Nut-cracking success and efficiency in two wild capuchin monkey populations

Abstract

Capuchins can employ several strategies to deal with environmental challenges, such as using stone tools to access encapsulated resources. Nut-cracking is customary in several capuchin populations and can be affected by ecological and cultural factors; however, data on success and efficiency are only known for two wild populations. In this work, using camera traps, we assessed palm nut-cracking success and efficiency in two newly studied wild bearded capuchin populations (Sapajus libidinosus) and compared them with other sites. We tested the hypothesis that the overall success and efficiency of nut-cracking would be similar between sites when processing similar resources, finding partial support for it. Although using hammerstones of different sizes, capuchins had a similar success frequency. However, efficiency (number of strikes to crack a nut) was different, with one population being more efficient. We also tested whether success and efficiency varied between sexes in adults. We predict adult males would be more successful and efficient when cracking hard nuts. We found no differences between the sexes in one site but found sex differences in the other, although also for the low-resistant nut, which was unexpected. Our data add to the knowledge of capuchin nut-cracking behaviour flexibility, variance and potential cultural traits.

Keywords: camera trap; hammerstones; population differences; sex differences; tool use; tradition.

Figure 1.

Maps of the research sites and the locations of the camera-trap installations.

Figure 2.

Palm nuts that capuchin monkeys (S. libidinosus) process with stones tools in the two areas. At UNP, (a) babaçu palm nut (Attalea speciosa) and (b) fresh macaúba palm nut (Acrocomia aculeata), and at CVNP, (c) palm nut from genus Attalea. The black line scales are 10 cm.