Answer
The scrotum is not found in all mammals, and it does not seem to be indispensable for the production of viable sperms. However, it seems that over thousands of years of evolution, human testes became adapted to producing viable sperms at below normal human body temperature of 37 deg C. Consequently, in the living (extant) human species, testes need a cooler environment (35 deg C) for optimal production of viable sperms.
Three mechanisms function to keep testes cool:
1. Cremaster muscle mechanism
These muscles (parts internal of the abdominal oblique) are attached to the spermatic cord. When the external environment gets too cold for the testes, the cremaster contracts and pulls the scrotum/testes closer to the body wall for warming. Conversely. if the body temperature makes the testicular environment too hot, the cremaster fibers relax and cause the scrotum to be lowered away from the warm body. This has the effect causing the testes to cool off.
Work Step by Step
2. The Dartos Muscle Mechanism;
The dartos is a layer of subcutaneous smooth muscle in the skin of the scrotum.. It contracts when the scrotum gets too cold. This causes the scrotal skin to wrinkle and promote warming of the testes in two ways:
a. Wrinkling reduces scrotal skin surface area- this, of courses, reduces heat loss
b. Wrinkling and tightening of scrotal skin pulls the scrotum/testes closer to the body, where their temperature is raised by the warmer body.
3, The Pampiniform Plexus Function
The pampiniform complexus is a network of veins that surrounds the testicular artery in the spermatic cord-- they join to form the testicular vein before entering the pelvic cavity. As a complex of veins around the entering testicular artery, the pampiniform plexus functions as a countercurrent mechanism to remove heat from the warm blood arterial blood. By removing heat from the blood of the testicular artery the pampinifom complex functions to prevent high temperature (Temp>35deg C) from inhibiting production of viable sperms by the testes.
