Characteristics of hammer stones and anvils used by wild bearded capuchin monkeys (Cebus libidinosus) to crack open palm nuts

Abstract

Capuchins living in Boa Vista (Piauì, Brazil) crack open hard palm nuts on hard, level surfaces (anvils) using stones (hammers) as percussive tools. This activity leaves diagnostic physical remains: distinctive shallow depressions (pits) on the surface of the anvil, cracked shells, and stone hammers on the anvil. To initiate comparison of percussive stone tool use and interpretation of the artifacts it produces across capuchins, chimpanzees, and hominins, we describe a sample of the anvils and hammer stones used by capuchin monkeys at our site. Anvils (boulders and logs) were located predominantly in the transition zone between the flat open woodland and ridges, in locations that offered some overhead coverage, and with a tree nearby, but not necessarily near palm trees. Anvils contained shallow, hemispherical pits with smooth interiors. Hammers represent a diverse assemblage of ancient rocks that are much harder than the prevailing sedimentary rock out of which they eroded. Hard stones large enough to serve as hammers were more abundant on the anvils than in the surrounding area, indicating that capuchins transport them to the anvils. Capuchins use hammers weighing on average more than 1 kg, a weight that is equivalent to 25-40% of the average body weight for adult males and females. Our findings indicate that capuchins select stones to use as hammers and transport stones and nuts to anvil sites. Wild capuchins provide a new reference point for interpreting early percussive stone tool use in hominins, and a point of comparison with chimpanzees cracking nuts.

Functional mastery of percussive technology in nut-cracking and stone-flaking actions: experimental comparison and implications for the evolution of the human brain

“…hammers and anvils of particular size, shape and materials, suggesting that they apprehend the functional properties of the nutcracking task [20,21,42,70]. This capacity of selecting functional tools has also been shown in capuchins [40,41,71]. However, this capacity to perceive the affordances of objects as potential tools needs to be learned through experience.…”

“…If we define anticipatory behaviour as behaviour that prepares for the forthcoming goal, humans as well as other primates must be capable of anticipatory behaviour. The choice of a hammer adapted to the hardness of the nut prior to the actual nut-cracking activity has been observed in capuchins as well as chimpanzees [21,40,42,71]. Anticipation may be observed at the level of the striking action as well.…”

Primate archaeology reveals cultural transmission in wild chimpanzees (Pan troglodytes verus)

“…Most tools used by non-human primates are made of organic materials and therefore cannot be used for investigations of past behaviour as preservation is irrevocably compromised. However, several primate species are known to use stone pounding tools to access encased food sources [15][16][17][18][19].…”

Synchronized practice helps bearded capuchin monkeys learn to extend attention while learning a tradition

“…The naturally occurring stones used by the monkeys to crack nuts weigh, on average, 1.1 kg (range: 250 g to 2.5 kg) (56). They are quartz, quartzite, siltstone, or harder sandstone.…”