First record of tool use by wild populations of the yellow-breasted capuchin monkey (Cebus xanthosternos) and new records for the bearded capuchin (Cebus libidinosus).

Reports on use of stones as hammers and anvils to open hard nuts by wild capuchin monkeys are scarce and limited to Cebus libidinosus. Here, we report for the first time data on tool use-stones as hammer and anvils to open nuts-in wild C. xanthosternos and a description of new tool using sites for C. libidinosus. Our records were made by visiting anvil sites and by information obtained from local residents. We surveyed three different biomes: Caatinga (dry forest and thorn scrub), Cerrado (Brazilian bush savannah), and Atlantic forest (wet forest), all records of tool use were from Caatinga or transitional areas between habitats. The behavior is suggested to be routinely performed and widespread among several populations. The fruits of six plant species in different localities were opened with hammer stones by C. xanthosternos. Hammer stones were of similar weigh as those described in other studies of C. libidinosus. Conditions found in Caatinga, such as a more frequent use of the ground by the monkeys and/or food scarcity, may play an important role in the acquisition of nut-cracking behavior. The absence of more reports of nut cracking and other forms of tool use in other species of wild Cebus is likely to result from a lack of surveys in very dry and food-limited habitats or intrinsic characteristics of other Cebus species.