Researchers observe that these different strategies are revealed in the size of an animal's testes. Chimpanzees and gorillas provide a nice example. Chimps are promiscuous, and gorillas are polygynous. Male gorilla bodies are about four times bigger that male chimp bodies, but the chimps' testes are four times larger than gorillas'--sixteen times larger, proportionally.(4)
The chimps have larger testes because it is important for them to have sex with many females and spread their seed. They are not committed to any one female, and they have no assurance of the survival of their children because they are not obligated to care for any particular one. Gorillas, on the other hand, are committed to a group of females. They devote less energy to spreading their genes and more to protecting their territory and providing for their females and their children. Thus, they do not need larger testes (4).
These same ratios are noted in butterflies, frogs, and many others. Based upon this criterion of relative testes' size, humans fall into the category of monogamous/polygynous (4).