Here we present 192 new records of 22 species and subspecies of primates, collected in 56 different localities during 10 field expeditions across the arc of deforestation between 2015 and 2018. Based on such new records, we extend the ranges of Alouatta puruensis, Ateles chamek, and Saimiri collinsi; identify potential hybridization zones in Alouatta and Ateles species; redefine the range of Plecturocebus moloch; and clarify the ranges of P. baptista and P. hoffmannsi.

The Djaffa Mountains guereza (Colobus guereza gallarum) is an endemic Ethiopian taxon. Information about its distribution and population size is missing. We collected data on population density and size in four forests in the Ahmar Mountains in a line-transect survey and estimated 109.6 individuals per square kilometer within the study area. This comparatively high density is thus not the main conservation concern, but the fragmentation of the guereza habitat and population is.

From November 2011 through to November 2019, we carried out seasonal road transect surveys and estimated population densities of Manyara monkeys and vervet monkeys in Lake Manyara National Park. Both species occurred at relatively high densities, and their population trends remained fairly stable over the study period. Our research highlights the value of long-term monitoring and the importance of Lake Manyara National Park as a stronghold for the endemic and endangered Manyara monkey.