Mammals are supposed to have testes outside the abdomen because their fertility would be impaired if they were kept inside at body temperature. But birds have internal testes, despite having a higher body temperature. Why and how did birds evolve in this way when mammals did not? And why doesn’t this affect the birds’ fertility?
• In mammals, the testes need to be outside the body to keep their temperature at around 34 or 35 °C. At 37 °C, the core body temperature of most mammals, the production of sperm fails, and the sperm die. The lower temperature is optimum…
