While you should never touch or interact with a wild fox, you can determine the sex of pet foxes by gently lay them on their back, and observing their genitals. The Mammal Research Institute report that male foxes called dogs have a sheath of skin and fur that contains the penis, which may be visible through the opening in the anterior side of the sheath. Additionally, males have enlarged, easily observed testes during the winter breeding season, although they become much less conspicuous during the summer. Females, who are often called vixens, obviously have no penis sheath; their genitals are less conspicuous and often obscured by fur. Additionally, mature females often exhibit conspicuous nipples.