Tufted capuchins (Cebus apella) use weight and sound to choose between full and empty nuts.

Wild capuchins feed on high-energy husked fruits. Field researchers suggested that capuchins select beforehand the fruits which 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 (phases 2 and 4), or sound (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.