Monkeying Around: Why Capuchins Accidently Making StoneTools is Causing a Headache

Updated: Aug 12, 2020

Article by Andoni Sergiou, MSc.

“…man alone relies mainly on extra-corporeal equipment of his own making which can be quickly changed or discarded as circumstances dictate. It is in making tools that man is unique.” – K. P. Oakley on Man the Toolmaker, 1944

Man the toolmaker. Oh yes, man the hunky, meat-eating, testosterone-fuelled, Flintstone car-driving toolmaker. It’s hard to read these treasured pieces from the old anthropological literature without detecting hints of the biased caveman stereotype from popular culture peeking through in-between the lines. Fortunately for anthropology though, and rather ironically as fate would have it, the myth of man the tool maker started to unravel during the 1960s after a female primatologist, Dr Jane Goodall, revealed to the world that her Gombe chimpanzees intentionally modified sticks to help procure ants as tasty snacks. Since then, the endless flood of animal tool and cultural transmission studies has made it clear that tool manufacture and use does not make us exceptional amongst the Earth’s wonderous collage of diverse organisms.

From coconut armoured octopi to fly-swatting elephants, behaviour that can and has been defined as tool use has been discovered in many. Whilst some of these species rely more heavily upon instinctual ingenuity for their toolusing proficiency, like coconut octopi, others instead combine their innate intelligence with high fidelity social learning, such as New Caledonian crows and the two-year old version of myself pictured above (although crows are admittedly still better at using a calculator than I am!). Marine biologists have even stumbled across pods of dolphins sharing small doses of puffer fish venom as a recreational instrument; they literally form communal gatherings to “pass the puffer fish on the left-hand side”! Hey, it’s 2020, the most enjoyable year of the century, who can blame them?

In the light of such discoveries (and of psychedelic dolphins), it comes as no surprise that anthropology’s ever rising bar of human uniqueness has moved from making and using tools with a few basic modifications (e.g. combining two rocks to crack open nuts in chimpanzees) to manufacturing complex tool kits involving demanding multi-step behavioural sequences. This new bar of human uniqueness comes with the additional assumption that these demanding tool making sequences are deliberate, or ‘intentional’, on the part of the tool maker, and thus often require a greater degree of cognitive sophistication. But given the precedent of Dr. Goodall’s findings during the intellectual climate and assumptions of the 1960s, one can’t help wondering whether even this assumption is a safe one? Turns out, it isn’t.

Why we thought that making stone tools was cognitively challenging

Until recently, it was thought that making stone tools epitomised the notion of a demanding multi-step tool making sequence. Now granted, stone tool kits like the ones possessed by our pre-sapiens ancestors don’t necessarily look super impressive when sat next to a SpaceX Falcon rocket, but they can be extremely arduous, cognitively taxing, and downright dangerous to make in their own right. For modern day university students, learning how to successfully turn a rock into a handheld tear-shaped axe can take hundreds of hours, and that’s with the best rock resources already brought to them, explicit teaching with language, really patient tutors, protective gear, and no scary predators trying to eat them! Now try to imagine that you are not Chuck Norris for second, and instead you are a naked or semi-naked ancient human ancestor with blunt canines and no claws to speak of. You have to spend a serious amount of time choosing and possibly carrying around some heavy stone cores. Then you have to bash those stones together noisily, at precise angles, and in the correct order of moves to make some sharp cutting flakes that may splinter off and get lodged in your eyes… not the kind of thing you want to be doing if your idea is to avoid being the lunch of a large cat or bird of prey, right?

Taking these multiple steps and costs of making stone tools into account (the long periods of learning with delayed reward, the possibility of ending up in sabre-tooth kitty litter… you know, the usual), this whole process of making stone tool kits would indeed seem to be a highly intentional business. That is, even for the earliest and most basic of stone tools that paleoarchaeologists have found, it has often been assumed that whoever made them did so with the express intention and cognitive foresight of making some form of cutting, digging, or hammering implements. After all, why go to the extraordinary lengths of making sharp stone flakes if not to make such tools? You can imagine the looks on everyone’s faces when yet another team of primatologists found an entire troop of wild capuchin monkeys regularly making perfect stone flakes… that they never even use as tools!

Enter the capuchins!

