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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). 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 using a scrotum, which holds the sperm-producing testes away from the heat of the body. However, 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 tissues that can generate heat. How then do marine mammals keep their sperm alive? It’s a mystery that Rommel and his colleagues solve in the course of their anatomical and physiological explorations of dolphins and seals.

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