Tufted Capuchins (Cebus apella) Use Weight and Sound to Choose Between Full and Empty Nuts

Abstract

Abstract Wild capuchins (Cebus apella) feed on high-energy husked fruits. Field researchers suggest that capuchins select beforehand the fruits that are worthwhile to crack open. We assessed whether captive capuchins use sound and/or weight as proximal causes to infer the fullness of a nut (full nuts are heavier and produce a low-pitched frequency sound when tapped). Two subjects had to choose between a nut shell containing food and another nut shell without food differing in weight and sound (Phase 1), or weight only (Phases 2 and 4), or sound only (Phase 3). Both subjects readily solved Phase 1 and 2. The only subject who spontaneously finger-tapped on the nut shells solved Phase 3. The minimal difference in weight perceived ranged between 2.1 and 3 g. Therefore, as expected by ecological psychology and optimal foraging theory, data suggest that capuchins seek information to discriminate effectively between full and empty nuts before going through the costly opening process.