Stone tools differences across three capuchin monkey populations: food’s physical properties, ecology, and culture

Robust capuchin monkeys (Sapajus) are known for processing mechanically challenging foods, having morphological adaptations to do so. However, several populations go beyond body limitations by using stone tools to expand their food range. Those populations use stones in a variety of ways, goals, and with different frequencies. Stone tool size correlates with the food’s resistance within some populations. However, we have no detailed comparisons to identify if this correlation is the same across populations. This study described and compared stone raw material availability, food’s physical properties (hardness and elasticity), and stone tool weight in three populations of bearded capuchin monkeys (Sapajus libidinosus), including a newly described site (Chapada dos Veadeiros National Park, CVNP). The differences we observed regarding stone tool weight selection among sites were not correlated to the food’s physical properties we analyzed. Lithic resource availability could partly explain some differences in the stone tools used. However, the tool weight differences are larger than the raw material variance across sites, meaning some distinctions are possible behavioral traditions, such as the same fruit (Hymenaea) being processed with bigger than needed tools in CVNP than in the other two sites. Capuchin monkey behavioral variability in stone tool use can be caused by several interacting factors, from ecological to cultural.

Robust capuchin monkeys (genus Sapajus) are widely and long known for processing mechanically challenging foods [1][2][3] . The genus Sapajus exhibits adaptation in the skull and postcranial skeleton that should produce increased masticatory or postcranial forces when feeding, such as thick enamel, large mandibular corpus and symphysis, and a broad face 4,5 . However, there are several degrees of morphological variance between species that do not straightforwardly correlate with species diet differences. For example, Sapajus apella has a more robust skull and skeleton than S. libidinosus, but the latter has a diet with tougher food 3 . This counterintuitive fact may represent behavioral adaptation in opposition to morphological adaptation since S. libidinosus' use stone tools to exploit the most challenging foods in their diet [6][7][8][9][10][11][12] . The same happens with yellow-breasted capuchins (S. xanthosternos), which also have a more gracile skull morphology than S. apella 5 , but can process hard food resources using tools 13 .
Sapajus is one of the few extant primate genera (together with Pan, Homo, Macaca and Cebus) to customarily use stone tools to process encapsulated food [14][15][16] . However, the use of stone tools is not the same in all populations, with variations in the material and dimensions of tools used and the food resources explored. Previous studies have suggested that the stone tools are chosen chiefly depending on the physical properties of the target food resource, and that was shown for populations of S. libidinosus, where heavier tools are used for harder food, as the stone tools used to process large palm nuts [17][18][19] . Additionally, chemical properties can also be a factor to influence the choice of a tool. For example, capuchins at Serra da Capivara choose larger tools to crack soft fresh cashew nuts, apparently, to protect themselves from the caustic liquid present at the fresh cashew nuts 20 . Differently, another population in which the same resource is explored without stone tools, uses a rubbing technique to avoid contact with the caustic liquid 21 . Therefore, information on ecology, lithic raw material availability, and behavior, including dietary ecology and food material properties, are needed to understand the evolutionary driving forces behind anatomical and cultural changes related to food processing.

Results
Stone tools dimensions of the three study sites are presented in Table 1. Stone tools at CVNP were statistically heaviest than the other sites (see section "Comparison between sites" for the statistical results). The stone tools were composed of quartzite, sandstone, and limonite, with different frequencies (χ 2 = 212.42, n = 486, df = 4, p < 0.001) at each site ( Table 2).
The lithic raw material characteristics available for each area are presented in Table 3, and were significantly different across the sites (χ 2 = 277.25, n = 577, df = 4, p < 0.001).
The physical properties test results (Indentation Hardness and Reduced Elastic Modulus) on the resources sampled in each area (Fig. 2), plus the mean weight of stone tools used to process each kind of resource, are presented in Table 4.
