Why Do Males Have External Testicles?
The answer is the reason men aren’t supposed to put laptops on their laps, at least for too long: sperm die when they’re heated. Body temperature is too high for them to be viable; the testes are external so that heat will dissipate into their surroundings, keeping them within the temperature range that keeps sperm alive. Fertility trumps inconvenience in this instance.
But what about animals that don’t have external testicles? Reptiles, amphibians, and fish are cool enough for sperm to be viable, it’s only warm-blooded animals that have the problem. Birds have developed a chemical mechanism to keep sperm viable at higher temperatures, so they can have internal testicles. External testicles are a mammalian problem.
So what about aquatic mammals, like whales and dolphins, that need to be streamlined? External testes certainly create drag, something they can’t and don’t tolerate. So what’s their secret?
They’ve developed a special circulatory structure where blood cooled on the surface of their skin flows in veins around the arteries leading to the testes, thereby cooling the arterial blood and reducing their temperature enough to keep sperm viable. It’s quite the elegant solution to the problem.
So now you know why males have balls, at least non-aquatic mammals.