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Group-specific archaeological signatures of stone tool use in wild macaques

  1. Lydia V Luncz  Is a corresponding author
  2. Mike Gill
  3. Tomos Proffitt
  4. Magdalena S Svensson
  5. Lars Kulik
  6. Suchinda Malaivijitnond
  1. University of Oxford, United Kingdom
  2. University College London, United Kingdom
  3. Oxford Brookes University, United Kingdom
  4. Max Planck Institute for Evolutionary Anthropology, Germany
  5. Chulalongkorn University, Thailand
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Cite this article as: eLife 2019;8:e46961 doi: 10.7554/eLife.46961

Abstract

Stone tools in the prehistoric record are the most abundant source of evidence for understanding early hominin technological and cultural variation. The field of primate archaeology is well placed to improve our scientific knowledge by using the tool behaviours of living primates as models to test hypotheses related to the adoption of tools by early stone-age hominins. Previously we have shown that diversity in stone tool behaviour between neighbouring groups of long-tailed macaques (Macaca-fascicularis) could be explained by ecological and environmental circumstances (Luncz et al., 2017b). Here however, we report archaeological evidence, which shows that the selection and reuse of tools cannot entirely be explained by ecological diversity. These results suggest that tool-use may develop differently within species of old-world monkeys, and that the evidence of material culture can differ within the same timeframe at local geographic scales and in spite of shared environmental and ecological settings.

https://doi.org/10.7554/eLife.46961.001

Introduction

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.

Research site in the Ao Phang Nga National Park, Thailand.

(A) Southeastern tip of Boi Yai Island and (B) Lobi Bay on Yao Noi Island, both within Ao Phang Nga National Park, Southern Thailand.

https://doi.org/10.7554/eLife.46961.002

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.

Results

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

Table 1
Total number of tools associated with specific prey species.
https://doi.org/10.7554/eLife.46961.003
PreyLobi BayBoi Yai Island
Monodonta labio93
Morulla spp.02
Nerita spp.1826
Oysters1026
Thais bitubercularis912
Total limestone hammerstones used4669
Total granite hammerstones used01

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).

Stone tools used by wild macaques in Ao Phang Nga National Park to exploit rock oysters (Saccostrea cucullate).

(a) Examples of stone tools used at Lobi Bay. (b) Examples of stone tools used on Boi Yai Island. (c) rock oyster prey species available on both islands and.

https://doi.org/10.7554/eLife.46961.004
Stone tools used by macaques in Ao Phang Nga National Park to exploit Thais bitubercularis.

(a) Examples of stone tools used at Lobi Bay. (b) examples of stone tools used on Boi Yai Island. (c) Thais bitubercularis prey species available on both islands.

https://doi.org/10.7554/eLife.46961.005

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).

Intensity of damage (UWG) compared for stone tools between two sites (Lobi Bay and Boi Yai Island) in the Ao Phang Nga National Park.

The size of the circle indicates the respective number of tools included. (For underlying data, see Source data 1).

https://doi.org/10.7554/eLife.46961.006

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).

Selected tools used to crack open marine prey by wild macaques in Lobi Bay and Boi Yai Island.

(A) Differences in selected tool weight to crack open marine snails. (B) Differences in selected tool weight to crack open Thais bitubercularis. The plots are showing all quantiles and the CIs (grey). (For underlying data, see Source data 1).

https://doi.org/10.7554/eLife.46961.007
Size of marine prey available at Lobi Bay and Boi Yai Island.

The plot is showing the mean with bootstrapped 95% confidence intervals.

https://doi.org/10.7554/eLife.46961.008

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).

Tools selected by wild macaques to crack open oysters at Lobi Bay and Boi Yai Island.

The plot is showing all quantiles and the CIs (grey). (For underlying data, see Source data 1).

https://doi.org/10.7554/eLife.46961.010
Size of oysters available at Lobi Bay and Boi Yai Island.

The plot is showing the mean with bootstrapped 95% confidence intervals.

https://doi.org/10.7554/eLife.46961.011

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).

Availability of raw material at Lobi Bay and Boi Yai Island.

(A) Weight of stones available at point transects. (B) Number of availability of stones. The plots show the mean with bootstrapped 95% confidence intervals.

https://doi.org/10.7554/eLife.46961.013

Discussion

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.

Materials and methods

Study sites

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We collected data in January and February 2017 and 2018 on two shorelines at islands in the Ao Phang Nga National Park. The first site, Lobi Bay, is located in the northwest tip of Yao Noi Island (8°10′ 51.06’ N, 98° 37′ 42.52’ E, Figure 1A). The second site is 9 km to the southwest, on the southern end of Boi Yai Island (8° 07′41.09’ N, 98° 33′ 25.04’ E, Figure 1B). Below the shoreline is an intertidal zone extending up to 200 m at spring tides. The area above the shoreline is comprised of steep coastal forest. Because the macaque populations at Boi Yai Island and Lobi Bay are not habituated to the presence of humans, the field of primate archaeology offers the most appropriate way of reconstructing tool selection and analyse damage pattern (use wear) (Haslam et al., 2013; Haslam et al., 2016a). This approach however also imposes limitations, as we are not able to differentiate between the sex and age of the tool user. However, tool use is a social activity in long-tailed macaques, with most of the group foraging together along the shore. Previous studies at Lobi Bay and Boi Yai Island using camera traps to document nut cracking behaviour confirmed that both groups comprised mixed demographics, where males and females in all age ranges use tools habitually (Luncz et al., 2017a). Furthermore, during low tide, we were able to observe shellfish foraging of monkeys on the shore from far away. Identification of individual group members was not possible, however since shellfish foraging is a social even in long-tailed macaques, most group members were counted simultaneously foraging together. Unfortunately, camera traps are not feasible in the ocean setting of shellfish foraging and therefore our study relies on indirect evidence of stone tools and prey remains. Based on the mtDNA phylogenetic analysis, these two populations are genetically similar (Malaivijitnond S., in prep.).

