Characterization of Stone Tool Use in Wild Groups of Critically Endangered Yellow-Breasted Capuchin Monkeys (Sapajus...

A hallmark of hominin evolution is the advent and diversification of stone tool use, and biological anthropologists have a particular interest in characterizing tool use behaviors in different hominin species. Robust capuchins, the genus Sapajus, are an excellent convergent model system for understanding tool use evolution, principally in the context of foraging, in terms of how they use stones to process hard fruits. However, most published studies of capuchin tool use in the wild focus on the species Sapajus libidinosus. In order to strengthen comparative analyses, it is important to do research across robust capuchin species to understand tool use capacities and variation across the genus. The first indirect evidence that reported tool use in wild S. xanthosternos was in 2009, but since that time no additional publications have described this behavior for this species. In this study we provide the first complete characterization of tool use in wild S. xanthosternos for two areas within Montes Claros, Minas Gerais state, Brazil: Santa Rosa de Lima district, and Lapa Grande State Park. We compare our findings to the stone tool use characteristics reported in wild Sapajus libidinosus and other capuchin monkey species.

In monthly surveys from January to September 2018, we walked trails through the two study areas and categorized tool use site characteristics, including anvil area, height, and perimeter; hammer stone weight; and species of fruit utilized. Additionally, we utilized camera traps at high-use sites.

We describe the tools used by S. xanthosternos at 169 sites clustered in seven Nutcracking Areas, for processing three different species of encased fruit: Macaúba (Acrocomia aculeata), Cansaçao (Cnidoscolus pubescens), and Guariroba (Syagrus oleracea). With camera traps, we directly observed tool use behavior for the first time in this species.

Sapajus xanthosternos displayed tool use patterns very similar to those of S. libidinosus. The main difference was in regards to physical force; in our study, the mean weight of stones used by S. xanthosternos was higher than the mean reported for S. libidinosus at most field sites; however, there was no difference in hammer weight between S. libidinosus and S. xanthosternos when hammering Macaúba. We also registered the heaviest hammer stone for tool use (7.675 kg) known for the robust capuchin genus. It is clear from our study that tool use capacity and characteristics are conserved across the radiation of extant Sapajus, suggesting a possible stone tool use cognitive capacity spanning up to 3.3 million years of diversification within this genus. Unlike stone tool use occurring throughout the year for S. libidinosus, at our study sites S. xanthosternos used nutcracking tools only during the dry season.