There was no correlation between the average values of stone tool weight and resource hardness (Spearman, S = 16, p = 0.297), or tool weight and resource elasticity (Spearman, S = 16, p = 0.297). This test was performed with the average values because the food sample could not be linked to each stone tool's weight values, as they were collected separately. www.nature.com/scientificreports/ stone weight was not significantly different (Mann-Whitney, W = 378.5, n = 138, p = 0.163). However, we take this result cautiously because only four tool sites used to process Hymenaea were recorded. In SCaNP, the resources (Hymenaea pods, Manihot seeds, and Anacardium cashew nuts) had significant differences in hardness, elasticity, and weight of the stone tools associated (Kruskal-Wallis, χ 2 = 37.348, n = 64, df = 3, p < 0.001; χ 2 = 33.005, n = 64, df = 3, p < 0.001; χ 2 = 8.533, n = 79, df = 3, p = 0.036). However, the weight of the stones was only significantly different in one pairwise comparison, revealing that the only difference was between hammer stones to process Hymenaea pods and Manihot seeds (Mann-Whitney, W = − 2.948, n = 79, p = 0.003), the latter processed with lighter stone tools.

Comparison between populations.
To analyze the differences between populations, we compared the resources present in more than one site and the stone tools used to process those items.
Attalea palm nuts, Buchenavia seeds, and the associated stone tools were not quantitatively evaluated because they are present in only one of the study sites.
The stones available in CVNP were 1.4 and 1.9 times heavier than the ones in SCoNP and SCaNP respectively (Table 3). However, the stone tools in CVNP were 5 to 8 times heavier compared to the same sites, suggesting that the heavier stone availability is not the only factor that causes the tool selection in CVNP and that an active choice for heavier tools is happening in that site.

Discussion
This work compared wild capuchin monkeys from three areas, describing the stone tool use in a new S. libidinosus population and comparing the resources processed within and between sites. We also described the physical properties of the resources and the stone tools used to process those foods and compared the data between the three sites and the literature. The differences we observed regarding stone tool use among sites, such as differences in stone tool weight to process the same resource (Hymenaea) in one of the studied populations, and the lack of differences within populations regarding the stone tools could be partially explained by ecological factors, such as raw material and resource availability. However, other differences appear to be more related to behavioral traditions. The new capuchin population described at CVNP, in a typical tropical savannah (Cerrado), presents a similar stone tool use pattern observed in other Cerrado populations. Most of the stone tools at CVNP were found to be used for processing high resistant palm nut (Attalea), similar to those processed in other savannah environments (e.g., Attalea barreirensis, Attalea speciosa) 7,13,17 . However, the weight of the tools and the physical properties of the food processed at this site was bigger than the other two populations analyzed (and also from the literature). Stone tools in CVNP had an average weight of 1672 g. The heaviest stone tool registered in our mapping was 5700 g (Table 1). In latter camera-trap surveillance (data not presented here, see Supplementary Video 1 for examples), we recorded monkeys effectively using a 4650 g stone tool (Fig. 5), an astonishing weight to be lifted by monkeys with a body mass average of 3500 g for adult males and 2100 g for adult females 34 . At the other two sites (SCaNP and SCoNP), the resources explored had, overall, lower hardness values, and the stone tools used to pound food resources were lighter, as expected based on SCaNP and SCoNP previous results 6,19 , as well from other S. libidinosus populations (Fazenda Boa Vista and Luis Gomes) where the monkeys choose stones according to the resistance of the resource processed 10,18 . CVNP has a relatively high density (215 stones/ha) of quartzite pebbles (the best raw material for stone tools) (Table 3), making it much more likely for the monkeys to encounter and use it than other stones available. Furthermore, the quartzite pebbles in this site are on average larger than the stones available at the other sites, maybe allowing CVNP´s monkeys to more often use heavy stones. Thus, this environmental difference could partially explain the use of larger tools for the CVNP population. However, the tools used are much heavier than the average stones available, suggesting an active choice for bigger stones.
In SCaNP, the highest availability of the best raw material for stone tools, quartzite, compared to the other sites (322 quartzite stones/ha, 50% higher than CVNP) could explain the more diversity of stone tool use observed in that area than any other site 6,19,23,35 , as the availability of this material would allow more interaction with those, and, over time, more innovation 35 . However, we probably do not have yet a complete picture of the repertoire of stone tool use at CVNP to thoroughly compare those sites and better understand the mechanism underlying tradition on the use of tools. Table 4. Physical properties (indentation hardness and elastic modulus) of the food resources processed with stone tools in each area and the average stone tool weight the capuchin monkeys use to process each resource type. Some values are not available (n.a.) because the monkeys were not observed to process the resource with stone tools in that population. *Data on stone tool weight for Hymenaea in SCaNP from 31  www.nature.com/scientificreports/ One of the resources explored with stone tools in CVNP is jatobá pods (Hymenaea sp.), although much less frequently than the palm nuts, at least in the study areas we mapped. The stone tools used for Hymenaea were lighter on average than the ones used to process the harder palm nuts, although not statistically significant (there was, however, a trend). This result must be taken with caution because of the small sample of Hymenaea tools.
