Liam Drew gropes for a better understanding of the evolutionary reason for external testicles, unsatisfied by the widely-cited “cooling hypothesis”:
Heat disrupts sperm production so effectively that biology textbooks and medical tracts alike give cooling as the reason for the scrotum. The problem is many biologists who seriously think about animal evolution are unhappy with this. Opponents say that testicles function optimally at cooler temperatures because they evolved this trait after their exile. If mammals became warm-blooded 220 million or so years ago, it would mean mammals carried their gonads internally for more than 100 million years before the scrotum made its bow. The two events were hardly tightly coupled.
The hypothesis’ biggest problem, though, is all the sacless branches on the family tree. Regardless of their testicular arrangements, all mammals have elevated core temperatures. If numerous mammals lack a scrotum, there is nothing fundamentally incompatible with making sperm at high temperatures. Elephants have a higher core temperature than gorillas and most marsupials. And beyond mammals it gets worse: Birds, the only other warm-blooded animals, have internal testes despite having core temperatures that in some species run to 108 degrees.
(Photo: a scrotum in a warm state by Brallion, via Wiki)