Moynihan (1976) mentioned an undescribed species of Callicebus between the ríos Caquetá and Orteguaza, Caquetá Department, Colombia. In August 2008, we confirmed the new species, which is phylogenetically related to C. ornatus and C. discolor to the north and south of the type locality respectively. We described this species as Callicebus caquetensis Defler et al., 2010 establishing its distribution through direct observations and information from local communities. Eighty-two animals were seen, including the holotype and paratype, which were collected. A review of historical archives of aerial photographs and satellite maps was carried out to assess the loss of the original vegetation of the area, and we found the habitat in an advanced state of fragmentation caused by extensive ranching and illegal crop cultivation. Existing coverage is now limited to small fragments of the original primary forest and secondary vegetation. We analyzed a Google Earth image of 75 km² of this primate's habitat and found only 32% of forest and secondary forest vegetation remaining in 2002, the year the image was captured. We propose this Colombian endemic species should be considered as Critically Endangered (CR), (based upon the criteria A3c,d,e, C2 of the IUCN). We suggest that the Colombian and Caquetá governments and private individuals give special attention to this endemic primate, most importantly in the creation of reserves and in environmental education initiatives. This is probably the most endangered of Colombia's primates.

A chromogenetic field analysis was performed with 25 of 29 of the known species of the genus Callicebus. Some species presented polymorphism, such as C. moloch, C. hoffmansii and C. cupreus. C. bernhardi presents the same distribution of color in chromogenetic fields as C. moloch, differing only in pigment amount, mainly in ventral surfaces, suggesting C. bernhardi is a junior synonym of C. moloch. C. hoffmansii presents two distinct phenotypes, but without a geographic barrier between them. Callicebus cupreus, C. dubius and C. caligatus are distinct species.

Two-phased seed dispersal by primates and dung beetles is crucial for tropical rainforest regeneration. Two species of howler monkey exist in the tropical rainforests of southern Mexico: the mantled howler monkey (Alouatta palliata), and the black howler monkey (A. pigra). Differences between these species in foraging and traveling behavior, as well as associated dung beetle assemblages, may influence seed dispersal patterns. In this paper we present the results of a preliminary four-month study comparing the above aspects between a group of A. palliata (N = 15) in Los Tuxtlas, Veracruz and a group of A. pigra (N = 7) in Palenque National Park, Chiapas, Mexico. We observed each group in alternating months using focal sampling, fecal examination, and trapping of dung beetles. Results showed that the A. palliata group consumed more mature fruit, but both groups dispersed similar numbers of seeds over the study period (ca. 13,000 seeds). The total number of seed species collected from the feces of the A. palliata group was lower than for the A. pigra group (13 vs. 31 species). The A. palliata group had a larger home range (33 vs. 6.25 ha) and average day range (202 vs. 126 m). More dung beetles from more species were associated with the A. palliata group (357 v. 99 beetles, 16 v. 8 species). The A. palliata group attracted more ball-rolling dung beetles (75.1% of total), while the A. pigra group attracted more burrowers (80.8% of total). Our results suggest important differences between black and mantled howler monkeys of southern Mexico as seed dispersers and highlight the need to consider foraging and ranging patterns, as well as associated secondary dispersers, when assessing seed dispersal by primates.