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The Amazon marmosets are poorly known and severely threatened – endemic to the arc of deforestation in southern Amazonia. Based on data from field expeditions across this global deforestation frontier, we provide new records and extend the range of Mico chrysoleucos, provide new records and refine the range of M. acariensis, report the first records of a species of Mico on permanently flooded forests, and hypothesize the existence of a hybrid zone of these two species and also M. melanurus.

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.

To our knowledge, this report represents the first description of thyroid gland tumors in an African Green Monkey (Chlorocebus aethiops). Two cystadenomas as well as a solid follicular adenoma are described in a 27-year-old female. No indications of excessive hormone production due to the tumors were detected.