During the breeding season the testes are often swollen and those of some Reds are darkly stained, which can help sex determination at close range. When squirrels sit up to feed the genitals are often visible, allowing accurate sexing. Females have eight teats and these may be particularly prominent if she is suckling kittens. There is, however, no sexual dimorphism in either species so, without the genitals or mammae being visible, they cannot be reliably sexed by appearance alone.
If handling squirrels, the distance between the genital opening and the anus can be used to determine the sex: in males, the distance between the two apertures is about 1cm (0.4 in.), while the two apertures are very close together in females.