Human beings used to be defined as “the tool-maker” species. 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. Tool use among wild animals holds interest for many scientists concerned with the origins and maintenance of skilled behaviours.
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. 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.
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. Since then scientists’ knowledge of animal tool use has expanded exponentially.
For years, scientists at the Smithsonian Tropical Research Institute (STRI) in Panama had whispered about the remote island where monkeys used stone tools. A botanist had witnessed the phenomenon during a long-ago survey, but, being more interested in flora than fauna at the time, she couldn’t linger to investigate. A return to the site would require new funds, good weather for a treacherous 35-mile boat ride, and days of swimming, hiking and camping amid rocky, wave-pounded shorelines and dense tropical forest.
“For a while, it kind of just stayed a rumor,” said Brendan Barrett, a behavioral ecologist at the Max Planck Institute for Ornithology in Germany and a visiting researcher at the STRI.
But when Barrett and his colleagues finally arrived at Jicaron Island in Panama’s Coiba National Park last year, what they found was well worth the effort: White-faced capuchin monkeys were using stones almost half their body weight as hammers to smash open shellfish, nuts and other foods.
“We were stunned,” said Barrett, lead author of a new paper on the discovery posted on the preprint website bioRxiv.
They practice a variety of complex behaviors. They rub plants over their bodies, potentially as medicine; defend themselves with sticks against snakes; play games by passing one another sticks and stones. The monkeys used their stone tools almost every day and often saved stones for repeated use. The monkeys collecting large, heavy cobbles from streams and shorelines and carrying them to broad, flat rocks or logs that could be used as “anvils.” Standing on two feet, using their tails to anchor themselves against the ground or a nearby tree, they monkeys grasp the sides of a stone, lift it to shoulder height, quickly move its hands to the top of the stone, then bring it down on the nut on nuts, crabs, snails and other foods, cracking open hard shells to reveal a tasty morsel. Moreover, the monkeys might avoid these imperfect outcomes by spinning, flipping and doing partial lifts with the stones to test different grips and find the one that’s most likely to be successful. The preparatory lifts didn’t necessarily help the monkeys crack open more nuts, but they might be linked to “tuning” muscular coordination as the monkeys prepare themselves for a heavy lift. Essentially, the preparatory lifts may help the monkeys get a sense of what they’ll need their muscles to do when it comes time to lift the stone and strike the nut in earnest.
Curiously, only male monkeys were seen wielding stone tools, even though females were often foraging nearby. This can’t be explained by females’ smaller size, given that juvenile males were able to use tools.
In 2003, bearded capuchin monkeys were the first South American primates that scientists ever observed using tools. Since this discovery, researchers have been studying the decision-making and strategies involved in capuchins’ stone tool use. Fazenda Boa Vista in Piauí, Brazil, wild bearded capuchin monkeys (Sapajus libidinosus) naturally use stones and anvils to crack open nuts. 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.
Studying how animals think about and use tool offers scientists like me an exciting glimpse into what human evolutionary history may have looked like, while also helping us to better understand animals in their own right.