Looking at the traces of wear on tools used by capuchin monkeys

Adrián Arroyo and coworkers have an upcoming paper in the Journal of Archaeological Science: Reports that describes their work quantifying use-wear on stone tools used by capuchin monkeys: “Use-wear and residue analysis of pounding tools used by wild capuchin monkeys (Sapajus libidinosus) from Serra da Capivara (Piauí, Brazil)”. Capuchins use stone tools for many purposes, from pounding nuts and seeds to digging. Arroyo and coworkers show that these different activities leave differential traces of wear on the tools themselves.

A paragraph from the introduction gives a helpful background on the range of activities for which the capuchins use stone tools:

Capuchins at [Serra da Capivara National Park (SCNP)] perform a range of stone tool use, including pounding cashew nuts (Anacardium occidentale), processing cactuses to consume the inner pith, and cracking seeds (Manihot sp. and Cordia rufescens) and fruits such as jatobá (Hymenaea courbaril) (Moura and Lee, 2004, Mannu and Ottoni, 2009, Falótico and Ottoni, 2016). In addition, stones are used to dig shallow holes to access small tubers (Thiloa glaucocarpa), roots (Ocotea sp.) and trapdoor spiders (Mannu and Ottoni, 2009, Falótico et al., 2017a), as throwing implements for sexual displays (Falótico and Ottoni, 2013), and as pounding tools to pulverize other quartzite cobbles (Mannu and Ottoni, 2009, Proffitt et al., 2016).

The authors focus some attention in their paper on the use of stone tools for digging. This idea hasn’t really come into the literature on stone tool use by early hominins, but it is obvious that some stone implements would make better digging tools than the small bone points used at sites like Swartkrans and Drimolen. The capuchin stone digging tools did not differ much from the stones used for other purposes, and the use-wear on stones used for digging was less distinctive than those used for pounding. The main observation is that capuchins don’t dig very long with stones, so they don’t have much time to accumulate signs of use-wear.

As usually the case, the paper discusses the relevance of these findings for understanding hominins. The discussion centers the idea that early hominin stone assemblages may represent activities that archaeologists haven’t thought about very much.

Overall, increasing evidence from the primatological stone tools use record supports the possibility that there are a range of archaeologically ‘hidden’ activities that early hominins may have performed in relation to their foraging strategies. Analyses such as the one presented in this study can be taken as a proof of concept for a means of identifying these types of activities. The future application of analyses designed to identify these percussive behaviours on primate stone tools to the hominin archaeological record may elucidate a wider exploitation of plant resources and fallback foods for our earliest ancestors. Having said this, however, merely identifying the characteristics of various percussive activities on a range of stone tools cannot be solely used to interrogate the ESA archaeological record. Future work must develop robust methods of understanding the effect that millennia scale post depositional factors have on ephemeral and fragmented hominins percussive assemblages.

That’s a valuable perspective. We may be subject to a failure of imagination by archaeologists about what stone tool assemblages represent. There is no reason to assume that the mix of activities represented by Pliocene and Early Pleistocene stone tool assemblages is the same as that represented by MSA and Middle Paleolithic toolkits, for example. Paleolithic archaeologists have a tendency to assume that the MSA/Middle Paleolithic toolkits represent improvements on earlier toolkits for basically similar subsistence activities. Looking at a broader array of tool-using behaviors across primates, it’s clear that some activities repeatedly emerge (like nut-cracking) while others are more idiosyncratic (like cactus pounding).

The present distribution of stone tool use by non-human primates is a small window on the traditions that may have existed in these and other primate species. Today’s traditions are not branches of a single technology tree. They are temporary florescences made possible by recurrent innovation and social learning. What matters in evolutionary time may not be the particular resources or inventions, but the behavioral plasticity and creativity that underlie them.

I keep in mind another lesson from the stone tool record of non-human primates that is not usually discussed in this kind of research. Capuchins, chimpanzees, and macaques all use stone tools on foods that make up a relatively small proportion of their overall caloric intake. Many capuchins, macaques, and chimpanzees lack stone tool traditions entirely. The traditions that primatologists observe today are products of local ecology, geology, and the histories of the primate groups. “Traditions” constitute millions upon millions of tools used by thousands and thousands of individuals, which may share little overlap in the activities they carry out.