The cultural evolution of early hominins is investigated through the identification of archaeological evidence, primarily in the form of stone artefacts and bones (Leakey, 1971; Schick and Toth, 1994). The emergence and evolution of tool use is generally accepted as a transformational process that led to a suite of adaptations and behaviours, enabling humans to become the most successful primate on the planet (Hovers, 2015). The earliest production of deliberately flaked stone tools dates to ~3.3 Mya (Harmand et al., 2015; Lewis and Harmand, 2016). It has, however, been hypothesized that simple pounding tools may have been a precursor to intentionally flaked technology, potentially originating around or before the period of our last common ancestor with chimpanzees (deBeaune, 2004; Marchant and McGrew, 2005; Rolian and Carvalho, 2017; Thompson et al., 2019).

Although rare, percussive stone tool use is also practiced by a few species of non-human primates. These include Western chimpanzees, Pan troglodytes verus, (Boesch and Boesch, 1990; Sugiyama and Koman, 1979), bearded capuchin monkeys, Sapajus libidinosus, (Fragaszy et al., 2004; Ottoni and Izar, 2008; Falótico and Ottoni, 2016; Luncz et al., 2016a), long-tailed macaques Macaca fascicularis, (Malaivijitnond et al., 2007; Gumert et al., 2009; Luncz et al., 2017a) and one group of white faced capuchin monkeys, Cebus capucinus imitator (Barrett et al., 2018). These non-human primates, similarly to hominins, create long lasting archaeological signatures comprised of lithic assemblages (Mercader et al., 2002; Mercader et al., 2007; Proffitt et al., 2018; Falótico et al., 2019). By merging the fields of archaeology and primatology it is possible to combine direct behavioural observations with the archaeological signature of tool use (Haslam et al., 2009; Rolian and Carvalho, 2017; Carvalho and Almeida-Warren, 2019). Lithic assemblages remain the most abundant source of evidence for understanding early hominin technological and cultural variation (Delagnes and Roche, 2005), as well as social learning (Stout et al., 2019). As such, the field of primate archaeology is well placed to help investigate variation of tool evidence and cultural evolution. Further, this new research field is providing the ability to test various archaeological hypotheses often applied to the Early Stone Age (ESA) archaeological record. Examples of such empirical work include the effect that distance from raw material source has on lithic reduction (Braun et al., 2008a; Luncz et al., 2016b); the effect of population size on resource depletion (Steele and Klein, 2005; Luncz et al., 2017b); and chaine-operatoire approaches to stone tools (Delagnes and Roche, 2005; Carvalho et al., 2008). Archaeological techniques applied to the primate record have also been used to identify evidence of past tool behaviours and expand the temporal signature of tool use in non-human primates (Mercader et al., 2002; Haslam et al., 2016a; Haslam et al., 2016b; Luncz et al., 2016b; Falótico et al., 2019). Furthermore, primate archaeology has proven an effective tool for studying the behaviour of wild non-habituated tool-using populations (Luncz et al., 2017a). Reconstruction of tool selection patterns in wild chimpanzees has for example, repeatedly revealed detailed group-specific behavioural patterns (Koops et al., 2015; Luncz et al., 2015; Pascual-Garrido, 2019). This research provides the ability to quantify behavioural diversity, allowing the identification of cultural variation between distinct populations.

Our close genetic relatedness to great apes has led to intense, decades-long research into the tool behaviour of chimpanzees (McGrew, 2010). More recently, however, attention is being paid to the tool behaviour of monkeys to broaden the comparative context for the evolution and origin of technology. Long-tailed macaques are the only Old-World monkeys who use stone tools in their daily foraging. This behaviour is mainly observed in populations that live along the ocean shores of Southern Thailand and Myanmar where long-tailed macaques use tools primarily to prey on shellfish, including oysters, marine gastropods, crabs and mussels (Gumert et al., 2009), as well as sea almonds (Falótico et al., 2017) and oil palm nuts (Luncz et al., 2017a). Long-tailed macaques, like humans, use stone tools to kill prey and access meat. These foraging behaviours leave distinct damage patterns (use wear) on the hammerstones. Use wear analysis on stone tools combined with direct behavioural observations has been used to associate tool variation with specific prey species (Haslam et al., 2013; Tan et al., 2015; Proffitt et al., 2018). For example, percussive damage on distal pointed ends of the hammerstone has generally been associated with processing rock oysters (Tan et al., 2015), whilst central pitting on a flat hammerstone face is associated with marine snail predation (Gumert et al., 2009; Haslam et al., 2013).

