Restraint

Separating mice/rats by gender. Separating mice/rats by gender can be difficult, especially when they are young and males' testicles have not descended into the scrotum. Compare the relative distance between anus and urinary papilla. The distance between the anus and the urinary papilla is greater in the male. This photograph illustrates that the mouse on the right has the lesser (shorter) distance between the anus and the urinary papilla, so it is the female.