Testicular torsion . "Torsion" means twisting -- and for a testicle, that's not a good thing. When testicular torsion occurs, the twisting kinks -- like a garden hose -- and blocks the blood vessels to one testicle. Certain men have a developmental problem that makes them susceptible to testicular torsion. Although testicular torsion is rare, it is an emergency. Sudden testicular pain demands an immediate trip to the emergency room. If treatment is delayed, the testicle can die. Torsion is most common during puberty - between ages 10 and 15 -- so it's important to let young teens know that any pain should be reported, even if they are embarrassed to say so.
Testicular pain is pain or discomfort that is felt in one or both testicles. The pain may originate from the testicle itself, or it may be the result of other conditions affecting the scrotum, groin or abdomen. Though there are numerous medical conditions that can cause testicular pain, it is important to understand that a few of them constitute medical emergencies that require immediate medical attention in order to prevent impairment or loss of testicular function. Testicular pain can be an acute (short-term) or chronic (long-term) condition. The testicular pain may be constant or intermittent.
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.