Function
The function of the scrotum appears to be to keep the temperature of the testes slightly lower than that of the rest of the body. For human beings, the scrotum temperature should be about 35-36 degrees Celsius (95-96.8 degrees Fahrenheit), i.e. one to two degrees Celsius below the accepted normal body temperature of 37 degrees Celsius (98.6 degrees Fahrenheit). Higher temperatures may be damaging to sperm count. The temperature is controlled by the scrotum moving the testicles closer to the abdomen when the ambient temperature is cold, and farther away when it is hot. Moving the testes away from the abdomen and increasing the exposed surface area allow a faster dispersion of excess heat. This is done by means of contraction and relaxation of the cremaster muscle in the abdomen and the dartos fascia (muscular tissue under the skin) in the scrotum.
However, this may not be the main function of the scrotum. The volume of sperm produced by the testes is small (0.1-0.2 ml). It has been suggested that if testes were situated within the abdominal cavity that they would be subjected to the regular changes in abdominal pressure that are exerted by the abdominal muscles. This squeezing and relaxing would result in the more rapid emptying of the testes and epididymis of sperm before the spermatozoa were matured sufficiently for fertilization. Some mammals — elephants and marine mammals, for example — do keep their testes within the abdomen and there may be mechanisms to prevent this inadvertent emptying.
Contraction of the abdominal muscles, and changes in intra-abdominal pressure, can often lift and lower the testicles within the scrotum. Contraction of the muscle fibers of the dartos tunic (or fascia) is completely involuntary and results in the appearance of increased wrinkling and thickening of the scrotal skin. The testicles are not directly attached to the skin of the scrotum, so this dartos contraction results in their sliding toward the abdomen. They also, in some men, can be lifted the same way by tightening the anus and pelvic muscles, doing Kegel exercises.
Although the ideal temperature for sperm growth varies between species, it usually appears, in warm-blooded species, to be a bit cooler than internal body temperature, making the scrotum necessary. Since this leaves the testicles vulnerable in many species, there is some debate on the evolutionary advantage of such a system. One theory is that the impregnation of females who are ill is less likely when sperm is highly sensitive to elevated body temperatures. An alternative explanation is to protect the testes from jolts and compressions associated with an active lifestyle. Animals that have stately movements — such as elephants, whales, and marsupial moles — have internal testes and no scrotum.
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