To determine the sex of mice and rats, examine the distance between the anus and the urinary/genital opening. This distance is longer in males than in females. In addition, females have nipples that are usually noticeable by 10 days of age. Males do not have nipples. In older females, the nipples are covered with fur, so a careful examination must be made, or a female could erroneously be called a male. Finally, in adult males, the testicles can be felt at the base of the tail.