At both sites, Boi Yai Island and Lobi Bay, wild macaques use stone tools to exploit rock oysters (Saccostrea cucullate) and a variety of marine snails including Nerita spp., Monodonta labio, Morula spp., and the larger Thais bitubercularis. Due to the similar morphological nature and size of Nerita spp., Morula spp. and Monodonta labio, we combined those species to analyse use wear patterns and they are hereafter referred to as marine snails. As macaques have been reported to frequently consume Cerith bifasciatus marine snails, we also collected information on their availability.

Boi Yai Island and Lobi Bay on Yao Noi Island are limestone outcrops within the Ao Phang Nga National Park which is underlain mainly with Permian carbonate rocks which were deposited about 295–250 million years ago (Kuttikul, 2015). The geological profile of available raw material across the shoreline is predominantly limestone, with the occasional rock formed of igneous rocks such as granite cobbles, which do not occur naturally on the Islands but may have been transported to the islands by wave action. There are occasional granite outcrops in other areas within Phang Nga bay (Charusiri et al., 1993), and this is a tsunami prone region (Kongapai et al., 2016) which could transport stone material to the research sites.

At both sites, Lobi Bay and Boi Yai Island, we conducted line transects along the shore and collected all stone tools previously used by the macaques to forage on marine prey. Stones were identified as tools if they showed fresh percussive marks and were found within 20 cm of identifiable prey remains. This distance was determined as the maximum to reasonably associate prey with tool. For each tool we identified which prey it had been utilised for. Where there was any doubt regarding the association of a tool with a particular prey, the tool was excluded from this study.

All tools were subsequently cleaned, weighed and all raw material types were documented. Use wear was recorded following a modified technique adopted by Haslam et al. (2013). Each tool was divided into ten zones and use wear (pitting, crushing, and fracture) was recorded for each zone. The 3-level system adopted by Haslam et al. (2013) to grade severity of use wear did not provide the necessary resolution to distinguish variations as it originally was designed to assess functional use. Therefore, for each type of use wear, we adopted a use wear grading system (UWG) describing five different use wear intensities (Table 2).

Table 2
Grading of use wear by zone (adapted from Haslam et al., 2013)
https://doi.org/10.7554/eLife.46961.015
Use Wear Grade (numerical)Use Wear Grade (descriptive)PittingCrushingFracture
DefinitionDistinct indentations from discrete strikes that can culminate in the creation of more general craters in the stone surfaceRounding and flattening of the tool surface, especially the protruding parts on the points and edges.A breakage of the tool caused by chipping or flaking
Grade 0NoneNo traceNo traceNo trace
Grade 1SlightTrace - minimal and isolated damage. Isolated points of impactTrace - minimal and isolated damageTrace - minimal and isolated fracture
Grade 2MediumOverlapping impact points that have formed a coherent platform of damageSurface clearly damaged but limited evidence of repeated use.Moderate - larger fractureaffecting < 30% of the use zone
Grade 3HighCumulative damage with a pitting to a depth of 2 mmCumulative - rounding can be observed and felt.Extensive - Fracture affecting between 30% and 60% of the use zone
Grade 4Very-HighCratered - a larger more extensive indentation that is more than 2 mm deep and 5 mm in diameter.More progressive rounding that has produced a flatter surface on the use zone.General - fracture extending beyond 60% of the use zone

The system designed by Haslam categorised use wear into the following three subsets: (1) Trace - superficial and isolated points of damage only, possibly incidental; (2) moderate – the stone surface was clearly damaged but this damage was spatially restricted with only minor evidence of repeated wear; and (3) extensive - cumulative damage that may cover a significant portion of the use-zone. The aim of Haslam et al was to use the wear patterns on the tools to identify foraging behaviours and therefore a limited scale of differentiation of damage intensity was sufficient. Here the objective was to specifically compare use wear intensity and potential re-use of tools used by two discrete populations of macaques. Therefore, a three grade scale was insufficient as it can skew results in the direction of amplifying observed differences. The emphasis was placed on designing a key that was reliable in differentiating between the higher end of the use wear scale to more accurately interpret the likelihood of reuse. Using the UWG definitions (Table 2), multiple separate analyses of the material was undertaken by two researchers to ensure replicability of results.

In respect of pitting, where use wear was rated as grade 4, we recorded the length, width and depth of each pit. Pit depth was measured digitally using a pit gage across an axis that lay across two opposing edges of the pit and crossed the deepest part of it. This method which anchors the arms of the gage at the pit edge and lays across the deepest part ensures that results are replicable. Given that the UWG distinguishes between grades 3 and 4 (Table 2) only based on a pit being greater than 2 mm deep, digital microscopic measurement was not considered necessary.

To compare use wear intensity, we identified the highest score of damage inflicted by the macaques on each tool and compared that between sites. To assess tool selection patterns between sites we compared the weight of each used tool in regard to the locally available raw material. To analyze whether the weights of stone tools used to crack open oysters or marine snails differed between the two observed populations we ran two linear models (LM), one for each prey species, which each included the population (Boi Yai Island and Lobi Bay) as fixed effect. As response we used the weight of the stone tools used to open oysters or snails. Prior to running the models, the response variable was log transformed to achieve a more symmetrical distribution. For both models we checked whether the assumptions of normally distributed and homogeneous residuals were fulfilled by visually inspecting a plot and the residuals plotted against fitted values. In both models we found no obvious deviations from these assumptions. We additionally tested model diagnostics using the R functions ‘‘dffits,’’ ‘‘dfbeta,’’ and ‘‘cooks.distance’’; and checked for leverage and found that no assumptions were violated.