Hymanaea pods were the only resource present in all three populations compared here. Our results showed that the physical properties of the pods were not significantly different across sites, meaning the challenge to open the pods would be similar in all three areas. However, the weight of the stone tools used in each population was different. CVNP monkeys used much larger stones to process the pods than the monkeys from the other two populations. One explanation could be the availability of the potential material to be used as tool. As mentioned before, CVNP has the larger/heavier raw stone material availability (Table 3). An alternative explanation would www.nature.com/scientificreports/ be related to the other resources explored. At the CVNP area, the monkeys explore, at the same nut-cracking sites, and with higher frequency, a much harder food (Attalea nuts), which could bias the tools selection to larger stones because of those more resistant palm nuts.
To better understand the tool use repertoire at CVNP, we still need to directly observe the monkeys or perform experimental tests to check the existence (or not) of tool use observed in other populations (e.g., probe tools, digging stones). The use of camera-traps recording nut-cracking sites (analysis ongoing) will also allow the observation of the stone tool use, its efficiency, and variation.
The first prediction of hypothesis 1 was not supported in any of the analyzed populations. At the populational level, heavier stone tools were not correlated to harder or less elastic food. In the case of CVNP, we had a small sample for stone tools used for Hymenaea, and both resources analyzed were found close to each other, so the stone selection in that area could have been biased to the process of the harder Attalea nuts. In the case of SCoNP, stone tools' weights did not differ between tools used for Hymenaea and Manihot seeds. Two things could explain this difference. First, in SCoNP, the lithic raw material is scarcer than in CVNP and SCaNP, restricting the selection of tools. Second, although small (Avg length, 16.42 mm), Manihot seed showed surprisingly higher hardness values, perhaps requiring a similar stone tool to the ones used to open the larger Hymenaea. Considering only the hardness value, we would expect that the monkeys would use heavier stones to process Manihot. However, in SCaNP, the stones used to crack Manihot were lighter than Hymenaea. One possible explanation is that although Hymenaea is much larger, hardness does not directly scale with size, as there is a combination of internal structures and material factors. Considering the structure of the food resources, it could be helpful in future studies to clarify the stone dimension and food physical properties.
The second prediction for H1 was also not supported. The anvil length did not vary by resource size. Our explanation is that an adequate anvil can be successfully used to process most resources. So, not being a limiting factor that needs to be different for most resources, e.g., the same anvil used to process an Attalea nut could be potentially used for Hymenaea pods, particularly when both species occur near each other. Even when a small target, such as Manihot, is being processed, the choice of an anvil could be driven by the larger resource in the area (rule of thumb to choose larger anvils), as this larger anvil can be used for any resource and there are no energetic costs, as it is not transported. A factor that should be analyzed in the future is the inclination of the anvil or the presence of pits, as some resources are rounder and more prone to roll from a tilted anvil 17 , meaning this factor could be more critical for anvil selection depending on the type of resource to be placed on the anvil.
Our results show a more complex association between food physical properties and tool selection than previous studies 10,18,19 . The data presented here, although limited by the small sampling and non-parametric univariate analysis, suggest that variables probably interact, such as stone availability and food hardness, and it reflects on tool selection. Our results suggest an approach to the tool choice considering food hardness, size, and stones availability in the environment. Other factors such the individual size, strength, age, and even position in the hierarchy could also influence tool choice and were not taken into account in this work. Future work testing the relationships between physical properties and tool selection in other taxa will improve the generalizability of these results, as well including other factors in the analysis.
