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.