Nut-cracking Brazilian monkeys could teach us about human evolution

. Jan 15, 2020
evolution monkeys farm brazil

Human beings were always defined as the “tool-maker species.” But our domination over this important skill was brought into question in the 1960s when renowned anthropologist Jane Goodall discovered that chimpanzees will pick and modify stems of grass in order to use them to collect termites. Her findings challenged Homo sapiens’ very place in the biological order. Since Dr. Goodall’s research, scientists’ knowledge of tool use by animals has expanded exponentially. We now know that monkeys, crows, parrots, pigs, and many other animals can use tools, and research on animal tool use has changed our understanding of how animals think and learn. Studying this process of animals using tools may also provide clues to solve the mysteries of human evolution.

</p> <p>Our human ancestors’ shift to making and using tools is linked to evolutionary changes in hand anatomy, a transition to walking upright, and <a href="">increased brain size</a>. But using found stones as pounding tools doesn’t require any of these advanced evolutionary traits, in fact, it likely came about before humans began to manufacture tools. By studying this percussive tool use in monkeys, researchers such as my colleagues and I can infer how early human ancestors practiced the same skills before the evolution of hands, posture, and brains.</p> <h2>Monkeys using tools</h2> <p>Understanding the memory, thinking, and problem-solving abilities of wild animals is unsurprisingly difficult. In experimental research where animals are asked to perform a behavior or solve a problem, there should be no distractions, such as the threat of predation. But wild animals come and go as they please, over large spaces, and researchers cannot control their environment.</p> <p>However, some field sites provide a unique opportunity to test wild animals’ cognition. Fazenda Boa Vista in the northern Brazilian state of <a href="">Piauí</a>, is one of those sites. There, wild bearded capuchin monkeys (<em>Sapajus libidinosus</em>) naturally use stones and anvils to crack open nuts.</p> <figure class="wp-block-image"><img src="" alt="monkeys brazil evolution" class="wp-image-30287" srcset=" 1000w, 300w, 768w, 610w" sizes="(max-width: 1000px) 100vw, 1000px" /><figcaption>Capuchin nut-cracking monkeys offer hints about human evolution. Photo: Luca Antonio Marino, CC BY-ND</figcaption></figure> <p>Along with fruit, insects, fungi, and tubers, the capuchin monkeys of Fazenda Boa Vista opportunistically crack open nuts as an additional food source. Though these monkeys only spend about 2 percent of their time using tools to access food, 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 cannot be cracked open without the use of tools. The monkey population 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 percent of their body weight.</p> <p>These bearded capuchin monkeys were the first South American primates that scientists observed using tools, only spotting this behavior in 2003. Since this discovery, researchers have been studying the decision-making and strategies involved in the capuchins’ use of stone tools.</p> <p>Because using stones to pound open food looks remarkably like what anthropologists imagine one of the earliest forms of human tool use, researchers are studying these monkeys as a way to understand the evolution of our species.</p> <h2>What happens with a new tool?</h2> <p>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 their hands around a box to decide how best to lift it, monkeys in Fazenda Boa Vista also feel their way through tool use.</p> <p>First, we placed unfamiliar stones and palm nuts around naturally-occurring wood or stone anvils. As the monkeys frequently use stones to crack open these tough nuts on the anvils, it was only a matter of time before they tested them out.</p> <figure class="wp-block-embed-youtube wp-block-embed is-type-video is-provider-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio"><div class="wp-block-embed__wrapper"> <span class="embed-youtube" style="text-align:center; display: block;"><iframe class='youtube-player' type='text/html' width='1200' height='675' src=';rel=1&#038;fs=1&#038;autohide=2&#038;showsearch=0&#038;showinfo=1&#038;iv_load_policy=1&#038;wmode=transparent' allowfullscreen='true' style='border:0;'></iframe></span> </div></figure> <p>We filmed slow-motion videos of 12 monkeys cracking nuts to understand how monkeys adjust to using an unfamiliar tool. The idea, stemming from perception-action theory, is that monkeys may obtain helpful information about the tool—such as its weight, and where they can securely hold it—by touching it before use. Similar to testing a hammer before banging a nail, this information may then help the monkeys to strike the nut forcefully and accurately.</p> <p>Back in the U.S., we spent months carefully watching the slow-motion videos and recording the monkeys’ quick behaviors while using tools. The videos showed that for nut-cracking, monkeys grasp the sides of a stone, lift it to shoulder height, quickly move their hands to the top of the stone, and then bring it down on the nut.</p> <p>Given that the stones can weigh about half as much as an adult female monkey, this is an impressive feat. But it’s not always done perfectly. If the monkey’s grip isn’t right, it might lose control of the stone; if the stone comes down at an angle, the nut is likely to fly off the anvil. When this happens, the monkeys lose precious time and energy trying to achieve their goal.</p> <p>What we found, however, is that 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.</p> <p>This same sort of haptic perception—the process of coming to understand an object by moving it around—plays a key role in our own ability to use tools with dexterity. In human beings’ evolutionary past, increasingly refined haptic perception may have contributed to advancing tool use.</p> <p>Studying how animals think about and use tools offers scientists like me an exciting glimpse into what human evolution may have looked like, while also helping us to better understand animals in their own right.</p> <hr class="wp-block-separator"/> <div class="wp-block-image"><figure class="alignleft"><img src="" alt="the conversation brazil article" class="wp-image-398" srcset=" 300w, 768w, 1024w" sizes="(max-width: 300px) 100vw, 300px" /></figure></div> <h6 style="text-align:right">Originally published on<br><a href=""><strong>The Conversation</strong></a></h6> <img src="" alt="The Conversation" width="1" height="1" style="border: none !important; box-shadow: none !important; margin: 0 !important; max-height: 1px !important; max-width: 1px !important; min-height: 1px !important; min-width: 1px !important; opacity: 0 !important; outline: none !important; padding: 0 !important; text-shadow: none !important" /> <p>

Kristen S. Morrow

Ph.D. Student in Anthropology and Integrative Conservation, University of Georgia

Our content is protected by copyright. Want to republish The Brazilian Report? Email us at [email protected]