While certain mammal lineages, such as deer, bovines and some primates, are noteworthy for their extreme sexual dimorphism, canids -- particularly those who are largely monogamous -- exhibit few obvious external differences. Accordingly, it is often difficult to determine the sex of wild foxes at a distance -- size, build and behavior are the only criteria helpful in such cases. To determine the sex of a pet fox, you can simply look at the animal's genitals.
Visual Genital Differences
While you should never touch or interact with a wild fox, you can determine the sex of pet foxes by gently flipping them on their back, and observing their genitals. 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.
Size and Build
Scientists have uncovered several subtle differences in size and build among various fox species. Male red (Vulpes vulpes) and gray foxes (Urocyon cinereoargenteus), as well as fennecs (Vulpes zerda), grow slightly larger and possess slightly larger skulls than females do. Still, these differences are quite subtle. For example, a 1995 study, published in the Journal of Mammalogy, found that Arctic fox males often weighed nearly 20 percent more than females, while their body lengths differed by less than 5 percent. In some species, for example red foxes, males have larger teeth than females do, but this is not a very helpful criteria for amateur pet keepers.
Males may exhibit several characteristic behaviors -- including heightened aggression, marking territory with urine and mounting other foxes. Neutered males may not exhibit these tendencies though, making behavior an unreliable indicator of sex. Because neutering or spaying foxes often provides health benefits, such as reduced rates of testicular cancer for males, and a decreased likelihood of developing breast cancer for females, owners often alter pet foxes.
If you are otherwise unable to distinguish the sex of your fox, consult your veterinarian. Your veterinarian can easily discern your pet's sex by examining his genitals. Your veterinarian may even be able to determine the sex of your fox without directly observing him: Researchers have developed a method for deciphering the sex of kit foxes by analyzing DNA collected from their droppings.