Chimpanzees understand material properties

Wild chimpanzees choose just the right tools for the job

To the point

• Intuitive understanding of material properties: chimpanzees deliberately select flexible plant materials to make tools for termite hunting.
• Flexible materials: the plant species used by chimpanzees are up to 175 percent less rigid than the unused materials in their environment. Flexible tools make it easier for chimpanzees to move through the winding tunnels of termite mounds and thus hunt more successfully.
• Supra-regional preferences: Certain plant species are preferred by chimpanzees, even if they live up to 5000 kilometers apart.

Scientists have discovered that chimpanzees employ a degree of engineering when making their tools, deliberately choosing materials based on their flexibility and mechanical properties. The research, carried out by a multidisciplinary team of scientists from the University of Oxford, the Max Planck Institute for Evolutionary Anthropology, the Jane Goodall Institute in Tanzania, the University of Algarve, the University of Porto and the University of Leipzig, has shown that chimpanzees regularly choose plant materials that create more flexible tools for termite fishing. This finding could help us understand how early humans developed their tool-making capabilities.

"This is the first direct evidence that wild chimpanzees select plant materials for termite fishing based on specific mechanical properties," says Alejandra Pascual-Garrido, who has been studying the raw materials used in chimpanzee tools in Gombe for more than a decade. Using a portable mechanical tester in the field, she measured how much force it took to bend plant materials used by chimpanzees to construct termite fishing tools compared to plant materials that were available but never used.

Termite fishing tools are relatively thin probes that chimpanzees make using different plant materials to fish the insects out of the mounds. Given the winding passages of termite mounds, the scientists hypothesized that flexible tools would be more effective at fishing out the insects than rigid sticks. To test this idea, the team showed that plant species never used by chimpanzees were 175 percent more rigid than their preferred materials. In fact, even among plants growing near termite mounds, those that showed obvious signs of regular use by the apes produced more flexible implements than nearby plants that showed no signs of use.

This was consistent with what researchers had long suspected from decades of observing chimpanzees fishing for termites: flexible tools could more easily navigate the winding tunnels of the termite mound, leading to more successful foraging. Remarkably, certain plant species, such as Grewia spp., are also favored by termite fishing chimpanzee communities living up to 5,000 kilometers apart, suggesting that the mechanics of these plant materials could be the basis for such ubiquitous preferences, and that rudimentary engineering may be deeply rooted in chimpanzee tool-making culture.

Combining biomechanics with animal behavior

Wild chimpanzees may therefore possess a kind of "folk physics" – an intuitive understanding of material properties that helps them choose the best tools for the job. This natural engineering ability is not just about using any stick or plant available; chimpanzees specifically select materials with mechanical properties that can make their tools more effective. “This novel approach, which combines biomechanics with animal behavior, helps us better understand the cognitive processes behind chimpanzee tool construction and how they evaluate and select materials based on functional properties,” says Pascual-Garrido.

The findings raise exciting questions about how this knowledge is learned and transmitted across generations, for example, by young chimpanzees watching and using their mothers' tools, and whether similar mechanical principles dictate chimpanzees' selection of materials for making other foraging tools, such as those used for eating ants or harvesting honey. "This finding has important implications for understanding how humans might have evolved their remarkable tool using abilities", explains Adam van Casteren, who specializes in biomechanics and evolutionary biology. "While perishable materials like wood rarely survive in the archaeological record, the mechanical principles behind effective tool construction and use remain constant across species and time."

By studying how chimpanzees select materials based on specific structural and/or mechanical properties, we can better understand the physical constraints and requirements that would have applied to early human tool use. Using such a comparative functional framework provides new insights into aspects of early technology that are not preserved in the archaeological record.