Final answer:
The testicles are situated externally in mammals to maintain a temperature conducive for sperm production and motility, which is about 2 to 4°C below body temperature. The scrotum's ability to regulate temperature is crucial for sperm viability, and improper descent of testes during development can lead to infertility.
Step-by-step explanation:
The testicles are situated externally in most mammals primarily to facilitate temperature regulation for sperm production. Sperm production, which occurs in the seminiferous tubules within the testes, is highly sensitive to temperature. It proceeds most efficiently when the testes are kept at about 2 to 4°C below the core body temperature. Given that sperm are immobile at body temperature, the external location of the testes in the scrotum ensures a cooler environment that is conducive for motility and viability of sperm. The scrotum can regulate its internal temperature via muscle contractions, pulling the testes closer to the body for warmth or allowing them to hang further away to cool down. Infertility can result in land mammals when the testes do not descend through the abdominal cavity during fetal development because they are unable to maintain the necessary cooler temperature.