Furthermore, we compared the intensity of the tool damage between the sites by comparing the occurrence of the different use wear intensities. The damage with the highest intensity rate was used in a Mann-Whitney U-test separated by prey species.

Environmental comparisons

Request a detailed protocol

Because the macaques are not habituated, direct observations during shellfish foraging along the shore are not possible. In order to collect used tools, we inspected the areas at the shore where monkeys were directly observed foraging from afar during the low tide and repeatedly returned after low tide to assess the area for more tool use. On Boi Yai Island this totalled a shoreline of approximately 1100 metres, whilst on Lobi Bay this resulted in a shoreline length of approximately 900 meters.

In order to assess the availability of stones for each population, we used point transects (plots) along line transects across the intertidal and tidal zone. The tidal zone describes the area between high and low water and the intertidal zone is located above the tidal zone. The intertidal zone floods only during the peaks of the tide cycles and remains dry for long stretches between high tides. It is inhabited by hardy sea life, such as barnacles, marine snails. Because the intertidal zone is accessible for much greater periods than the more frequently flooded tidal zone macaques would have greater opportunities in a given timespan to access raw material and prey. For this reason, we compared the stone availability separated within the intertidal and tidal zone at a given longitudinal positions.

Each plot was randomly selected 100 meters apart along the line transects. This resulted in 20 plots in each zone in Lobi Bay and 24 plot on Boi Yai Island. Within each plot all stones larger than 2 cm in maximum length were weighed (stones smaller than this were considered too small for macaques to utilise as tools). Due to the large amount of available stones each transect plot was limited to 20 by 20 cm. Plots were arranged in pairs where one plot was located in the tidal zone and one in the intertidal zone at the same longitudinal position on the transects. Tidal and intertidal plots were at least 20 meters apart from one another. Separated for island (Boi Yai Island and Lobi Bay) and location (tidal and intertidal) we bootstrapped i) the number, ii) the weight of the stones we found per transect 1000 times and compared the confidence intervals at the level of 95% between the islands. Additionally, we applied two sample t-tests.

We assessed the availability and size of prey species at both sites within 1 m2 plots along the same line transects as the stone plots, spread 100 meters apart from each other. This resulted in 10 plots in Lobi Bay and 12 plots on Boi Yai Island. Within each plot the frequency, length and width of available prey species were documented. To increase the sample for the size measurements we additionally searched for marine prey along the shore at both sites.

To compare the size of oysters, we measured the maximum length and maximum width and calculated a volume for 341 oysters in Lobi Bay and 436 oysters on Boi Yai Island. We bootstrapped the volume for each site 1000 times and compared the confidence intervals at the level of 95% to each other. Additionally, we applied a two-sample t-test. To test for differences in snail size between the sites, we conducted the same procedure for four snail species (Cerith bifasciatus (NLobi = 21, NBoi = 73); Nerita spp. (N Lobi = 120, NBoi = 119); Thais bitubercularis (NLobi = 14, NBoi = 88)).

To investigate the availability of marine snails we bootstrapped the number of snails we found at each site and on each transect 1000 times and compared the confidence intervals at the level of 95% between the sites. Additionally, we applied two sample t-tests. As oysters are abundant throughout the shoreline and did not pose a limitation of availably, they were excluded from the comparisons of prey availability.

All models, bootstraps and tests were implemented in R version 3.5. (R Development Core Team, 2015). The LM was fitted using the function lm, and the Mann-Whitney U-test with the function wilcox.test.

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Decision letter

  1. Jessica C Thompson
    Reviewing Editor; Yale University, United States
  2. Detlef Weigel
    Senior Editor; Max Planck Institute for Developmental Biology, Germany
  3. Jessica C Thompson
    Reviewer; Yale University, United States
  4. Amanda Tan
    Reviewer; Nanyang Technological University, Singapore

In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.

Thank you for submitting your article "Group specific stone technology suggests cultural diversity in Old-World Monkeys" for consideration by eLife. Your article has been reviewed by four peer reviewers, including Jessica C Thompson as the Reviewing Editor and Reviewer #2, and the evaluation has been overseen by Detlef Weigel as the Senior Editor. The following individual involved in review of your submission has agreed to reveal their identity: Amanda Tan (Reviewer #4).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

Summary

This manuscript represents a detailed and well-written investigation into the archaeological signature of macaque tool use on two neighboring populations in Thailand. It is impressive in its scope and detail. The factors investigated here have direct relevance for tool use across the primate record and provide tantalizing details of how we might use the meticulous documentation of this behavior amongst living primates to understand aspects of early hominin behavior that are not directly observable.

The authors do a good job of working their way through layers of logic to demonstrate that species/population differences, raw material availability, and resource package size (in short, ecological/environmental and raw material constraints) do not explain all the differences that they observe in tool use. Thus, it challenges us to consider the variables in the archaeological record that might drive assemblage composition, but can't be immediately tied to ecological or raw material constraints. The authors also make an excellent point about conservation, as this behavior in wild primates may be threatened by increasing human interactions with them. For these reasons, it is a worthy contribution. However, it needs revisions before advancing.

From their results, the authors conclude that cultural differences – transmission of distinct and population-specific behaviors through social learning – is responsible for the patterns that cannot be explained by other variables. This is the same logic used by archaeologists when documenting traces on objects or their associations with one another. The difference is that it is impossible to observe the behavior that led to these traces and associations in the archaeological record, but it is possible to observe this amongst living organisms. Since direct observation was not possible with the macaques, this is essentially an archaeological study carried out in the present day.

