Researchers spent 12 years documenting the behaviors exhibited by a population of Eastern chimpanzees (Pan troglodytes schweinfurthii) whose range extends across more than 50,000 square kilometers (over 19,300 square miles) of northern Democratic Republic of Congo.

The paper published this month in the journal Folia Primatologica detailing the team’s findings includes a description of an entirely new chimpanzee tool kit featuring four different kinds of tools: a long ant probe, a short probe, a thin wand, and a digging stick.

These tools are used to harvest five different food types, including a variety of driver and ponerine ant species as well as honey from the nests of ground-dwelling and arboreal bees. And they’re not the only evidence of unique behaviors discovered among this chimp population.

Scientists have discovered a new chimpanzee “behavioral realm” in the Bili-Uéré region of the Democratic Republic of Congo (DRC).

A research team led by Thurston C. Hicks of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany and the University of Warsaw in Poland spent 12 years documenting the behaviors exhibited by a population of Eastern chimpanzees (Pan troglodytes schweinfurthii) whose range extends across more than 50,000 square kilometers (over 19,300 square miles) of northern DRC.

The paper published this month in the journal Folia Primatologica detailing the team’s findings includes a description of an entirely new chimpanzee tool kit featuring four different kinds of tools: a long ant probe, a short probe, a thin wand, and a digging stick. These tools are used to harvest five different food types, including a variety of driver and ponerine ant species as well as honey from the nests of ground-dwelling and arboreal bees.

“We found that the tool types each have different distinctive characteristics,” Hicks told Mongabay. “The tools used by some of our chimpanzee communities to harvest driver ants are the largest known for insect prey in Africa, on average 1.2 meters long, but some are 2.5 meters! Given the painful bites these ants inflict, adding some extra length makes sense, but we still don’t know why they make them so long compared to other populations.”

According to Hicks, just a few decades ago, chimpanzees’ various behavioral traditions appeared to be distributed in a fairly haphazard manner, with few discernible patterns. But now that scientists have collected data from many additional chimp study sites, patterns are emerging: chimps in Central Africa use clubs to pound open beehives, for instance, while the use of stone and wooden hammers to crack nuts is limited to chimps in a certain region of West Africa.

“With this paper we add the Bili-Uéré,” Hicks said. In addition to their novel tool use, he added, the Bili-Uéré chimps are also distinguished by their construction of ground nests. “Up to 28 percent of nests in some areas are made on the ground, gorilla style,” he explained. “This is rather puzzling, because in most areas chimpanzees sleep in trees, presumably to avoid predators. In [the Bili-Uéré] region, predators like leopards and hyenas are abundant — and there are even lions!” (A student of Hicks’ is currently studying the Bili-Uéré chimps’ use of ground nests.)

But Hicks says that he thinks the most interesting quirk of the Bili-Uéré chimps is what he calls their “expanded ‘pounding behavior.’” Many chimpanzee groups are known to pound fruits against branches or roots in order to open them up, but the Bili-Uéré population also pounds open two different kinds of termite mounds, Cubitermes and Thoracotermes, across the entire 50,000-square-kilometer region Hicks and team surveyed.

“What is intriguing is, this population seems to totally ignore the widespread and abundant Macrotermes termite mounds, which are preyed upon using stick tools in many other chimpanzee populations such as, famously, at Gombe,” Hicks said. “Instead, they are pounding open the mounds of two other widespread termite types which are ignored by most other chimpanzee populations (with one exception: Tai Forest). This to me is consistent with human-like quirky food preferences and supports the proposal of my colleague Thibaud Gruber that chimpanzees, like humans, have ‘cultural override.’”

Potential ecological factors have to be ruled out before we can be sure that the Bili-Uéré chimps are just being choosy about their meals, Hicks is quick to add, but genetic explanations are unlikely, he said, “as all Eastern chimpanzees appear to have diversified very recently in prehistory, with little genetic differentiation.”

Though the behavioral similarities demonstrated by the Bili-Uéré chimps across such a large region could have arisen through other means, Hicks believes that social learning is the most likely explanation for the development of this unique animal culture.

“Although it is possible that these patterns could reflect ecological factors or genetic differences, I find them to be much more consistent with cultural diffusion,” he said. In other words: “A chimpanzee in a particular community invented a certain behavior, it became the community norm and then it spread through the surrounding forests via territorial expansions or immigrating females, until hitting some natural barrier.”

The Bili-Uéré chimpanzee population’s range actually occupies two very different habitat types on the two sides of the Uele River: savanna woodland forest mosaic to the north and tropical moist forest to the south. Though the behaviors of the population overall are strikingly similar, it’s perhaps not surprising that Hicks and team did discover some geographic variation, such as a lack of honey-digging tools to the south of the river even though the same types of bees are common there. Meanwhile, long driver ant probes and fruit-pounding sites were found only to the north of the Uele River.

“One would think, given the diversity of chimpanzee behaviors elsewhere, these chimpanzees would have reacted to the different habitat types and invented a different material culture,” Hicks said. It’s possible that the Bili-Uéré chimps have only recently colonized the area. “Perhaps chimpanzee traditions are rather conservative, and they have not had time to differentiate their behaviors to match the very different habitats they were colonizing,” Hicks speculates. He and his colleagues plan to visit the Ituri Forest, which lies about 400 kilometers southeast of the Bili-Uéré region, later this year to test the theory that the Bili-Uéré chimps are part of an even more widespread behavioral realm that occurs across northern DRC.

Whatever the reason for the distinct behavioral similarities shared amongst the Bili-Uéré chimpanzee population, Hicks says that this research can help us understand how culture spread through generations of our ancient hominin ancestors. “It could be that we are observing the first stages in behavioral differentiation between subpopulations,” he said. “Whatever the case, we are very lucky to have this ‘natural laboratory’ in Northern DRC to study the spread of chimpanzee traditions.”

The team’s findings about the Bili-Uére chimp culture could also have important implications for the conservation of chimpanzee populations. “We have evidence from Bili-Uére, for instance, that chimpanzees living in areas hunted more heavily by humans no longer make ground nests and reduce their amount of vocalizing, probably to avoid detection by hunters. This must have an impact on their social lives,” Hicks said.

“To the south of Uele River the apes are being slaughtered as gold and diamond mines spread across the region. This has not yet happened in the more remote Bili-Gangu Forests north of the Uele. If they disappear, the tragedy of their loss will be compounded by the accompanying loss of a priceless opportunity allowing us to understand how great ape traditions spread across time and space. Our activities are robbing these apes not only of their forests but their ‘wisdom of the elders,’ their ingenious cultural solutions to survival in the forests and savannas developed over hundreds or thousands of ape generations.”

CITATION

• Hicks, T. C., Kühl, H. S., Boesch, C., Dieguez, P., Ayimisin, A. E., Fernandez, R. M., … & Hart, J. (2019). Bili-Uéré: A Chimpanzee Behavioural Realm in Northern Democratic Republic of Congo. Folia Primatologica, 90(1), 3-64. doi:10.1159/000492998

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