Everywhere chimpanzees roam, scientists are uncovering more fascinating behaviors such as harvesting algae with long poles, mining honey out of the ground using sticks and accessing hidden reserves of water inside trees with sponges fashioned from chewed-up leaves.
These and other behaviors make up the basis of what scientists increasingly describe as chimpanzee “culture” — learned traditions that vary by location. And humans might be killing it, according to a study published Thursday in the journal Science.
The study suggests that people and their disturbances of ecosystems may be hindering the transmission of chimpanzee culture, and in some cases, destroying it altogether.
To reach this conclusion, more than 75 scientists and other researchers compiled data from 144 chimpanzee communities found across 15 countries in West and Central Africa. They then compared the behaviors each group was known to exhibit against various ways of measuring human disturbance, such as population, infrastructure, roads and forest cover.
Chimpanzees living in areas with the most human disturbance had less varied behavioral repertoires, the study found. On average, animals in areas with the most disturbance saw an 88 percent reduction in behavioral diversity when compared with animals living where human impact was low.
The reasons behind this loss of behavioral diversity or cultural traditions are probably many, said Ammie Kalan, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Germany and senior author of the study.
When chimpanzee groups become fragmented or reduced in size, Kalan said, it creates “a physical break in the social transmission chain.” Worse still, when a population disappears altogether, its knowledge dies with it.
People are also changing chimpanzee habitats in ways that make certain behaviors less valuable or even obsolete. For instance, when humans cut down a forest to plant papaya trees, local chimpanzees may abandon the more difficult practice of cracking nuts with rocks in favor of raiding crops — a behavior with an easier payoff.
Chimpanzees may also become more nocturnal as a result of human activity, which can include hunting the apes for the bushmeat trade. This, in turn, might affect their access to certain resources or the time available for select behaviors.
Cat Hobaiter, a University of St. Andrews primatologist who was not involved in the study, said the scale of the data used was impressive and that it paints a clear picture of how humans are affecting chimpanzee cultures.
“The impact of some human activities might be very limited in terms of the total population of chimpanzees, but they could wipe out a unique culture, something that is irreplaceable, no matter what we're able to do in terms of trying to restore habitat in the future,” Hobaiter said in an email.
The idea that chimpanzees possess distinct cultures is still relatively new, with the first scientific paper describing the idea published in 1999. Most of what is known about chimpanzees has come from researching just a handful of populations that have been habituated to humans, Kalan said, but scientists have little insight into many other chimp communities that remain wary of people.
“There’s still so much more to know,” Kalan said.
Remote cameras have begun to reveal some of the primates’ tricks, but researchers don’t always understand what they are seeing. Take a behavior called accumulative stone throwing, which Kalan and her colleagues described in 2016.
“This is where chimpanzees in some populations will go up to a tree and they will throw a stone at it,” Kalan said. “They repeatedly hit the same tree, and what happens is these trees end up with a lot of different marks, like wounds, and you get a pileup of stones around the base of these trees.”
Unlike algae scooping, termite fishing or honey digging, camera traps show that the chimpanzees aren’t getting food out of accumulative stone throwing. Nor does it seem to be a game. Instead, Kalan said she believes the thud of the rock against wood plays some role in communication.
“The reason why I say that is because the same behavior is accompanied with a ‘pant-hoot’ vocalization, which is a chimpanzee long-distance vocalization,” Kalan said. “So often if they do this behavior you sometimes see them sit down and they kind of wait, and they might even repeat the behavior again.”
Whatever is going on, it’s clear from camera trap footage (and the piles of stones) that the trees are important to the chimpanzees. Kalan said that is why she and her co-authors argue that, in addition to trying to protect a species and its habitat, humans must also protect the places that mean something to the animals — something the paper refers to as “chimpanzee cultural heritage sites.”
Likewise, the authors argue chimpanzee conservation shouldn’t just focus on protecting species and subspecies, but also on “culturally significant units.” If chimpanzees survive as a species but populations with unique cultures disappear, they say, then something critical has still been lost.
“I think this is a really important approach, and one that applies to many species,” Hobaiter said. Some populations of dolphins use sponges to protect their noses as they forage, for example, and “there is incredible cultural variation in whale and bird songs.”
“Each population, each community, each generation is different,” she said. “In the rush to conserve the species, we also need to remember the individual.”