The reviewers concur that the strength of this paper lies in demonstrating how arguments that are being used in archaeology can have a resonance in living animals, as well as application to the archaeological record. This paper therefore could represent a significant advance in the joining of the fields of archaeology and animal behavior, but this would require a reframe of the paper to emphasize the verifiable aspect of the paper, which is the use of archaeological methods to suggest culture, rather than on demonstrating culture itself is the cause of the tool differences. The work would do more to advance the field by contemplating a wider range of possible mechanisms for variation in tools instead of sticking with the current dichotomy of culture (generally considered exciting) or not culture (generally considered boring).

The reviewers also concurred that in the absence of direct observational data, the paper cannot be convincingly framed in terms of primate "culture" (although this may be true, it does not meet the observational tests required in animal behavior research). We cannot be even sure that effects found for the marine snail are not the result of the activity of a single individual (or a matriline), who would favor the use/re-use of certain stones. Later in the manuscript, the reviewers mention that previous studies have documented that make and female macaques will use different sized tools. Could this account for some of the variation? As Bandini and Tennie, 2017 have suggested, it is particularly important to show that any example in the wild is not the result of re-invention by individuals, which would make the argument weaker for all animal cultures in general.

Again, all reviewers saw much value in the fact that macaques faced with similar ecological conditions and similar requirements, with similar genetics, do not produce identical archaeological records. Whether this is attributable to culture or not at this point becomes less important than the fact that this is exactly what we must deal with in the archaeological record. This manuscript suggests that investigations of primate tool use may provide us with insights into the mechanisms of social learning. This would be particularly interesting because of the importance that evolutionary psychologists place on certain types of social learning (imitation, etc.). However, again the social context of the tool use and learning is not directly observable here, and the authors do not have the social data that would tell us exactly how this information is transferred between individuals. Therefore, they should remove sentences about social learning, because they infer, rather than observe, that this happened.

In the end, all reviewers agreed that the authors should reframe this manuscript to emphasize its strengths relative to the archaeological record and back away from claims of culture and social learning as the primary focus. In light of this reframing, the manuscript mainly requires changes to the background and discussion. "Culture" or social transmission processes more broadly are undoubtedly involved in macaque tool use, but these can be discussed more thoughtfully with regards to how social learning might influence behavior and artifact production. Proposing these possible explanations would direct further research, something that is actually possible with the macaque study system, unlike with the hominin archaeological record. Therefore, the authors should use the opportunity to report observed differences in material objects and environments, and set up hypotheses about what different stone traces COULD be reflecting, much as archaeologists do. This could also involve suggestions for future work that actually uses archaeological methods. For example, if some are multi-use tools, which is in and of itself very interesting, it is especially important to be able to document all the resources on which a single tool was used and not just the one that was last used and discarded nearby. In future, and in the absence of observational data, the authors might consider taking samples from the rock surfaces for proteomic or residue analysis, much as archaeologists do.

The reviews been consolidated below into the major points that should take place to make this manuscript acceptable for publication in eLife. Because many of the points include useful discussion and suggestions for additional references, much of the original wording has been retained, along with the helpful suggestions therein. This makes for a somewhat long decision letter that can be boiled down to nine major points.

Essential revisions

Below is a summary of the key changes that should take place to make this manuscript acceptable for publication in eLife:

1) Clear integration of this research with the previous study. As a Research Advance, a more substantial review of the results of the previous study would be beneficial. How and why are these studies different and how are they alike? It is not clear enough to readers who are unfamiliar with macaque tool use sites that the two studies were conducted with two entirely separate populations at different sites. Also, since the original study found that ecological parameters do appear to be driving some of the variation in tool use, the deviation from this in the current study becomes even more interesting. The previous study capitalized upon the size differential between the two macaque populations to examine whether habitual tool-aided exploitation by macaques could lead to reduction in size and abundance of shellfish prey. This contributed to the long-standing debate in coastal archaeology as to whether the reduction in shellfish sizes in the Middle Stone Age could have been due to human exploitation of marine resources. Here, this study is showing that primate tool artifacts also vary between populations. The authors should emphasize these potential contributions via primate archaeology, and how further behavioral work could help with interpreting the lithic record (particularly with respect to the concerns raised above).

2) Re-framing the significance of the work. This should be with respect to what is directly observable, and more specifically address its implications for the archaeological record of early hominins, rather than simply labelling patterns as cultural or non-cultural.

3) Clarifying possible alternative explanations than "culture". The lack of information about the sex and age of individuals does not invalidate this study, and in fact makes it more akin to archaeological work. However, all possible sources of variation should be mentioned as possible explanations. One possibility is providing information on the number of individuals or different age on the two islands. If the islands do not differ in their age and sex demographics then it would seem unlikely that this is a reason for these differences. However, the authors should still discuss other possibilities. How confidently can we disentangle the effects of use frequency from users' strength/proficiency/strike accuracy on wear patterns? Macaques do differ in proficiency, and some tool users will be able to crack food items with fewer strikes or might be more accurate with their strikes. These users would conceivably create fewer and less diffuse use wear than users who require many strikes to crack food items. The latter might create use wear that would be scored as more intense or extensive but interpreting this as re-use would be erroneous. Perhaps differences in use wear patterns between both populations could indicate differences in population level proficiency? In spite of the authors' claim that the snails are morphologically similar, there could be subtle ecological differences that could explain a different use of stones by the monkeys on the two islands. For instance, there might be a higher proportion of a certain type of snail in one of the two islands, which would make the re-use of certain stones more efficient. There is also a cognitive argument that can be made about tool selection, e.g. functional fixedness. Macaques on Boi Yai have to use the heavier tools to crack oysters because they are bigger than on the other island. That means the heavier tools can be functionally fixed to be used with oysters. Macaques then use lighter rocks (the ones that are not functionally fixed) to crack the snails. On the other hand, on Lobi Bay, there is no functional fixedness at work, and there is an additional problem: stones appear to be rather lighter in general, in comparison to the ones in Boi Yai, at least according to Figure 8: it is unclear is a rock of less than 100g is enough to break a snail, so macaques could be drawn there to take heavier stones. On that note, there is confusion about the difference in weight that can be found in the figures. In Figure 8, weight ranges from less than 100 to around 200g. On Figure 5, weights are rather around 400-800g. If these are estimates obtained from the model, the authors should say it.

