External Testes Evolution Mystery

Scientists still do not have one definitive answer for why many male mammals carry their testes outside the body in a scrotum. The old cooling explanation helps in some cases, but it does not cleanly explain why animals like elephants and birds keep their testes inside or why several different evolutionary patterns exist across mammals. Researchers have proposed competing ideas involving temperature, sperm competition, mutation rates, display, and locomotion, but modern reviews still describe the full evolutionary reason as unresolved.

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