• dietary overlap;
  • gracile capuchin monkey;
  • Sapajus;
  • conservation biology;
  • behavioral ecology

The Ka'apor capuchin, Cebus kaapori, is perhaps the most endangered primate of the Brazilian Amazon. Endemic to a region with extreme intensification of habitat-degrading activities, it survives in remnant populations in a completely fragmented landscape. Before now, the only data available were isolated observations of feeding, locality records, and information on population densities and group size obtained during census. Here we present the first data on the activity budget, diet, and daily path length of the species, and compare our preliminary results with those for other capuchin monkeys. A group of nine Ka'apor capuchins was monitored over a period of four months during the dry season in the Goianésia do Pará municipality, Pará, Brazil. We used instantaneous scan sampling (n = 4,647 scans) to construct an activity budget for the monkeys, and we identified the plants in their diet to species level (n = 41 plant taxa), allowing us to compare dietary overlap with other gracile capuchin species, as well as with the robust capuchin (Sapajus spp.), a potential competitor present throughout the range of the Ka'apor capuchin. Like other species of gracile capuchins, C. kaapori was highly frugivorous, with the vast majority of the feeding records of arils and fruit pulp (74%), supplemented by arthropods (13%) and seeds (10%), although diet composition was highly variable across months. The group used a total area of 62.4 ha during the study period, and average daily path length was 2,173 m (±400 m), with the entire home range utilized in every month of the study. We found significant overlap in the diet of the Ka'apor capuchin and Sapajus, highlighting the urgency to increase knowledge of the ecological needs of C. kaapori and understand synergistic effects of sympatry with competitive species, increasing forest fragmentation, and widespread human impact on C. kaapori sustainability. Am. J. Primatol. 76:919–931, 2014. © 2014 Wiley Periodicals, Inc.