Many would think that the testes of a 450-pound gorilla would dwarf the size of our own, but in actuality, this is completely untrue. Out of all the great apes (chimpanzees, gorillas, and orangutans), gorillas and orangutans have the smallest testes. Don't let your ego get too big yet! The 100-pound chimpanzee makes our 1.5 ounce testes look like marbles compared to their own which are approximately 4 ounces. How can this be so?

Modern physical anthropologists have studied the surprising differences among the genitalia of apes and men to a great extent. These scientists have identified two explantions for this mystery: species that copulate routinely need bigger testes; and promiscuous species in which several males mate frequently with one female need especially large testes (because the male that implants the most semen has the best chance of fertilizing the egg). This is called the Theory of Testis Size.

A female gorilla is only receptive for a couple of days a month, and after she becomes pregnant and gives birth, she will not resume sexual activity for another three to four years. Because of this, a male gorilla experiences sex very rarely, and his tiny testes are more than enough to meet those demands. The sex life of a male orangutan is only slightly more demanding. Consequently, the orangutans testes are slightly larger than those of the gorilla. Now on the other hand, chimpanzees are a very promiscuous species. The male chimpanzee has a chance to copulate almost daily, and usually, several male chimpanzees will copulate with the same female. It is necessary for the male chimpanzee to outdo other males in semen output in order to fertilize the promiscuous females. This explains their need for gigantic testes. Humans copulate more often than gorillas and orangutans but less often than chimpanzees, and typically, men are not in sperm competition to fertilize a female. This explains our medium-sized testes.

Each species has big enough testes to do their job. Anything larger would divert energy from other tissues, and add more costs than benefits.
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Source:
The Third Chimpanzee by Jared Diamond.

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