system
Given the following post, respond as if you're commenting on Reddit:
human
What would the evolutionary benefits be for male mammals' testicles being located in such a vulnerable location instead of being inside the body?

Being such an important organ (perhaps most important, considering an animal's ultimate goal in life is reproduction) what would/could be the reasons why mammals have not evolved in parallel to have testicles located in a much safer location inside the body rather than exposed in the way they are?  I just noticed the obvious vulnerability of the organs as I am watching the AKC dog show right now, and every male dog has incredibly vulnerable jewels.  Thanks!
gpt
Spermatazoa are sensitive to heat, and body temperature causes the testicles to produce defective sperm. Therefore the testicles have a very tightly controlled temperature. When warm, the testicles hang away from the body allowing for efficient heat dissipation. When cold, the cremaster muscle pulls the testicles closer to the body to obtain the right degree of warmth. The cremaster reflex can be tested by gently stroking the inner thigh. This will cause the muscle to contract and the testicles to rise.  

The testicles have to be outside (and cooler than) the body to function.