4) More in-depth treatment of the archaeological and primate archaeological literature. The mention of pounding tools as part of the toolkit of the LCA could likely include references to Rolian and Carvalho, 2017. The authors should also reference John Shea's recent paper (Evol Anthropology, 2018) about tool use as habitual versus occasional, and perhaps Thompson's (2019 Current Anthropology) piece on pounding tool use as the precursor to flaked stone tool use. Elsewhere in the Introduction the authors mention the idea of the extraordinary advantages that tool use provides those individuals that implement these strategies. While this makes sense to me theoretically, there are only a few examples where this has been explicitly tested. One example would be Morgan et al., 2016 who explicitly tested the significance of tools. Similarly, the final sentence of paragraph four on the Introduction that mentions that fact that primate tool use leaves an archaeological record could benefit from references to Carvalho et al., 2012. This would however call for a review of other papers that have been published on the topic, and what this particular paper brings in addition.

5) Clarification and de-emphasis of multiple use tools (or removal of this argument). The argument on multitool or curated (in the sense used by the authors here, but see point 6 below) usage is, right now, too weak. The idea of multi-use tools is interesting, but cannot be used here as one of the main arguments for differences between groups. How do we know it was used on multiple prey-species? The authors state that if a specimen could not be identified as being associated with a single species then it was not included in the study, so this would contradict that. Furthermore, there are only 7 stones observed, and the researchers haven't been able to observe that they were really used for different purposes. In general, the discussion of the Boi Yai island tool is problematic (L298+). Following the argument of the authors, there is not much use wear for snails because little force is needed. But then, the same argument should apply for oysters: because they are bigger, then more force should apply and thus likelihood of use wear increases. And once again, functional fixedness or conservatism, which are both mechanisms acting at the individual (rather than 'cultural') level, can kick in, and explain the re-use, because it is unclear what macaques consider the same substrate and what is not. In fact, it is unclear whether re-use is evidence of culture in any animal species so far (see for example Hobaiter et al., 2014, Plos Biology), rightly so because of the possibility of individual characteristics (it is much more parsimonious than to claim norms for instance). In addition, if some tools were indeed reused, how can the authors be sure that the findings are not contributed by single or several individuals? The authors do not have behavioral data from this population to confirm this, but observations of tool use by macaques at other sites should make them aware that some individuals more than others have stronger tool preference and retain tools for longer periods of time than others. This possibility must at least be discussed. The authors interpret scarring on different tool surfaces as a repurposing of the tool for different food items. This is based on previous findings on one population of macaques on Piak Nam Yai island that tool points were predominantly used on oysters while faces were used on gastropods (Haslam et al., 2013, Tan et al., 2015). More current research on yet another population on Koram Island however, is finding that the macaques there predominantly use tool points regardless of whether or not they were processing sessile oysters or motile gastropods. This work is preparation and not yet published, so it is understandable that the authors not consider this in their interpretations of use wear at present, but it does signal a need for greater caution. A final possibility might be that the difference in use intensity between two populations is related to differences in tidal conditions. From Figure 1, it appears that the Boi Island site is relatively more sheltered from the open sea compared to Lobi Bay. We know tool-aided coastal foraging can take place only during the low tides, and used tools left along the shore can be washed away by wave action. Is it possible that the macaques on Boi Island have greater access to the coast, and that tools on Boi Island are less likely to be washed away by wave action?

6) Clarification of terminology to connect archaeology and animal behavior literatures. The authors use the word "curation" several times in the manuscript. This is an interesting use for the word in this context, but the authors need to be more explicit about what they are describing. The word curation in relation to tool use stems from Lewis Binford's investigation into the variance in resource and land use by modern hunter-gatherers and his gatherer vs. forager dichotomy (Binfrod, 1973; 1979; 1980). However, largely stemming from the vague nature of this description, Shott and others have reviewed this concept. The most current concept of curation in archaeological contexts is the realized utility of a tool relative to the actual utility of a tool. This is after an extensive review of what is meant by curation by Shott (1996 as well as subsequent reviews of use life Shott and Sillitoe, 2006). It would be exciting to see primate archaeology begin to use the vocabulary of Paleolithic archaeologists in this way, but if the authors intend to do so they should describe what they mean by curation and implement it into this manuscript (with references to the original literature). If they are not going to do that, then they should use a different vocabulary altogether. For example, the term "repeated curation" is not how archaeologists would use this term, and curation would not be something that is repeated. This would conflate curation with re-use. If they use the term in the way that Shott describes it they could quite easily accommodate the idea that the Boi Ya macaques are extracting a greater amount of utility from the tools then those at Lobi Bay. In archaeological literature this would be described as differing levels of use life and site use intensity usually associated with ecological differences. However, I think the authors have convincingly shown that this is unlikely, especially since the greater levels of intensity of use are not seen in all species. Another example of terminology that does not connect the two bodies of literature is in how the authors describe transects as "background material" which is used to describe the locally available stones that were not selected for tool use. It is excellent that they are providing the data for this, which is directly comparable to archaeological data which do the same thing. However, in that case it should be referred to as "locally available raw material" as opposed to "background material" because it will make conversations between archaeologists and primate archaeologists more productive to be using the same vocabulary.

