Cetaceans (whales and dolphins) have internal testicles. It's easy to guess why: they would incur a significant decrease in hydrodynamic efficiency if they had

Cetaceans (whales and dolphins) have internal testicles. It's easy to guess why: they would incur a significant decrease in hydrodynamic efficiency if they had external testicles like other mammals. However, they risk not having viable sperm due to the warmer internal environment--even worse than in most land animals since cetaceans have a thick layer of insulating blubber. As a result, they have developed an alternative cooling system where blood that has been cooled in capillaries close to the skin converges on the testicles, effectively cooling them. All of this is to say that being in an easy-to-hit place apparently does not lead to as much evolutionary pressure as contributing to poor hydrodynamics.