Many human males who have been hit "below the belt" would like to know if there is a mammalian alternative to testicular descent. As males of many mammals mature, their testicles descend in front of the pelvic girdle, right through the abdominal wall, and hang dangerously, like pendulums, outside the body in the scrotum.

Why is testicular descent a characteristic of most mammalian species? How can this process impart any fitness advantage to those species that employ it? Matt Young and Paul K. Strode in their recent book Why evolution works and creationism fails (Rutgers University Press, New Brunswick, 2009) discuss this matter and reason the evolution. The leading hypothesis for external gonads, they say, has been that moving the testicles to the outside of the body was an advantageous structural modifi- cation as the ancestors of mammals evolved endothermy which eventually created an environment that was hostile to proper sperm development. While sperm development was only an annual process to our fish ancestors, related to an annual increase in water temperature, the body temperature of mamma- lian ancestors, however, soon began to exceed the optimum temperature range for efficient and safe sperm development. In humans, this optimum is 2 degC below the normal human body temperature of 37^oC. At a mere 38^oC, mutation rates in sperm DNA reach lethal levels - and something had to be done, by the evolution, that is.

So goes the leading hypothesis; there are others. The display hypothesis: the testicles move into the scrotum outside the body cavity because the presence of a brightly colored scrotum of many species indicates a male’s readiness to mate. The training hypothesis: the testicles move because the scrotum presents the sperm with a hostile environment that, if survived, will result in only the highest quality sperm being available for fertilization.