A taxonomic review of the titi monkeys, genus <i>Callicebus</i> Thomas, 1903, with the description of two new species, <i>Callicebus bernhardi</i> and <i>Callicebus stephennashi</i>, from Brazilian Amazonia
Authors: Van Roosmalen Marc G. M. , Van Roosmalen Tomas , Mittermeier Russell A. 
Published: December 2023

This paper provides a taxonomic review of the titi monkeys, genus Callicebus, and describes two new species from central Brazilian Amazonia, Callicebus bernhardi and Callicebus stephennashi. Previous revisions include Hershkovitz (1988, 1990), Kobayashi (1995), Kobayashi and Langguth (1999) and Groves (1993, 2001). Here we arrange the titi monkeys, genus Callicebus Thomas, 1903, into five Species Groups or clades, and a total of 28 species. The Callicebus donacophilus Group is represented by the following species: Callicebus modestus Lönnberg, 1939, Callicebus donacophilus (d’Orbigny, 1836), Callicebus pallescens Thomas, 1907 (treated by Hershkovitz as a subspecies of C. donacophilus), Callicebus olallae Lönnberg, 1939, and Callicebus oenanthe Thomas, 1924. The Callicebus moloch Group is represented by the following species: Callicebus cinerascens (Spix, 1823), Callicebus hoffmannsi Thomas, 1908, Callicebus baptista Lönnberg, 1939 (treated by Hershkovitz as a subspecies of C. hoffmannsi), Callicebus brunneus (Wagner, 1842), Callicebus moloch (Hoffmannsegg, 1807), and the new species here described as Callicebus bernhardi. The Callicebus cupreus Group is represented by the following species: Callicebus cupreus (Spix, 1823), Callicebus discolor (I. Geoffroy & Deville, 1848) (treated by Hershkovitz as a subspecies of C. cupreus), Callicebus ornatus (Gray, 1866) (treated by Hershkovitz as a subspecies of Callicebus cupreus), Callicebus caligatus (Wagner, 1842), Callicebus dubius Hershkovitz, 1988, and the new species here described as Callicebus stephennashi. The Callicebus torquatus Group is represented by the following species: Callicebus torquatus (Hoffmannsegg, 1807), Callicebus lugens (Humboldt, 1811) (treated by Hershkovitz as a subspecies of C. torquatus), Callicebus purinus Thomas, 1927 (treated by Hershkovitz as a subspecies of C. torquatus), Callicebus lucifer Thomas, 1914 (treated by Hershkovitz as a subspecies of C. torquatus), Callicebus regulus Thomas, 1927 (treated by Hershkovitz as a subspecies of C. torquatus), and Callicebus medemi Hershkovitz, 1963 (treated by Hershkovitz as a subspecies of C. torquatus). The Callicebus personatus Group is treated separately from the Callicebus moloch Group and is represented by the following species: Callicebus personatus (É. Geoffroy, 1812), Callicebus melanochir (Wied-Neuwied, 1820) (treated by Hershkovitz as a subspecies of C. personatus), Callicebus nigrifrons (Spix, 1823) (treated by Hershkovitz as a subspecies of C. personatus), Callicebus barbarabrownae Hershkovitz, 1990 (treated by Hershkovitz as a subspecies of C. personatus), and Callicebus coimbrai Kobayashi & Langguth, 1999. Callicebus bernhardi was discovered in the Rio Aripuanã basin in 1996 by M. G. M. van Roosmalen, who also discovered Callicebus stephennashi in 2001, while traveling on the Rio Purus. The geographic distributions of all hitherto recognized Callicebus species are updated, and the validity of the river barrier hypothesis for titis and other Amazonian primates is discussed, along with their conservation status.