WASHINGTON — Some chimpanzee groups are stone-throwers. Some use rocks to crack open tree nuts to eat. Others use sticks to fish for algae.
As researchers learn more about Homo sapiens’ closest living genetic relatives, they are also discovering more about the diversity of behaviors within chimpanzee groups — activities learned, at least in part socially, and passed from generation to generation.
These patterns are referred to as “traditions” — or even animal “culture.” In a new study , scientists argue that this diversity of behaviors should be protected as species themselves are safeguarded, and that they are now under threat from human disturbance.
“What we mean by ‘culture’ is something you learn socially from your group members that you may not learn if you were born into a different chimpanzee group,” said Ammie Kalan, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
“As chimpanzee populations decline and their habitats become fragmented, we can see a stark decline in chimpanzee behavioral diversity,” said Kalan, co-author of the sweeping new study published Thursday in the journal Science.
The 10-year study, led by researchers at the Max Planck Institute and the German Center for Integrative Biodiversity Research, examines data on 144 chimpanzee communities in Africa and the occurrence of 31 specific behaviors, such as tool usage or rock throwing.
The regions with the least human impact showed the greatest variety in chimp behaviors. But areas greatly altered by logging, road-building, climate change and other human activities showed markedly less behavioral diversity — an 88 percent lower probability of exhibiting all behaviors.
Multiple factors drive the loss, the authors say.
“With the increase of human disturbance, chimps may not be able to live in such large groups anymore — and it has been shown that group size is connected with social learning,” said Hjalmar K?hl, also a primatologist at the Max Planck Institute and a co-author.
For example, researchers studying chimpanzee groups in parts of West Africa encountered mysterious piles of stones alongside battered tree trunks.
The rocks had been thrown against the trees by chimpanzees for reasons still unclear to the scientists who first documented the behavior in 2016. Perhaps the purpose was to mark territory, or proclaim dominance within a group, or start a game, or something else, the biologists surmised.
But not all chimpanzees are stone-throwers.
Some groups use stones to crack open tree nuts. Researchers recently discovered an archaeological site in West Africa that showed chimpanzees had used stones there for nut-cracking for more than 4,000 years.
Elsewhere in West Africa, sticks were the tools of choice, with young chimps in Guinea learning from their elders to use them to “fish” in lakes for long strands of algae to eat. Or, in Nigeria, to poke termite mounds to gather the insects for food.
Sixty years ago, scientists had limited knowledge of chimpanzees in the wild, until researcher Jane Goodall first recorded behaviors like tool usage, which previously were associated only with humans.
In 1999, Goodall and other scientists popularized the phrase “chimpanzee cultures” in an article in the journal Science. The use of the term has ignited debate ever since — including resistance from some anthropologists– but also launched further research.
Most likely genetics and socially learned behavior interact to form animal “culture” in chimpanzees and other species, said Carl Safina, an ecologist and author of several books on animal behavior who was not involved in the study.
This has implications for conservation.
“We have come to understand that behavioral diversity matters for protecting species,” said Andrew Whiten, an evolutionary psychologist and zoologist at the University of St. Andrews in Scotland, who was not involved in the study.
“The greater the diversity of behavior, the more likely a species will be able to deal with future changes and challenges in their environment,” he said. “It’s not good news when their options are limited.”
Last month, Whiten co-authored a “Policy Forum” article in Science, entitled “Animal cultures matter for conservation,” arguing that policy-makers should include behavioral diversity alongside other measures of biodiversity.
“Culture is not the tip of the iceberg for these great apes — some kind of nice luxury — but an intrinsic and essential part of their local adaptation,” Carel van Schaik, an anthropologist at the University of Zurich who was not involved in the new study, wrote in an email.
Lydia Luncz, a primatologist at the University of Oxford, agrees.
“We are far from understanding yet what is the cultural repertoire of chimps,” said Luncz, who also was not involved in the study. “It would be a tragedy to lose more of the cultural heritage of our closest living relatives.”