7) Description of raw material. The description of the rock types used by the macaques indicates that they use mostly limestone but also use calcite (which seems like a very bad option for cracking shells but I guess it is what you need to use if you have no other options). However, they also mention the use of granites and other volcanics. This is a bit concerning and something that must be addressed, because stone raw materials matter so much to tool selection. This is especially because the main differences identified between the two study sites is intensity of tool use. Granites and limestones will have very different mechanical properties. There could be a scenario whereby the differences in raw material structured the degree of utilization because some rocks were softer and would need to be utilized further. However, if the proportions of different lithologies were the same on both islands then this would be a moot point. The authors might also reconsider use of the word "traction" when they referred to the process by which these non-limestone rocks made it to the island. Do they mean here that they were brought to the coast as cobbles/pebbles by wave action, but are not actually lithic materials found on the islands themselves as outcrops?

8) Clarification of the tidal and intertidal significance. There needs to be a little bit better explanation as to why the intertidal versus the tidal zone matters. It is clear that the authors went out of their way to document this in great detail, but it is not quite clear why. Are there ecological differences between these zones that might impact the intensity of use of the resources in them? There do appear to be compelling differences in the variability of rock size availability at the two different locales, but why does this matter in terms of which rocks would be selected by macaques?

9) Application to the archaeological record in the discussion and conclusions. The conclusion mentions there are few ways of testing cultural transmission in the archaeological record. However, this is technically also true of this study. What the authors have documented is differences that archaeologists usually identify as functional (degree of utilization). Several studies have focused on degree of utilization as measures of site use intensity (e.g. Dibble, 1995 in Dibble and Lenoir volume). However, as the authors here have argued, these differences may be cultural IF we can determine that ecological factors and genetic factors can be ruled out. The genetic factors will unlikely be identified in the archaeological record. However, the contextual ecological factors may be identified. The one caveat with the Luncz et al. is that these are groups on islands. Might we expect similar kinds of diversity in Paleolithic contexts whereby information can pass between groups more easily? In Paleolithic contexts the assemblages are heavily time averaged. Given what Luncz and colleagues have already shown with resource depression that tool use can begin to wane because the prey items decrease. What are the possible time depths of the behaviors that macaques are exhibiting now? What would the time-averaged signature of the behaviors on Lobi Bay look like? Some discussion of this would make the paper even more archaeologically applicable.

Title

The title should reflect the new emphasis on the application to archaeology.

https://doi.org/10.7554/eLife.46961.020

Author response

Essential revisions

Below is a summary of the key changes that should take place to make this manuscript acceptable for publication in eLife:

1) Clear integration of this research with the previous study. As a Research Advance, a more substantial review of the results of the previous study would be beneficial. How and why are these studies different and how are they alike? It is not clear enough to readers who are unfamiliar with macaque tool use sites that the two studies were conducted with two entirely separate populations at different sites. Also, since the original study found that ecological parameters do appear to be driving some of the variation in tool use, the deviation from this in the current study becomes even more interesting. The previous study capitalized upon the size differential between the two macaque populations to examine whether habitual tool-aided exploitation by macaques could lead to reduction in size and abundance of shellfish prey. This contributed to the long-standing debate in coastal archaeology as to whether the reduction in shellfish sizes in the Middle Stone Age could have been due to human exploitation of marine resources. Here, this study is showing that primate tool artifacts also vary between populations. The authors should emphasize these potential contributions via primate archaeology, and how further behavioral work could help with interpreting the lithic record (particularly with respect to the concerns raised above).

We have expanded the review of our previous study and further highlighted the importance of our new findings.

2) Re-framing the significance of the work. This should be with respect to what is directly observable, and more specifically address its implications for the archaeological record of early hominins, rather than simply labelling patterns as cultural or non-cultural.

We have reframed the significance of this work in regard to the implications for the archaeological record.

3) Clarifying possible alternative explanations than "culture". The lack of information about the sex and age of individuals does not invalidate this study, and in fact makes it more akin to archaeological work. However, all possible sources of variation should be mentioned as possible explanations. One possibility is providing information on the number of individuals or different age on the two islands. If the islands do not differ in their age and sex demographics then it would seem unlikely that this is a reason for these differences. However, the authors should still discuss other possibilities. How confidently can we disentangle the effects of use frequency from users' strength/proficiency/strike accuracy on wear patterns?

We have included additional information on the demographics of the studied populations and highlighted possible alternative explanations for the observed differences in tool variation in regard to the primate archaeological record.

Macaques do differ in proficiency, and some tool users will be able to crack food items with fewer strikes or might be more accurate with their strikes. These users would conceivably create fewer and less diffuse use wear than users who require many strikes to crack food items. The latter might create use wear that would be scored as more intense or extensive but interpreting this as re-use would be erroneous. Perhaps differences in use wear patterns between both populations could indicate differences in population level proficiency? In spite of the authors' claim that the snails are morphologically similar, there could be subtle ecological differences that could explain a different use of stones by the monkeys on the two islands. For instance, there might be a higher proportion of a certain type of snail in one of the two islands, which would make the re-use of certain stones more efficient.

