Monkeys smashing nuts with stones hint at how human tool use evolved
Posted by admin on 9th January 2020

Human beings used to be defined as “the tool-maker” species. But the uniqueness of this description was challenged in the 1960s when Dr. Jane Goodall discovered that chimpanzees will pick and modify grass stems to use to collect termites. Her observations called into question homo sapiens‘ very place in the world.

Since then scientists’ knowledge of animal tool use has expanded exponentially. We now know that monkeys, crows, parrots, pigs and many other animals can use tools, and research on animal tool use changed our understanding of how animals think and learn.

Studying animal tooling – defined as the process of using an object to achieve a mechanical outcome on a target – can also provide clues to the mysteries of human evolution.

Our human ancestors’ shift to making and using tools is linked to evolutionary changes in hand anatomy, a transition to walking on two rather than four feet and increased brain size. But using found stones as pounding tools doesn’t require any of these advanced evolutionary traits; it likely came about before humans began to manufacture tools. By studying this percussive tool use in monkeys, researchers like my colleagues and I can infer how early human ancestors practiced the same skills before modern hands, posture and brains evolved.

Monkeys using tools

Understanding wild animals’ memory, thinking and problem-solving abilities is no easy task. In experimental research where animals are asked to perform a behavior or solve a problem, there should be no distractions – like a predator popping up. But wild animals come and go as they please, over large spaces, and researchers cannot control what is happening around them.

However, some field sites provide a unique opportunity to test wild animals’ cognition. Fazenda Boa Vista in Piauí, Brazil is one of those sites. Here, wild bearded capuchin monkeys (Sapajus libidinosus) naturally use stones and anvils to crack open nuts.

Young capuchin monkey observes adult male monkey eating nuts cracked open using a stone tool.
Luca Antonio Marino, CC BY-ND

Along with fruit, insects, fungi, and tubers, the Fazenda Boa Vista capuchin monkeys opportunistically crack open nuts as an additional food source. Although these monkeys only spend about 2% of their time using tools to access foods, the nuts they eat are an important secondary food item that are available year-round. The challenge is that these nuts have tough shells that can’t be cracked open without a tool. This population of monkeys has figured out how to crack nuts by placing them on a wood or stone anvil and then smashing them with rocks that weigh around 25-50% of their body weight.

These bearded capuchin monkeys were the first South American primates that scientists ever observed using tools – only spotted in 2003. Since this discovery, researchers have been studying the decision-making and strategies involved in capuchins’ stone tool use.

Because using stones to pound open food looks remarkably like what anthropologists imagine one of the earliest forms of human tool use looked like, researchers study these monkeys as a way to understand our own evolutionary past.

What happens with a new tool?

My colleagues and I carried out an experimental field study that focused on understanding how these monkeys prepare to use their tools. Just as a person might move her hands around a box to decide how best to lift it, monkeys at this site feel their way through tool use.

First, we placed unfamiliar stones and palm nuts around naturally-occurring wood or stone anvils. Since the monkeys frequently use stones to crack open these tough nuts on the anvils, it was only a matter of time before they tried out the experimental stones.