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How do male cetaceans and seals keep their testes cool without a scrotum? It turns out to be the same mechanism that keeps the fetus cool in a pregnant female
Many cells and tissues of the body have an ideal temperature at which they perform best. Muscles are most efficient a few degrees above their resting temperature, hence the need for a warm-up before a rigorous workout. Neural tissue, on the other hand, is very sensitive to rising temperatures-an increase of only 4-6 degrees Celsius can disturb brain function, leading to convulsions and even death. To avoid such catastrophes mammals have evolved elaborate physiological mechanisms to regulate their core body temperatures (about 37 degrees in human beings). Unfortunately this core temperature is about three degrees above the ideal temperature for the production and storage of viable sperm cells. Many terrestrial mammals have solved this problem by evolving a scrotum, which holds the sperm-bearing testes away from the heat of the body.
Two groups of marine mammals, cetaceans and seals, do not have a scrotum; they maintain their testes inside the body where they are surrounded by blubber and muscle. The body temperatures of resting seals and dolphins are commonly between 36-38 degrees-high enough to inhibit the production of sperm in most other mammals. The core temperature of an active animal may be higher still. Active locomotory muscles can generate heat 10 to 100 times faster than resting muscles. Much of this heat flows into the abdominal cavity rather than away from it. Moreover, when these marine mammals dive, the flow of blood is redistributed away from the muscles and visceral organs, thus decreasing the amount of heat that can be transferred away from these tissues. Such conditions raise the question of how seals and cetaceans keep their reproductive tissues cool.
It's a question that applies equally well to female marine mammals. All mammalian females need to regulate the temperatures of their reproductive organs during pregnancy to produce viable offspring. Pregnancy may increase the female's metabolic rate as much as 40 percent, and the mass-specific metabolic rate of the fetus may be twice that of the mother's. The excess heat associated with the growth of the fetus must be transferred to the mother and out...