Hypothesis 2 was supported for Hymenaea pods and fresh cashews but not for dry cashews and manihot seeds. Hymenaea pods and fresh cashews did not differ in the physical properties between sites, even though the Hymenaea in each place are from different species. That allows an interesting comparison between the use of  www.nature.com/scientificreports/ stone tools to open this resource, as it is processed with stones of different weight even being similar regarding physical properties. The same is true, in part, for the green cashew nuts, although in this case, the chemical characteristics could also be a factor of influence 20 . We found differences in hardness and elasticity between the study sites for Manihot and dry cashews. The difference between physical properties for the same species in different sites can be related to the time lying on the ground. Cashew nuts can lay on the floor for a long time (days or even months) before being picked by the monkeys (T.F. personal observation), and our sampling reflects that, as we collected a mix of dry nuts from the ground and directly from the tree. Therefore, we would expect the loss of water content to turn the tissues less elastic and harder 30 . The influence of the effects of the environment on the physical properties could also explain the difference found for Manihot seed; even if similar to the naked eye, we may have sampled seeds in different stages of desiccation.
Robust capuchin monkeys are known to explore mechanically tough food 2,3 . Capuchins also have features that may facilitate the production of large muscle and bite forces without compromising gape and so can exploit mechanically challenging foods at relatively large jaw gapes 36,37 . However, food such Attalea and Hymenaea are larger/bigger than the jaw gape of the monkeys and could only be opened by hand or using tools, and that appears to be the case. The monkeys at the analyzed populations (and other previously studied) have shown that the use of stone tools provides access to hard-encased food that would not be accessible otherwise 6,7,9,19,38 . Even for smaller food targets, e.g., Manihot seeds and cashew nuts, the efficiency of using tools to process those targets could be higher, especially if the population already has the tool use behavior in their repertoire because of the more challenging targets that obligatory need stone tools to be accessed, as appears to be the case of the populations in this study.
The differences we observed regarding stone tool use between the newly described capuchin population (CVNP) and the previously studied ones could be partially explained by ecological factors, such as the availability of lithic material and food resources. However, the differences in the availability of lithic material are not big www.nature.com/scientificreports/ enough to fully explain the behavioral variances. Our results show that capuchins choose, among the raw material available in the environment, suitable stone tools to open the food. Although the presence of other food resources can bias the tool choice, as the harder Attalea nuts presence appears to do at CVNP regarding Hymenaea stone tools. Moreover, some differences are more subtle, such as the cashew nut processing. Even though cashew nuts were available, SCoNP monkeys do not eat cashew nuts, with or without stone tools 33 . Since the absence of consumption of this food resource in SCoNP is neither because of availability nor impossibility of access, one could argue that this difference is a cultural behavior. The same can be said about the bigger stones used to process Hymeanea at CVNP 20 . Animal culture can be defined as all that is learned from others and is repeatedly transmitted in this way, forming traditions that may be inherited by successive generations 39 . The social learning of stone tools by capuchin monkeys has been shown several times 27,40,41 , but we still need to understand the ecological factors that also influence this stone tool's cultural variance. Identifying behavioral differences across populations and the factors that lead to this variance is the first step to better understanding capuchin cultures. SCaNP has an area of 130,000 ha, with an average temperature of 28 °C (range 10-47 °C) and a mean annual rainfall of 689 mm. This region is composed of Serra Grande formation, which has sandstones and conglomerates, and Canindé Group, which has laminated shale, sandstones, tempestites, mudstones, cobbles, and siltstones (for more details, see 31,42 ).

Methods
SCoNP has 823,435 ha of area, in which the average temperature is 28 °C (range 12-45 °C), and the mean annual rainfall is 650 mm. There are also more humid areas (Cerrado) in the east and north of SCoNP and some intermittent rivers in the valleys. This park presents three geological formations: Cabeça, which has hard sandstones; Pimenteiras, having red shale, sandstones, and siltstones; and Serra Grande, which has conglomeratic sandstone and quartz pebbles (for more details, see 6 ).