Our recent research published in eLife (Luncz et al., 2017b) focused on providing explanations for observed differences in tool selection between two long-tailed macaque populations. These groups live on neighbouring islands (Koram and Nom Sao Island) on the east coast of Thailand, within the Khao Sam Roi Yot National Park. The results from this study showed that both macaque groups used stone tools to process the same species of shellfish, however the two groups selected different sized stone tools (Luncz et al., 2017b). After comparing underlying ecological factors, potentially affecting this disparity in tool size selection, our data suggested that the differences in tool selection were related to the varying prey sizes on each island. Differences in prey size were associated with overharvesting and resource depletion through increased predation pressure on one island. Maximising foraging efforts, macaques appear to preferentially target larger prey, leading to a reduction in the available prey populations and resulting in an overabundance of smaller, sexually immature prey specimens on the more heavily populated Koram Island, in turn leading to the use of smaller hammerstones. Conversely, on the less populated neighbouring island of Nom Sao, the prey population remained abundant and significantly larger in size, resulting in the use of larger hammerstones.

The tool-assisted feed-back loop observed amongst the macaque population at Khao Sam Roi Yot National Park has interesting hominin archaeological similarities. The earliest repeated shellfish exploitation is known to have occurred in South Africa during the Middle Stone Age (MSA) and Later Stone Age (LSA) (Steele and Klein, 2008). Chronological variations in shellfish sizes found in archaeological middens from the both MSA and LSA have been used as proxies for hominin population increases. Increased shellfish collection during the LSA compared to the MSA (Sealy and Galimberti, 2011) has been used to argue for and against a long and short chronology for the appearance of modern human behaviour (Mcbrearty and Brooks, 2000; Klein, 2008; Steele and Klein, 2005). However, others have also suggested that changing environmental factors may have been a contributing factor in this shellfish size variation (Sealy and Galimberti, 2011). The primate archaeological evidence from Khao Sam Roi Yot provides a direct primate analogy to a phenomenon previously observed through the archaeological record, allowing the ability to directly observe and monitor the effect that population size of tool using primates has on the available shellfish population.

This Research Advance paper builds on and expands our previous research to include further investigation into variation in tool selection and use patterns between long-tailed macaques. By comparing tool selection and use wear patterns between two adjacent islands in the Ao Phang Nga National Park in Southern Thailand we present new information on the possible underlying reasons behind tool variation seen in macaques (Luncz et al., 2017b). This area of investigation is not only important for understanding inter-group primate tool variation, but also allows testing of expectations derived from archaeological theory to understand possible underlying factors associated with stone tool variation in the ESA. These include the cultural historic approach and the use of typological analysis to document and understand variation between hominin lithic assemblages (Leakey, 1971; Isaac, 1977a), the application of a palaeoecological approach, where external environmental factors are seen as important drivers of stone tool variation (Isaac, 1977b; Toth, 1982; Toth, 1985; Braun and Harris, 2003; Braun et al., 2010). And finally, a chaine-operatoire or technological approach, where emphasis is placed on understanding the complete technological production and use of lithic assemblages, with variation seen to some extent as culturally or socially mediated (Delagnes and Roche, 2005; Roche and Texier, 1996; Roche et al., 1999; de la Torre and Mora, 2005).

During exploratory pilot fieldwork on the western side of peninsular Thailand in the Andaman Sea, approximately 500 km southeast of the previous study site in Khao Sam Roi Yot National Park, we identified two island dwelling populations in the Ao Phang Nga National Park, at Boi Yai Island and Lobi Bay (~9 km apart; Figure 1) who routinely use stone tools to process the same marine resources. Initial findings suggested possible inter-group tool use differences and as such a possible variation in the macaque archaeological record.

