I learned in histology that the outer testicular capsule has more pain receptors than any other mammalian tissue. Which would make evolutionary sense in terms of protecting the “family inheritance.” What is less obvious is, if they are so important, why do we have to carry them around outside the body? The textbook answer is to maintain them at the optimal temperature for spermatogenesis. However, this isn’t entirely satisfactory. For one thing, some mammals, e.g., elephants and whales, and some southern hemisphere species, don’t have scrotal testes. And birds have internal testes despite having a higher body temperature than we do. Furthermore, there are other stem cell populations, e.g. in bone marrow and stomach lining, that function perfectly well inside the body. So if sperm cells do best at slightly below body temperature it is probably because that is the environment they evolved in. So why? A clue comes from the fact that those few species that don’t have scrota have very little sexual dimorphism. Forget whales; they’re an exception to every rule, but male and female elephants have almost the same average size. In addition, scrotal males tend to do lots of fighting. So perhaps extracorporal testes evolved as a way for females to ward off unwanted advances. Also, if the male is expected to play a providential role and protect the brood, it would be ideal for the female if he were bigger and stronger than she. However, extracorporal (and pain-sensitive) testicles give her a means of bringing him to heel should she turn on him, or of sending a strong signal should the need arise.