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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鈥檛 this affect the birds鈥� fertility?

鈥� In mammals, the testes need to be outside the body to keep their temperature at around 34 or 35 掳C. At 37 掳C, the core body temperature of most mammals, the production of sperm fails, and the sperm die. The lower temperature is optimum for their survival.

鈥淚n mammals, testes need to be outside the body to lower their temperature for sperm production鈥�

Some mammals, for example elephants and rhinos, do have internal testes because they would be too large to hang from the body. Whales have them internally for streamlining. These mammals have evolved other mechanisms to cool the testes to the optimum temperature for sperm production.

Birds maintain a similar body temperature to mammals, and have internal testes for aerodynamics. But there is no evidence to suggest that they have any mechanisms to cool the testes. In fact, research suggests that sperm production in birds can occur efficiently at core body temperature.

Peter Scott, Hove, East Sussex, UK