Figure 1

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By focusing on intensity of use wear as a proxy for tool re-use and by accounting for underlying ecological and environmental factors of both Boi Yai Island and Lobi Bay, we aim to suggest possible drivers for the observed stone tool variation between the two populations. If environmental circumstances are similar and the factors influencing tool selection, such as prey size, raw material availability and prey availability do not differ between islands, the possibility that tool use behaviour in long-tailed macaques is socially learned cannot be dismissed.

In total, we analysed 115 stone tools combined from both locations (Table 1).

The stone tool variation of the macaque populations of Boi Yai Island and Lobi Bay are separated into two categories; use wear by prey and size of tools selected by prey. Figure 2 shows examples of the distinct differences of use wear intensity of tools used to process rock oysters (Figure 2B) between Lobi Bay (Figure 2A) and Boi Yai Island (Figure 2C). Similarly, Figure 3 illustrates the profound differences between use wear grading (UWG) ratings for tools used to exploit Thais bitubercularis (Figure 3B) on Lobi Bay (Figure 3A) and Boi Yai Island (Figure 3C).

Figure 2

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Figure 3

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The comparison between UWG revealed differences between the sites. Stone tools used to crack open oysters are more intensively used on Boi Yai Island compared to Lobi Bay (Figure 4, Mann-Whitney U test: U = 32, N1 = 10, N2 = 26, p<0.001). Similar results can be found for UWG on stone tools used to crack Thais bitubercularis shells (Figure 4, Mann-Whitney U test: U = 1.5, N1 = 9, N2 = 12, p<0.001). The three morphologically similar marine snails (Nerita spp., Monodonta Labio and Morulla spp.) however, produced little in terms of inter-group UWG variation of tool intensity (Figure 4, Mann-Whitney U test: U = 381.5, N1 = 27, N2 = 29, p=0.832).

Figure 4

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A comparison of tool size selection associated with prey type revealed differences between Boi Yai Island and Lobi Bay in several important aspects. For marine snails, we found that macaques in Lobi Bay selected heavier tools than on Boi Yai Island (Figure 5A, Linear model: E = −1.100 SE = 0.176 F_(1,54)=38.898 P<0.001). When preying on Thais bitubercularis however, both populations selected similarly sized tools (Figure 5B, Linear model: E = −0.099 SE = 0.194 F_(1,19)=0.263 P=0.614). The size of the available prey was similar at both sites (Figure 6, site comparison C. bifasciatus: Mann-Whitney U test: z = −1.368, N1 = 21, N2 = 73, p=0.086; island comparison marine snails: Mann-Whitney U test: z = −0.511, N1 = 123, N2 = 129, p=0.305, island comparison Thais bitubercularis: Mann-Whitney U test: z = −0.458, N1 = 14, N2 = 88, p=0.324).

Figure 5

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Figure 6

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Figure 6—source data 1

Maximum length and maximum width of marine snails from Boi Yai Island and Lobi Bay.

https://doi.org/10.7554/eLife.46961.009

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To exploit oysters, however, macaques on Boi Yai Island used heavier tools than those used for processing the same prey in Lobi Bay (Figure 7, Linear model: E = 0.451 SE = 0.181 F_(1,34)=6.197 P=0.018). Oyster size on Boi Yai Island were, in general, larger than in Lobi Bay (Figure 8, Mann-Whitney U test: z = −2.862, N1 = 341, N2 = 436, p=0.002).

Figure 7

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Figure 8

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Figure 8—source data 1

Maximum length and maximum width of oysters on Boi Yai Island and Lobi Bay.

https://doi.org/10.7554/eLife.46961.012

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Several (n = 7) stone tools on Boi Yai Island showed multiple use wear patterns (on the face and on the tip of the tool). Using the methods of Haslam et al. (2013), these damage patterns are indicative of being used on more than one prey species.

Average weight of available raw material (bootstrapped 95% confidence intervals), separated between intertidal and tidal zones differed between sites (Figure 9A, Mann-Whitney U test: z = −5.107, N1 = 270, N2 = 432, p<0.001). The raw material availability also differed between sites (bootstrapped 95% confidence intervals over observed plots) (Mann-Whitney U test: z = −1.911, N1 = 20, N2 = 24, p=0.028). However, when separated by tidal and intertidal zones only, availability on Boi Yai Island shows a trend towards more stones available in the intertidal zone (Figure 9B, Mann-Whitney U test: z = −1.374, N1 = 20, N2 = 23, p=0.085). The availability in the tidal zone is similar at both sites (Figure 9B, Mann-Whitney U test: z = −1.214, N1 = 20, N2 = 24, p=0.112).

