Studying animals can help in understanding things about people. In mammals , testicles can be very big or very small in relation to the size of the animal itself. The size has to do with the amount of sperm that the male animal needs to make. Some types of male animal generally only have one sexual partner at a time and are called monogamous . Some types of animals generally live in a herd or flock where one male will have many female sexual partners. These males are called polygamous . More sperm is needed by polygamous males than by monogamous males. Polygamous males generally have larger testicles than monogamous males. The testicles grow larger to make more sperm.
2) Irreversible adaptation to sperm competition . It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster ) for sperm competition and thus had to place its testes outside the body.  This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo , a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.