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Why Do Most Mammals' Testes Hang Outside of Their Bodies?
The best answer to any question
There are many theories out there as to why sperm needs to be kept cooler and why the testes of most mammals hang outside their bodies, inside the scrotal sac. In my opinion, the least likely is the "handicap principle." The easiest way to illustrate this principle is the peacock's flashy feathers.
Caster Semenya: Tests Reveal Sprinter Has Both Male, Female Characteristics
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