Figure 9

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Figure 9—source data 1

Measurements and weight of stones available in the tidal and intertidal zone on Boi Yai Island and Lobi Bay.

https://doi.org/10.7554/eLife.46961.014

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This study explores potential factors behind the variation in percussive stone tool use of two wild macaque populations from adjacent islands in the Ao Phang Nga National Park in Thailand. Macaques on these islands showed differences in their tool selection and degrees of tool reuse when foraging for marine prey. Tool selection was dependent on the species preyed upon. Macaques on Boi Yai Island selected heavier stone tools than those at Lobi Bay to process oysters. Comparisons of available prey species between the sites showed that oysters at Boi Yai Island were indeed larger. The heavier weight of oyster tools used on Boi Yai Island is therefore not surprising and reiterates previous findings, supporting that macaques adjust tool size to prey size (Luncz et al., 2017b; Gumert et al., 2013). Functional fixedness in regard to stone tool behavioural patterns and target foods can be observed across all tool using non-human primates (Carvalho et al., 2008; Fragaszy et al., 2010; Luncz et al., 2016a). For the larger gastropod (Thais bitubercularis) prey, both macaque populations selected similar sized tools to exploit similar sized resources. However, this pattern differs in respect to the processing of marine snails, where the Lobi Bay population selected heavier tools to exploit prey of a similar size to that recorded at Boi Yai Island. Variation in available raw material does not explain the differences in stone tool selection, as the mean weight of available raw material was in fact greater on Boi Yai Island. Furthermore, suitable raw material was available in greater quantities along the shore (especially in the intertidal zone) on Boi Yai Island. Based on this evidence, it is possible to suggest that purely ecological or environmental factors only account for some of the stone tool variation observed between these populations.

When assessing use wear intensity across the two sites, however, additional factors must be explored to explain the apparent variation. Compared to Lobi Bay, the Boi Yai Island tools showed a higher degree of use wear intensity. This pattern was recorded across oyster and Thais bitubercularis tools. The differences in oyster tools are most likely influenced by the oyster size difference reported between the locations. The intensity of use wear seen on oyster tools of Boi Yai Island, however, is unlikely to have developed during a single foraging event (Haslam et al., 2016c). Thais bitubercularis tools used on Boi Yai were characterised by the development of substantial central pits, suggesting multiple re-use events, whilst the hammerstones used to process the same species on Lobi Bay showed few signs of percussive damage. For marine snails (Nerita spp., Monodonta Labio and Morulla spp.), there was no significant difference in use wear intensity. It is reasonable to suggest that the relative fragility of these prey species means that little force was required to open them. Consequently, use wear intensity is low for these tools on both islands.

The lack of evidence of re-used hammerstones on Lobi Bay could be influenced by the tidal forces that potentially remove used tools from their original location. Lobi Bay however, appears relatively sheltered compared to Boi Yai Island as it lies within a large bay. Future research will be able to document the effect of tidal forces on tool movement and will clarify the availability of used tools between populations. A single example of a granite hammerstone was identified within the Boi Yai assemblage (Table 1). This igneous rock is more dense than the available limestone rocks available to the macaques which may have been a factor in its selection and re-use as a hammerstone. Of course, the re-use of this rock may be coincidental, and it should be remembered that any available granite has been geologically transported to Boi Yai Island from elsewhere and is therefore very rare (Luncz et al., 2017a).

As shown previously, raw material availability is consistent between both islands, with limestone constituting most of the available raw material. Scarcity of raw material, cannot, therefore, explain the apparent preference for tool re-use on Boi Yai Island. Indeed, raw material availability on Boi Yai Island is greater than that found at Lobi Bay. This would suggest that macaques on Boi Yai Island, unlike their neighbours, preferentially re-use tools rather than choose from readily available and suitable local stones.

