellan Ford and Frank Beach in their book Patterns of Sexual Behaviour suggested that 84% of the 185 human cultures they had data on engaged in polygyny.