Stone Tool Use by Black-Horned Capuchin Monkeys (Sapajus nigritus cucullatus) in an Urban Park in Londrina, Brazil

Tool use to crack open palm nuts has been observed extensively in some capuchin monkey species. However, for southern black-horned capuchin monkeys (Sapajus nigritus cucullatus), there is only one published record of stone tool use from the 1990s, from an urban park in Londrina, Brazil. In the present study, we returned to this urban park to systematically investigate the hammer-and-anvil sites used to crack nuts by this capuchin monkey population. We analyzed the fruit and shell remnants resulting from hammering to test the following hypotheses: (1) hammers at tool-use sites are actively chosen for size and/or weight, (2) tool use sites that are closer to a tree of the processed plant species are more frequently used, and (3) there is no seasonal effect modulating hammer-and-anvil use for processing nuts. We located and identified 205 tool-use sites through active search within the park, and we measured and weighed the tools involved in hammering activity. We compared hammer weight and size to that of the comparable material available in the environment and found that hammer size and weight differed significantly from that of the raw materials found in the environment, suggesting active choice of the hammers. We identified three plant species for which nuts were processed through tool use: Syagrus romanzoffiana, Acrocomia aculeata and Terminalia catappa. Through measuring the distance between each tool-use site and the nearest tree of the plant species processed there, we found that closer proximity between the tool-use site and the nearest target tree (Syagrus and Acrocomia) increased cracking frequency. We returned to each site three times during the study year to determine if new tool use activity had occurred, and our data indicate habitual use of tools to crack open nuts throughout the year, with no large differences between seasons.