Hammer-stones to open macaúba nuts and unintentionally flake production in wild bearded capuchin monkeys (Sapajus libidinosus) at Ubajara National Park (Brazil): An archeological approach

Robust capuchin monkeys are among the few animals to use stone tools to process encased food items in the wild. So far, hammer-stones used by capuchins have only been described in detail, with archeological approaches, in the long-term study site of Serra da Capivara, where capuchins use lithic tools to crack open low-resistance food items, dig the soil to access embedded resources, or pound on conglomerate cliffs to pulverize them (stone-on-stone). Our work provides the first technological and techno-morpho-functional, use-wear and residue analysis of a sample of lithic materials collected at six nut-cracking sites used by bearded capuchin monkeys (Sapajus libidinosus) living in the Ubajara National Park (Ceará, Brazil), a population not habituated to the presence of researchers at the time. Shell remains at the sites were dominated by macaúba (Acrocomia aculeata) nuts. Technological and techno-morpho-functional analysis identified six lithic hammer-stones, four tool fragments and fifteen flakes on the bases of their morphology, their technological traits (flakes), their potential percussion marks of use observed at the naked eye, and their potential function. Use-wear and residue (e.g., starch grains) analyses unambiguously linking lithic tools to the processing of food items have been found on two hammer-stones, one hammer-stone fragment, two flakes and two micro-flakes. Our study adds one more geographical site where an archeological approach has been taken to describe tools used by capuchins. We report that cracking of hard-shelled nuts by wild robust capuchins may unintentionally produce flakes like those produced by stone-on-stone behavior observed in the same species, by long-tailed macaques cracking Elaeis guineensis nuts, by western chimpanzees cracking Panda oleosa nuts and by Pliocene/Pleistocene hominins. The detailed analysis of lithic tools used by capuchin monkeys to process hard-shelled nuts, therefore, represents a significant improvement towards the construction of a representative reference collection of tools for this important model taxon for stone tool use in non-human primates