The absense of a scrotum is certainly an advantage in streamlining the body. But the position inside the body may also be connected to the fact that average body temperature is fairly low. It averages abour 35.5 degrees Celcius. This is similar to the body temperature of other testicond mammals, where the body temperature varies between 33.0 and 35.0C. (The elephant has the highest at 36.5C). The average body temperature of non-testicond mammals is 38.0C. The critical temperature for producing sperm is estimated at about 38.0C, so there is obviously no danger with the internal position when the body temperature is kept fairly low.
In the largest cetacean, the blue whale, the testes may be 45cm long and weigh 45kg each.
There appears to be an annual cycle of sperm production. This is thought to possibly be triggered by an increase in day length. Many species show a definite season during which the male is producing sperm. The average sperm length of the humpback whale is 52 microns, the sperm whale 40 microns, and those of the common porpoise average 73 microns, against 55 microns for human sperm.
The vasa deferentia (sperm tubes, so to speak) go back from the testes and combine separatly with the ureter. It has been noted that these ducts are highly convoluted, ranging from spiral valves in the blue whale to simple folds in the pygmy sperm whale. With one exception, all terrestrial testicond mammals have highly convoluted ducts. It has been suggested that the shortness of the tubes (due to the internal position of the testes) may be made up for by the convolutions.
The only accessory genital gland is the prostata, which surronds the urogenital canal. This gland is surronded by a thick layer of muscle tissue, called the compressor prostatae. This gland secretes the additional fluids which mix with the sperm.
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