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.
Each sperm produced by the testes takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat that keeps the testes and sperm at the correct temperature. It may be surprising that the testes should lie in such a vulnerable place outside the body, but it is too hot for them inside. Spermatogenesis requires a temperature that is three to five degrees Fahrenheit below body temperature. If it becomes too cool on the outside, the cremaster muscle will contract to bring the testes closer the body for warmth.