The scrotum is a mystery. Why do most male mammals have their reproductive glands so vulnerably located in a sack of skin and muscle outside the body? According to new research, the answer might be found in those unusual mammals that have testicles located inside the abdomen. These includes elephants, aardvarks, and others from a group that originated in Africa, known as the Afrotheria.
Testicles function best when slightly below body temperature. But we can’t say for sure this is why mammals evolved descended testicles, not least because males without scrota can still successfully reproduce. External testes may also be a way of showing off to potential mates, or to protect them from pressure inside the body created by movement.
The new study, published in PLOS Biology, examined 71 placental mammals for two key genes—RXFP2 and INSL3 —that are needed for the development of ligaments involved in testicular descent. They found that in many afrotherian mammals without external testes these genes had mutated to the point where they would no longer function.