Mapping nut‐cracking in a new population of wild capuchin monkeys (Sapajus libidinosus) at Ubajara National Park, Brazil

Abstract

Populations of bearded capuchin monkeys (Sapajus libidinosus) vary in their tool use behaviors, with some of this divergence regarded as culturally determined. The use of stone tools, primarily to crack open encased foods, is widespread among bearded capuchins living in dry habitats (Caatinga and Cerrado). Significant diversity in targets, processed foods, material, and size of tools is observed across populations. However, so far, only a few sites have been systematically studied, and we are still distant from a representative picture of the range of variation in capuchins’ culture. In this study, we did a systematic assessment of stone tool use sites in the Ubajara National Park (UNP), in the Caatinga region of Ceará, Brazil, recording and measuring stone tools, processed foods, and available lithic resources as part of an extensive comparative research, the CapCult project. We found indirect and direct evidence that capuchin monkeys at UNP customarily use hammerstones and anvils to process at least two species of palm nuts, macauba (Acrocomia aculeata) and the harder babaçu (Attalea speciosa). Most of the anvils were rock surfaces and had leftovers of only one palm nut species. The hammerstones used to process both palm nuts were not significantly different in weight, although the ones used for Ac. aculeata were longer. We found a higher frequency of nut‐cracking sites in the drier lowland area of the park, reflecting differences in the density of the most common palm species, Ac. aculeata, and availability of raw stone material. The stone tool use observed in UNP is within the scope of previously reported in savannah capuchin populations. Our study widens the knowledge of stone tool‐use diversity in wild capuchin monkeys, which could contribute to shaping conservation policy, including cultural traits.

Wild capuchin monkeys use stones and sticks to access underground food

“…We collected and measured 50 digging stone tools (see details on dimensions for each target resource in Table S2 ). Most of them are made of sandstone materials and are smaller and lighter (average weight 128 g) than the pounding tools used for palm nut cracking (average weight 1142 g) by the same population 20 .

Figure 1 Tools used by the capuchin monkeys at Ubajara National Park.

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“…In contrast, the lowland is hotter (28.2 °C), drier (average rainfall: 939 mm), and presents vegetation composed of dry forest and stepic savannah 37 , 38 . The wet season occurs from January to June, and the dry season occurs from July to December 20 . Plants species reported to be dug with tools by SCaNP population (farinha-seca, Combretum glaucocarpum , and louro, Ocotea sp.)…”

“…The bearded capuchin monkeys ( Sapajus libidinosus ) living in this location are known to use stone tools as hammers and anvils to crack open encased resources, mostly palm nuts 20 , 21 . The Sertão Group was composed of 31 individuals for most of the period sampled, 3 adult males (more than 7 years), 3 subadult males (5–7 years), 10 adult females (more than 5 years), 9 juveniles (2–5 years) and 6 infants (0–2 years).…”

“…We did not register episodes of digging for dry macaúba nuts ( Acrocomia aculeata ). Those are nuts they crack with stones 20 and that accumulate on the ground around palm trees and nut-cracking sites. The monkeys often use their hands to remove the surface layer of the soil and access the buried nuts.…”

“…Here, we report another population of bearded capuchin monkeys ( S. libidinosus ) that uses stones for digging, and this is the first population observed to use the ‘stick-probing’ technique to obtain underground food. These monkeys live in Ubajara National Park (Brazil), a much wetter savannah area, and are already known for using hammerstones for palm nut-cracking 20 , 21 . We describe two techniques used for extracting underground storage organs and four techniques used for the capture of trapdoor spiders.…”