there is some confusion surrounding the number of valid subspecies, with some listing four, and others listing two; the western "erythrocebus patas patas" (common patas; with a black nose) and the eastern "erythrocebus patas pyrrhonotus" (nisnas; with a white nose). others, however, have suggested that at least some of the features used to separate these subspecies are merely variations in the female's facial pattern during pregnancy. on the other hand, the change in the nose during pregnancy occurs only in the west african populations.