The testes in male mammals descend into the scrotal sac because

Analyze the question: It asks why the testes move out of the abdomen into the scrotal sac in male mammals.

Explain the concept: Sperm production (spermatogenesis) in the seminiferous tubules works best at a temperature slightly lower than normal body temperature, about 2-3°C below it. The scrotum hangs outside the abdominal cavity, so it keeps the testes cooler. Two mechanisms help regulate this:

• The cremaster muscle contracts in cold conditions to pull the testes closer to the body for warmth, and relaxes in warm conditions to let them hang lower.
• The dartos muscle in the scrotal skin wrinkles or relaxes the skin for the same purpose.

Step-by-step solution:

• A. Congestion in the lower abdomen: There is no such physiological reason, so this is wrong.
• B. Risk of being destroyed: If anything, the scrotum exposes the testes to more risk of injury than the abdomen does, so this is wrong.
• C. Need special support: Support is not the purpose. The testes descend for temperature reasons, so this is wrong.
• D. Require a relatively low temperature: This is correct. Normal body temperature is too high for effective sperm production, and the scrotum provides the cooler environment.

Answer: The testes descend into the scrotal sac because they require a relatively low temperature, which is option D.

Exam tip: Remember "cool testes, healthy sperm." If a question asks about the scrotum's function, think temperature regulation. Undescended testes (cryptorchidism) can lead to sterility because the sperm-producing cells are damaged by the heat inside the body.