- Groups of chimpanzees in West Africa use stone tools in distinctly different ways to crack open nuts.
- Researchers used 3D scans to trace wear patterns on the tools, called “hammerstones” and “anvils.”
- The different tool uses may help archaeologists identify signs of early stone tool technology in human ancestors more than 3 million years ago.
The dents and dings on the surfaces of rocks used by chimpanzees to crack open nuts preserve a history of how the animals used the tools and how those patterns change from one troop to the next, researchers reported recently in Royal Society Open Science. This archaeological data can offer insights into the ways similar stone tool technology arose millions of years ago among hominins, the ancestors of humans.
Scientists examined nut-smashing rocks used by chimpanzees (Pan troglodytes verus) in Taï National Park, in southwestern Côte d’Ivoire in West Africa. Measurements and 3D scans of the tools revealed distinct patterns of damage related to how the animals wielded the rocks.
The team then compared these marks to those from prior study that described nut-cracking tools used by chimpanzees about 300 kilometers away in Bossou, Guinea — the only other study to examine tools in this way. Even though the Guinea chimps used their stone tools for the same purpose, differences in the damage patterns clearly showed which tools came from which troop.
The study focused on “percussive” tools used for hitting or pounding. Most archaeological research on tool use has focused on “flake” technology — tools used for cutting and slicing. However, percussive tools probably came first, researchers believe.
Studying the details of how chimpanzees use percussive tools today helps researchers better understand hominin behavior from over 2 million years ago, said lead author and archaeologist Tomos Proffitt of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Hominins then were “running around and cutting things, but also, similarly to chimpanzees, probably cracking open nuts,” Proffitt said.
Researchers have conducted only two long-term studies of how wild chimpanzees use stone tools, for the troops in Taï National Park near the village of Djouroutou, and in Bossou, Guinea.
The Djouroutou chimps crack a variety of nuts, most often soft coula nuts, as well as hard panda and parinaric nuts. The animals use two types of tools: hammerstones and anvils. Hammerstones, aptly named, are hammers made of stone that chimpanzees wield to break open nuts. Anvils are the workbenches for the nut-cracking operation. The study included 11 hammerstones and 7 stone anvils.
The team weighed the hammerstones and measured the lengths of both hammerstones and anvils. They also took hundreds of photographs of both tool types and converted them into 3-dimensional renderings. These detailed maps allowed the scientists to visualize and quantify the damage patterns on the stones — in essence, the stories of how the chimps used them to smash their meals.
Researchers found that hammerstones and anvils differed in key ways. For example, the Djouroutou chimpanzees use hammerstones that are much smaller than their anvils. The hammerstone nut-cracking craters are in the middle of one or both sides of the tool. In contrast, the anvils sustain scars on just the top side, where they are pockmarked all over.
Damage patterns on the anvils and hammerstones in Guinea were notably different from those in the Republic of Côte d’Ivoire, the team showed. These differences likely arose from differences in stone availability and types of nuts cracked, Proffitt notes. Despite these differences, anvils at both sites had deeper and steeper craters than their respective hammerstones.
Understanding differences and similarities in nut-cracking tools from one troop to the next is “essential to understanding cultural diversity within chimpanzees,” said Adrián Arroyo, an archaeologist at the Institut Català de Paleoecologia Humana I Evolució Social in Tarragona, Spain, who was not involved with the research.
Similar cultural meanings are encoded in the stone tools left behind by our ancient ancestors, Proffitt said. “Every stone tool that has ever been used by a human or a hominin is still out there,” he said. In the same vein, “Every stone tool used by a chimpanzee is still somewhere in the forest in which it was used.”
Proffitt, T., Reeves, J., Pacome, S., Luncz, L. (2022) Identifying functional and regional differences in chimpanzee stone tool technology. Royal Society Open Science, 220826. doi:10.1098/rsos.220826