Species, subspecies, or color morphs? Reconsidering the taxonomy of Callicebus Thomas, 1903 in the Purus-Madeira interfluvium.

There have been recent disagreements as to how many taxa of titi monkeys, genus Callicebus, occur in the region between the Purus and Madeira rivers in western Brazilian Amazonia. Three parapatric taxa were proposed for the area: Callicebus caligatus, Callicebus stephennashi, and Callicebus dubius, but the latter has recently been considered a synonym of C. caligatus, even though both form monophyletic groups and are morphologically distinct. We analyzed the geographic variation in the pelage of Callicebus occurring between the Madeira and Purus rivers and concluded that the phenotypes attributed to C. caligatus and C. dubius are not individual morphs, but rather well-marked and geographically restricted varieties. For this reason, we classify Callicebus caligatus as a polytypic species with two subspecies: Callicebus caligatus caligatus and Callicebus caligatus dubius. This classification is corroborated by molecular evidence as well. The morphological and distributional data indicate that Callicebus stephennashi is a hybrid form of C. c. caligatus and C. c. dubius, due to the presence of intermediate characters. Therefore, until more precise locality records are provided and further evidence is presented, we consider Callicebus stephennashi to be a homonym of the two parental forms.