Population densities of black-and-gold howler monkeys (Alouatta caraya) in northeastern Argentina and the possible influence of a yellow fever outbreak.

Habitat disturbances, including stochastic events such as disease outbreaks, can significantly affect primate populations by reducing population density and demographic structure. The black-and-gold howler monkey (Alouatta caraya, IUCN: Near Threatened) is the southernmost species of the genus Alouatta and inhabits a variety of environments throughout its distributional range. In Argentina, A. caraya is classified as vulnerable, but following a yellow fever outbreak (YFO) between 2007 and 2009, the subpopulations in the province of Misiones were reclassified as Endangered. This study provides, for the first time, density estimates for two sites in southern Misiones where monkey mortality was reported during the 2007-2009 YFO, as well as for four sites in northern Corrientes, where no YFO-related mortality has been documented. All sites are located within the Southern Cone Mesopotamian Savanna (SCMS) ecoregion. We identified 34 groups of A. caraya-17 in Misiones and 17 in Corrientes. In Misiones, we recorded one of the lowest A. caraya densities reported in Argentina, with only 0.33 individuals per hectare (mean = 0.31, SD ± 0.12). In contrast, densities in Corrientes reached up to 1.34 individuals per hectare (mean = 1.11, SD ± 0.18), consistent with previously reported values for the species. These preliminary results suggest that density variation within the same ecoregion may be linked to population declines in areas affected by yellow fever (YF). Nonetheless, the presence of three groups with only three individuals each in Misiones may indicate early signs of recolonization or population expansion in the aftermath of the outbreak.