The amount to tools for each prey species that we found was approximately similar. In order to identify differences in use wear intensity we applied a modified use wear grading system published by Haslam et al., 2013. This system was initially developed to assign the behaviour to the tool but not to assess the intensity of use. We added additional clarification to the Materials and methods section of the paper to address this methodology.

There is also a cognitive argument that can be made about tool selection, e.g. functional fixedness. Macaques on Boi Yai have to use the heavier tools to crack oysters because they are bigger than on the other island. That means the heavier tools can be functionally fixed to be used with oysters. Macaques then use lighter rocks (the ones that are not functionally fixed) to crack the snails. On the other hand, on Lobi Bay, there is no functional fixedness at work, and there is an additional problem: stones appear to be rather lighter in general, in comparison to the ones in Boi Yai, at least according to Figure 8: it is unclear is a rock of less than 100g is enough to break a snail, so macaques could be drawn there to take heavier stones.

We highlight in the text that tool selection is partly influenced by the prey size. This is consistent with other studies which show that tool size is adjusted to prey size. However this fact holds true for both populations. Raw material availability however is not a limiting factor for tool selection. We added more details regarding this to the text but highlight the fact that this factor does not explain all differences found in their tool evidence.

On that note, there is confusion about the difference in weight that can be found in the figures. In Figure 8, weight ranges from less than 100 to around 200g. On Figure 5, weights are rather around 400-800g. If these are estimates obtained from the model, the authors should say it.

The plot is based on the original data. Figure 9 is not showing the range of the original data, only the 95% confidence interval. We have clarified this in the text.

4) More in-depth treatment of the archaeological and primate archaeological literature. The mention of pounding tools as part of the toolkit of the LCA could likely include references to Rolian and Carvalho, 2017. The authors should also reference John Shea's recent paper (Evol Anthropology, 2018) about tool use as habitual versus occasional, and perhaps Thompson's (2019 Current Anthropology) piece on pounding tool use as the precursor to flaked stone tool use.

We have included the suggested literature.

Elsewhere in the Introduction the authors mention the idea of the extraordinary advantages that tool use provides those individuals that implement these strategies. While this makes sense to me theoretically, there are only a few examples where this has been explicitly tested. One example would be Morgan, 2016 who explicitly tested the significance of tools. Similarly, the final sentence of paragraph four on the Introduction that mentions that fact that primate tool use leaves an archaeological record could benefit from references to Carvalho et al., 2012. This would however call for a review of other papers that have been published on the topic, and what this particular paper brings in addition.

We have deleted this statement.

5) Clarification and de-emphasis of multiple use tools (or removal of this argument). The argument on multitool or curated (in the sense used by the authors here, but see point 6 below) usage is, right now, too weak. The idea of multi-use tools is interesting, but cannot be used here as one of the main arguments for differences between groups. How do we know it was used on multiple prey-species? The authors state that if a specimen could not be identified as being associated with a single species then it was not included in the study, so this would contradict that. Furthermore, there are only 7 stones observed, and the researchers haven't been able to observe that they were really used for different purposes. In general, the discussion of the Boi Yai island tool is problematic (L298+). Following the argument of the authors, there is not much use wear for snails because little force is needed. But then, the same argument should apply for oysters: because they are bigger, then more force should apply and thus likelihood of use wear increases. And once again, functional fixedness or conservatism, which are both mechanisms acting at the individual (rather than 'cultural') level, can kick in, and explain the re-use, because it is unclear what macaques consider the same substrate and what is not. In fact, it is unclear whether re-use is evidence of culture in any animal species so far (see for example Hobaiter et al., 2014, Plos Biology), rightly so because of the possibility of individual characteristics (it is much more parsimonious than to claim norms for instance). In addition, if some tools were indeed reused, how can the authors be sure that the findings are not contributed by single or several individuals? The authors do not have behavioral data from this population to confirm this, but observations of tool use by macaques at other sites should make them aware that some individuals more than others have stronger tool preference and retain tools for longer periods of time than others. This possibility must at least be discussed. The authors interpret scarring on different tool surfaces as a repurposing of the tool for different food items. This is based on previous findings on one population of macaques on Piak Nam Yai island that tool points were predominantly used on oysters while faces were used on gastropods (Haslam et al., 2013, Tan et al., 2015). More current research on yet another population on Koram Island however, is finding that the macaques there predominantly use tool points regardless of whether or not they were processing sessile oysters or motile gastropods. This work is preparation and not yet published, so it is understandable that the authors not consider this in their interpretations of use wear at present, but it does signal a need for greater caution.

We have removed the multi-use tool argument from the manuscript.

A final possibility might be that the difference in use intensity between two populations is related to differences in tidal conditions. From Figure 1, it appears that the Boi Island site is relatively more sheltered from the open sea compared to Lobi Bay. We know tool-aided coastal foraging can take place only during the low tides, and used tools left along the shore can be washed away by wave action. Is it possible that the macaques on Boi Island have greater access to the coast, and that tools on Boi Island are less likely to be washed away by wave action?

This is unlikely, Lobi Bay (as it lies within a bay) is essentially more sheltered than Boi Yai island. Further research will test for the duration of tool evidence along the shore. We included this discussion into the manuscript.