Chapada dos Veadeiros National Park (CVNP, − 14.093696, − 47.373306) was created in 1961 in the northeast of Goiás State (Fig. 1), in a central region of Brazil. This site has an area of 240,611 ha characterized by Cerrado biome and a tropical savanna climate. It has annual rainfall ranging from 1500 to 1750 mm, most of it occurring during the wet season (November to March), and average temperatures ranging from 20 to 27 °C 32 . The region encompasses waterfalls and three rivers descending from high plateaus of altitudes up to 1676 m, erosional scraps, and intra-plateau depressions 43 . This high altitude and water availability enable a mosaic of vegetation, including more open areas as grassland and open scrubland, wet areas as gallery forests, until more dense areas with taller trees, as forest and wooded savanna 32 . In terms of geomorphology, CVNP is in the Brazilian Central Plateau and is formed by Araí Group, that includes the basement granite gneiss and contains quartzites, conglomerates, calcareous-pelitic rocks, sandstones, basalts, and siltstones; and also by Paranoá Group, which is above the Araí group and contains metasediments, including quartzites, metasiltstones, limestones and dolostones 43 . Tool sites and environmental sampling. The stone tool use site mapping at CVNP was done following the same method of previous work 6  A "processing site" was characterized by the following items: (1) "anvil", a flat surface used as a substrate for the processed encased food, (2) "hammer" on top or beside the anvil (within 1 m), and (3) remains of the processed encased food atop or adjacent (within 30 cm) to the anvil 44 .
When a processing site was identified, we recorded the following variables: GPS coordinates, anvil material and size (maximum length and width), stone tool material, weight and size (length, width, and thicknesses), and processed encased food. We used digital scales (to nearest 0.1 g), calipers (to nearest 0.1 mm), and measuring tapes (to nearest mm). We visually identified processed encased foods in each site and collected samples from nearby trees for botanical identification. When two resources were processed on the same anvil, we considered, for this analysis, the one that had more leftovers on and around the anvil. We collected, when possible, the remaining of processed resources at the anvils for the physical properties test (see below). However, most of the resources needed to be later collected fresh, to be in a similar stage to the one the monkeys process them, and thus were not associated with a cracking site, but they were collected opportunistically in nearby areas.
Samplings of processing sites and description of S. libidinosus tool use at ScaNP and SCoNP were done in previous studies 6,19 . Still, we revisited both locations in October/2019 to collect food samples for physical properties testing and plot samples (see below). We collected Manihot, Annacardium, and Hymenaea samples in the same areas of the previous works. At SCaNP, we collected additional behavioral data on tool use throughout two study periods with the Pedra Furada (PF) group (2018 and 2019). TF followed this group for a total of 242 h, recording all occurrences of tool use and measuring the tools used with the same protocol as above after the monkeys left. The PF group was also studied and described in previous studies in this area 19,20,23,24 . The data on the stone tools is available in Supplementary Table 1.
To characterize the availability of raw lithic material, we did plot samples at the three sites, in the living area of groups Pedra Furada (SCaNP), Gruta do Boi (SCoNP), and Mariri (CVNP). Forty-five plots of 10 × 10 m were www.nature.com/scientificreports/ done along 4.5 km transect lines, in 100 m intervals, covering the living area of at least one group of capuchin monkeys as part of a broader survey for ecological description. We identified stone tool use sites with the same methodology as above in each plot. To evaluate stone material and the amount available in the areas, we did a 50 × 50 cm subplot at the SW corner of each plot, where all stones with more than 3 cm (the lower size limit a capuchin monkey would use it as a tool 20 ) were counted and classified by rock type, measured, and weighted.
Food resources physical properties test. Mechanics were measured with a Lucas Scientific FLS-2 portable mechanical tester 45,46 . Although hardness is not a property per se, it is a derived concept traditionally used to measure the resistance to deforming under indentation. The technology of the equipment used can measure indentation of millimeter dimensions bringing more accurate results for such tests. The test used is called sharp indent (for more details, see 47 ) and, in addition to the hardness values, gives us the elastic modulus, defined as the ratio of stress to strain in the elastic region 30 . Following an initial force loading, the displacement is held constant while recording the force decay for further 90 s or until the load stabilizes, forming a curve. Fitting a curve to the relaxation behavior allows the calculation of both an instantaneous (Ei) and infinite (E∞) elastic modulus. "Ei" represents the elastic modulus of a material if it could be loaded instantly, while "E∞" estimates the elastic behavior under an infinitely slow loading regime. In this study, we use "Ei" measurements, which are likely more relevant when investigating how food behaves under chewing loads 48 .

Data availability
The data on stone tools and food physical properties analyzed during this study are included in this published article as supplementary information files. We also include the R script with the statistical tests performed. The remaining dataset is available from the corresponding author on reasonable request.