The tool use variation observed between these two island primate populations meets the criteria for tool curation as discussed in the hominin archaeological literature. Shott and Sillitoe (2005) define lithic tool curation as the ‘ratio of realized to maximum utility’ of a tool. The varying degrees of repeated use, shown by use wear assessments of the hammerstones between the two islands, coupled with the similarities in primate species, raw material availability, processed prey species, and local ecological settings, suggest that the macaques on Boi Yai Island are successfully extracting a greater degree of utility from their hammerstones compared to those used on Lobi Bay. These variations in the intensity of tool use suggest a primate example of curated stone tool use on Boi Yai Island compared to those on Lobi Bay.

Previously, tool differences between wild macaque groups were only known to be influenced by prey size (Tan et al., 2015; Luncz et al., 2017b; Gumert and Malaivijitnond, 2013) or sex of the tool user (Gumert et al., 2011). Furthermore, it has been shown that the frequency of tool use in long-tailed macaques might be influenced by genetics, where the Burmese subspecies (Macaca fascicularis aurea) and hybrids expressing a more Burmese phenotype use tools in higher frequencies (Gumert et al., 2019). Our research sites in the Ao Phang Nga National Park lie within a natural hybrid zone and phylogenetic differences between groups cannot yet be fully excluded. Preliminary data show similar degrees of hybridization between groups (Malaivijitnond et al., 2007). The demographics (assessed from video footage, Luncz et al., 2017a) do not differ between the two populations. Males and females of all age classes use stone tools, it therefore seems unlikely that the difference in stone tools between the two islands stem from these factors. Muscle strength, individual proficiency and strike accuracy may all be contributing factors to the development of use wear, as these tools were most likely repeatedly used by multiple individuals over numerous tool use episodes. However, similar demographics between both macaque populations make it unlikely that these factors contributed to the differences found in use wear intensity. Additionally, by combining our new results here with our previous findings of tool use by macaques in the Gulf of Thailand (Luncz et al., 2017b), we were able to identify that macaques of Lobi Bay and Boi Yai Island do not forage on Cerith bifasciatus, despite its widespread abundance. Macaques in the Gulf of Thailand, however, regularly exploit this species.

These results partly support previous research that has shown that tool selection can be explained by ecological, biological and environmental factors (Gumert et al., 2011; Gumert and Malaivijitnond, 2013; Luncz et al., 2017b). However, the differences in use wear intensity and therefore frequency of re-use of hammerstones on Boi Yai Island remains unexplained by these factors.

Our new results show differences in use wear intensity and the extent to which macaques unintentionally modify the shape of their tools through use. This would suggest that tool use across a synchronous landscape need not be uniform, and highlights the potential for the development of synchronic tool use variation within species. Indeed, tool variation may be conditioned by cultural factors as well as responses to ecological conditions. Tool use has been shown to be influenced by social learning in several species (van Schaik et al., 1999; Whiten, 2000). To date, distinct cultural differences in technological behaviour in living primates has mainly been associated with the ape clade (Whiten et al., 1999; Whiten and Boesch, 2001; van Shaik, 2004; Luncz et al., 2012).

As this study, however, is dependent on primate archaeological data, as opposed to direct behavioural observations, it is important to note that its interpretations are subject to the same or similar constraints as found in the early hominin archaeological record. Identifying underlying reasons for behavioural variation in the animal kingdom often requires direct observations of differential tool use despite similar ecological situations between populations. Direct observation of behaviour is, however, impossible when attempting to assess cultural variation in the early archaeological record, and as such a cultural driver for observed lithic variation should only be invoked once non-cultural factors have been ruled out. Non-cultural factors may include, raw material availability and quality, ecological variation, adaptation to climatic change and genetic variation. In the absence of direct observations primate archaeology must also strive for similar goals in identifying possible cultural variation.

The observed artefactual variation between the two island macaque populations described here, although offering an opportunity for investigating the reasons for variation in tool using macaque populations, also provides a unique primate analogy to discuss possible drivers behind early lithic variability in the hominin archaeological record from a similar perspective as that available to archaeologists. Thus, it challenges us to consider the variables in the archaeological record that might drive assemblage composition but cannot be immediately tied to ecological or raw material constraints. Within the field of palaeolithic archaeology, understanding the cultural factors behind lithic variation has been at the forefront of research and has led to two primary theoretical frameworks, the cultural historic approach, which stressed the importance of typological tool categories to develop sequences of hominin cultural traditions (Leakey, 1971; Isaac, 1977a) and the chaine-operatoire approach (Roche and Texier, 1996; Roche et al., 1999; Delagnes and Roche, 2005; de la Torre and Mora, 2005; de la Torre, 2011), which has been used to reconstruct the technical processes and the social acts involved in tool use. At its core, however, is the working assumption that hominin lithic variation is a product of cultural differences both between geographically and temporally distinct groups as well as between hominin species (Sellet, 1993).

