Original: For the human, a temperature should be one or two degrees below the body temperature - 36.8 degrees Celsius (98 degrees Fahrenheit); higher temperatures may be damaging to sperm count.
Simplified: Human testicles work best at 36.8 degrees Celsius (98 degrees Fahrenheit) when normal body temperature is 37 degrees Celsius (98.6 degrees Fahrenheit). Higher temperatures than 36.8 degrees will damage the sperm inside the testicles.