Bearded capuchins (pictured below) have long been stars of the nonhuman tool-using world, and are well known among primatologists for their capacity to crack open nut shells using rocks that can weigh up to a third of their own body weight! Nevertheless, it is typically the chimpanzees that steal the lime light when it comes to the nut cracking literature. Indeed, chimpanzee behaviour is frequently used by anthropologists as a very rough approximation for the behaviour once exhibited by our own ancestors. Some researchers have even gone as far as suggesting that nut cracking-esque behaviour may have given rise to stone cutting tools in our ancestors after flakes splintered off of hammer stones when missing a nut and striking either the ground or an anvil stone upon which a nut can be placed. Yet careful comparison has shown that the shapes and scars of early stone flakes found within the paleoarchaeological record do not match those that characterise the flakes produced when chimpanzees are nut cracking. With our chimpanzee cousins unable to produce rudimentary stone flakes this chapter seemed closed… our mistake! If capuchins are anything, they are surprising and bizarre. I studied the cheeky fur balls for my undergraduate dissertation, trust me!

This initial discovery actually dates back to the Covid-free days of 2014, when Doctor Tomos Proffitt and colleagues observed a troop of wild bearded capuchin monkeys engaging in a strange ritual at Serra da Capivara National Park in Brazil. These capuchins would grab loose A bearded capuchin monkey (Sapajus libidinosus) cracking open a pod with a hammer stone. Just look at those biceps. cobbles from mixed earthy mounds and then use these as hammers to pound ‘target’ cobbles that still laid half exposed inside the mounds. The teams’ video footage showed that in roughly half of the recordings the monkeys would lick the cobbles after striking a targeted cobble. It still remains an open mystery as to why the monkeys do this (p. s. any primatologists out there, this is your chance for major research!), but it has been theorised that the powdered cobbles provide a rich source of dietary silica to meet micronutrient requirements.

What intrigued the team more were the flakes and core stones produced as a by-product of this behaviour, which the capuchins never seemed to interact with. Since then, analysing and sharing these stone fragments with paleoarchaeologists has shown that much of the scarring patterns on the hammer stones and flakes matches those of an early stone tool type known as the ‘Oldowan industry’, which first appeared in Ethiopia some 2.6 million years ago. This discovery is a double-edged sword; on the one hand it provides some clues as to how our own ancestors may have first discovered cutting flakes, but on the other many of these scarring patterns are used by paleoarchaeologists as criteria for identifying intentional tool manufacture and cognitive planning, both of which were apparently absent in the capuchin troop whilst making the flakes. In essence, lurking beneath all the cross-species and archaeological data accumulated since the 1960s was a highly watered down and cerebral (hu)man the tool maker, still tugging away at the assumptions of contemporary academia.

As shown in the image traces displayed below (for the original images see the link to the study provided at the end of the article), the target cobbles have a scar at the point of impact followed by small ripples called ‘conchoidal ripples’. These are found in both the capuchin flakes (on the left) and those of our ancestors (on the right) because the force of a directional hammer stone strike spreads throughout a target in a cone of kinetic energy – hence the conchoidal ripples. The capuchins also produced several interlocking flake removals one after another called a ‘lithic reduction sequence’, which can be seen in first minute of this 3D computer model.

We used to see these reduction sequences as evidence for a kind of basic cognitive plan or strategic recipe for making multiple flakes from a single stone core in our ancestors, but the capuchins have shown us that no such strategies are needed to make this pattern.

The fact that many of the capuchin flakes and hammer stones meet standard paleoarchaeological criteria for intentional tool making and are indistinguishable from many Oldowan samples raises many questions for the future. Are all of the Oldowan flakes really tools or are some of them by-products of other less intentional behaviours? In some cases, the Oldowan samples have been found near animal fossils that exhibit characteristic cutting marks, and this context helps us when interpreting the purpose of stone flake deposits. But in the absence of such fossil animal evidence, the capuchin data has shown us that we cannot rely on the tools alone to diagnose intentional production for all samples. Furthermore, unless the tools are found in association with hominin remains (fossils of our ancient pre-sapiens ancestors and their sister species), we cannot rule out the possibility that ancient non-hominin primates made the tools. The Brazilian national park in which the capuchin troop currently lives is also home to a series of archaeological sites containing hammer stones that date back as far as 700 years (recording more than 100 generations of capuchin cultural transmission). For paleolithic stone tools then, it can become a challenge as to whose archaeology we are actually examining: hominin? Ape? Monkey? One thing is for sure, it’s causing paleoarchaeologists a headache!

I rest my case. Cheeky fur balls.

Original research article: Wild monkeys flake stone tools

Footage downloads come from the open source supplementary material of this research article.

Capuchin image marked for redistribution with modification on Flickr.

Tool use collage contains Wikimedia images collected and put together by Andoni Sergiou

Short bio: Andoni is an alumnus from the University of Kent specialising in Collective Brain theory. He uses computer simulations and artificial intelligence to study how human social networks process cultural information over multiple generations in order to produce cultural adaptations, technological innovations, and socioecological development.

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