The other side of the theoretical divide is an ecological approach to lithic variation. After identifying limitations in the cultural historic approach for understanding the wider effects that the environment and ecology had on hominin behaviour and stone tools a palaeoecological approach has been put forward (Isaac, 1977b; Toth, 1982; Toth, 1985; Braun and Harris, 2003; Braun et al., 2010). This framework stressed the importance of viewing hominin stone tools as being intrinsically linked to external environmental and ecological factors. These included, ecological factors (Braun and Harris, 2003; Braun et al., 2010), raw material availability (Stout et al., 2005; Harmand, 2009; Braun et al., 2008b; Braun et al., 2009a; Braun et al., 2009b; Goldman-Neuman and Hovers, 2012) and functional factors (Toth, 1985).

One of the strengths of primate archaeological studies is the ability to test long standing fundamental archaeological theories and assumptions by applying them to stone tool using living primates (Haslam et al., 2009; Luncz et al., 2016b). Our findings suggest that when raw material availability, species differences, functional differences and environmental and ecological conditions are shared and do not explain stone tool variation the effect of cultural differences as a driving factor behind lithic technological variation in hominins must not be overlooked (Koops et al., 2015; Luncz et al., 2015; Pascual-Garrido, 2019). Our results highlight the possibility that tool selection in old world monkeys might also be affected through social learning and therefore might classify as a cultural behaviour.

This study builds a platform for long-term research focused on the group specific behaviours in populations of long-tailed macaques through the archaeological analysis of tool. Further archaeological research at the site coupled with direct observations will reveal the stepwise development of use wear patterns in long-tailed macaques. This highlights the potential of primate archaeology for answering questions regarding the ecological and socio-cultural factors which affected the development of stone tool use in our own lineage, given the absence of direct observations on early hominins (Westergaard, 1998; Wynn et al., 2011).

Unfortunately, the number of islands where long-tailed macaques are known to use tools are few and, in each instance, the cultural behaviour is possibly unique. The Andaman Sea coast of Thailand is renowned for its natural beauty and attracts increasing numbers of tourists. Increased human contact is known to affect stone tool behaviour by altering natural foraging behaviours (Gumert et al., 2013; Luncz et al., 2017a). Long-tailed macaques are experiencing profound population threats (Eudey, 2008). However, it is clear that these potentially rare, and certainly distinctive, examples of tool behaviour in old world monkeys need to be preserved and protected from human interference. Otherwise examples of extraordinary material culture may be lost before they are even discovered.

All data generated or analysed during this study are included in the manuscript and supporting files.

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Author details

1. Lydia V Luncz

   Primate Models for Behavioural Evolution Lab, University of Oxford, Oxford, United Kingdom

   Contribution

   Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing—original draft, Project administration, Writing—review and editing

   For correspondence

   Lydia.Luncz@anthro.ox.ac.uk

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-2972-4742

2. Mike Gill

   Primate Models for Behavioural Evolution Lab, University of Oxford, Oxford, United Kingdom

   Contribution

   Data curation, Writing—original draft

   Competing interests

   No competing interests declared

3. Tomos Proffitt

   The Institute of Archaeology, University College London, London, United Kingdom

   Contribution

   Investigation, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

4. Magdalena S Svensson

   Department of Social Sciences, Oxford Brookes University, Oxford, United Kingdom

   Contribution

   Data curation, Writing—review and editing

   Competing interests

   No competing interests declared

5. Lars Kulik

   Department of Primatology, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany

   Contribution

   Investigation, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

6. Suchinda Malaivijitnond

   1. Faculty of Science, Chulalongkorn University, Bangkok, Thailand
   2. National Primate Research Centre of Thailand, Chulalongkorn University, Saraburi, Thailand

   Contribution

   Conceptualization, Project administration, Writing—review and editing

   Competing interests

   No competing interests declared

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