Chimps Shine Light on Mystery of How the First Humans Used Tools to Dig for Food

chimpanzees, tool use
Tool use behaviors that emerged during the excavation of underground food. Motes-Rodrigo et al., 2019

Scientists have observed chimpanzees in captivity spontaneously working out how to use tools to dig up buried food, according to research published in the open-access journal PLOS ONE.

It was once thought that using tools in this way was unique to humans and our ancient hominin ancestors. However, recent studies of wild chimpanzees and bearded capuchin monkeys have indicated that these animals can use tools to excavate underground food, such as plant roots, corms and tubers—known technically as "underground storage organs" (USOs).

Despite this recent research, direct observations of chimps using tools to excavate food in the wild are limited.

"During my Ph.D. research in Tanzania several years ago, I discovered that chimpanzees used tools to extract underground storage organs," Adriana Hernandez-Aguilar, lead author of the study from the University of Oslo, told Newsweek. "This was the first time that this type of tool use was reported for chimpanzees, and until then it was a behavior considered uniquely human."

"However, the chimpanzees were not habituated to human observers and thus we never saw them excavating," she said. "We based our discovery on indirect evidence such as holes, partially bitten USOs, wadges, knuckle prints, etc. Later the same was reported for another site in Senegal, but again the chimpanzees could not be observed, because they were not used to people."

The digging up of this type of food from the ground is considered critical in human evolution, according to Hernandez-Aguilar.

"This behavior is part of a debate about what foods our ancestors ate to fuel the large brain characteristic of humans," she said. "Was it meat from hunting and/or scavenging, or was it roots and tubers? Many human hunter-gatherer societies today rely on the roots and tubers that women gather because hunting can be unpredictable."

"Thus, the excavation of roots and tubers is considered important in our evolution and since chimpanzees are—together with bonobos—our closest living relatives, studying how they excavate these resources can provide data that can help us understand how our hominin ancestor may have done it."

Because researchers could not observe chimps excavating in the wild, Hernandez-Aguilar and her team decided to study a 10-strong chimp colony at the Kristiansand Zoo—located just under 200 miles southeast from the Norwegian capital Oslo—to gain a better understanding of how this behavior developed.

"We wanted to know first, if they would excavate and second, how they would do it—i.e., which techniques they would use," she said.

According to the paper, none of the chimps in the colony had previously been observed carrying out excavating behavior. The team gave the apes the opportunity to dig up food that had been artificially placed underground in their naturally forested outdoor enclosure.

Specifically, the researchers dug five small holes and placed a whole fruit in each, ensuring that the chimps could see the food before covering them up. In the first experiment, they gave the apes prepared tools made from sticks and tree bark, while in the second they did not provide any tools. No guidance or demonstration was given to the chimps at any time.

The team found that the chimps spontaneously used tools to excavate the buried foods. In the first experiment, nine out of the 10 chimps successfully dug up the buried fruit at least once, with eight choosing to use the prepared tools over their own hands. In the second experiment, the chimps collected their own tools from around their enclosure.

Intriguingly, the team observed the chimps using six different tool use behaviors to dig up the fruit, which are described as probing, perforating, digging, pounding, enlarging and shoveling.

"Chimpanzees were selective in their choice of tools that we provided, preferring longer tools for excavation," the authors wrote in the study. "They also obtained their own tools mainly from naturally occurring vegetation and transported them to the excavation site. They reused some tools throughout the study."

Furthermore, the authors observed the chimps taking turns to excavate a hole, and even sharing the fruit once it was dug up. It should be noted that these results may not necessarily be applicable to wild populations, and the chimps still excavated with their hands more often than they did with tools.

Nevertheless, the findings could have implications for our understanding of the evolution of such behaviors in humans. While modern chimps are not perfect models for studying the behavior of ancient hominins, the researchers say that our ancestors may have worked out how to use simple tools to dig up underground food in a manner similar to these chimps.

"Since Jane Goodall's discovery almost 60 years ago, we know that chimpanzees use tools," Hernandez-Aguilar said. "Chimpanzees exhibit an impressive diversity of tool use in many different contexts. They crack nuts with stone tools, extract honey, fish for termites and algae, use leaf sponges to clean themselves, or as 'symbols' to communicate with other chimps, just to name a few. However, using tools for excavating roots and tubers has been only found in two study sites, as I mentioned above."

"The fact that the chimpanzees in our experiment exhibited six different behaviors in the context of excavating underground food, suggests that the extraction of roots and tubers in the wild is a complex behavior," she said. "This adds to the growing body of evidence that shows that humans are not the only animals who possess complex tool use. In this way, we can see human tool use more like a continuum from what exists already in other animals, rather than as something completely unique to humans and separated from the rest of the animal kingdom."