6) Clarification of terminology to connect archaeology and animal behavior literatures. The authors use the word "curation" several times in the manuscript. This is an interesting use for the word in this context, but the authors need to be more explicit about what they are describing. The word curation in relation to tool use stems from Lewis Binford's investigation into the variance in resource and land use by modern hunter-gatherers and his gatherer vs. forager dichotomy (Binfrod, 1973; 1979; 1980). However, largely stemming from the vague nature of this description, Shott and others have reviewed this concept. The most current concept of curation in archaeological contexts is the realized utility of a tool relative to the actual utility of a tool. This is after an extensive review of what is meant by curation by Shott (1996 as well as subsequent reviews of use life Shott and Sillitoe, 2006). It would be exciting to see primate archaeology begin to use the vocabulary of Paleolithic archaeologists in this way, but if the authors intend to do so they should describe what they mean by curation and implement it into this manuscript (with references to the original literature). If they are not going to do that, then they should use a different vocabulary altogether. For example, the term "repeated curation" is not how archaeologists would use this term, and curation would not be something that is repeated. This would conflate curation with re-use. If they use the term in the way that Shott describes it they could quite easily accommodate the idea that the Boi Ya macaques are extracting a greater amount of utility from the tools then those at Lobi Bay. In archaeological literature this would be described as differing levels of use life and site use intensity usually associated with ecological differences. However, I think the authors have convincingly shown that this is unlikely, especially since the greater levels of intensity of use are not seen in all species. Another example of terminology that does not connect the two bodies of literature is in how the authors describe transects as "background material" which is used to describe the locally available stones that were not selected for tool use. It is excellent that they are providing the data for this, which is directly comparable to archaeological data which do the same thing. However, in that case it should be referred to as "locally available raw material" as opposed to "background material" because it will make conversations between archaeologists and primate archaeologists more productive to be using the same vocabulary.

We have added the suggested literature and discussed how curation could be applied to primate archaeology.

7) Description of raw material. The description of the rock types used by the macaques indicates that they use mostly limestone but also use calcite (which seems like a very bad option for cracking shells but I guess it is what you need to use if you have no other options). However, they also mention the use of granites and other volcanics. This is a bit concerning and something that must be addressed, because stone raw materials matter so much to tool selection. This is especially because the main differences identified between the two study sites is intensity of tool use. Granites and limestones will have very different mechanical properties. There could be a scenario whereby the differences in raw material structured the degree of utilization because some rocks were softer and would need to be utilized further. However, if the proportions of different lithologies were the same on both islands then this would be a moot point. The authors might also reconsider use of the word "traction" when they referred to the process by which these non-limestone rocks made it to the island. Do they mean here that they were brought to the coast as cobbles/pebbles by wave action, but are not actually lithic materials found on the islands themselves as outcrops?

We have modified the text of the manuscript to make explicit that there is no calcite included in this study. Additionally, we have expanded Table 1. to show the frequencies of hammerstones of each raw material for both islands. Granite hammerstones only account for a single artefact and as such does not have an effect on the statistical analysis presented in this study. We added a short discussion regarding how granite material might have appeared at the field sites.

8) Clarification of the tidal and intertidal significance. There needs to be a little bit better explanation as to why the intertidal versus the tidal zone matters. It is clear that the authors went out of their way to document this in great detail, but it is not quite clear why. Are there ecological differences between these zones that might impact the intensity of use of the resources in them? There do appear to be compelling differences in the variability of rock size availability at the two different locales, but why does this matter in terms of which rocks would be selected by macaques?

We have clarified this in the text.

9) Application to the archaeological record in the discussion and conclusions. The conclusion mentions there are few ways of testing cultural transmission in the archaeological record. However, this is technically also true of this study. What the authors have documented is differences that archaeologists usually identify as functional (degree of utilization). Several studies have focused on degree of utilization as measures of site use intensity (e.g. Dibble, 1995 in Dibble and Lenoir volume). However, as the authors here have argued, these differences may be cultural IF we can determine that ecological factors and genetic factors can be ruled out. The genetic factors will unlikely be identified in the archaeological record. However, the contextual ecological factors may be identified. The one caveat with the Luncz et al. is that these are groups on islands. Might we expect similar kinds of diversity in Paleolithic contexts whereby information can pass between groups more easily? In Paleolithic contexts the assemblages are heavily time averaged. Given what Luncz and colleagues have already shown with resource depression that tool use can begin to wane because the prey items decrease. What are the possible time depths of the behaviors that macaques are exhibiting now? What would the time-averaged signature of the behaviors on Lobi Bay look like? Some discussion of this would make the paper even more archaeologically applicable.

We have added this point of discussion to the manuscript.

Title

The title should reflect the new emphasis on the application to archaeology.

We have adjusted the title to reflect a more archaeological take on the data.

https://doi.org/10.7554/eLife.46961.021

Article and author information

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
    ORCID icon "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

Funding

Leverhulme Trust

  • Lydia V Luncz

Primate Center Göttingen

  • Lydia V Luncz

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Acknowledgements

The National Research Council of Thailand permitted LVL and MS to conduct research in Thailand, and the Department of National Parks, Wildlife and Plant Conservation gave permission to enter and conduct research in Ao Phang Nga National Park. We thank the National Park rangers of Boi Yai Island for their logistical support. We thank the editor, reviewers Amanda Tan and Jessica C Thompson, two additional anonymous reviewers and Alexander Mielke for helpful comments on this manuscript. This research was funded by the Leverhulme Trust and by the German Primate Centre in Goettingen, Germany. During writing TP was supported by the British Academy Postdoctoral Fellowship (pf170157).

Senior Editor

  1. Detlef Weigel, Max Planck Institute for Developmental Biology, Germany

Reviewing Editor

  1. Jessica C Thompson, Yale University, United States

Reviewers

  1. Jessica C Thompson, Yale University, United States
  2. Amanda Tan, Nanyang Technological University, Singapore

Publication history

  1. Received: March 18, 2019
  2. Accepted: October 4, 2019
  3. Version of Record published: October 22, 2019 (version 1)

Copyright

© 2019, Luncz et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

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