The chimps use their teeth to form the brush-tipped termite-fishing tool (Footage: Biology Letters/C Sanz/J Call/ D Morgan)
Scientists believe they have solved the mystery of why some chimpanzees are so good at catching termites.
A team working in the Republic of Congo discovered that the chimps are crafting brush-tipped "fishing rods" to scoop the insects out of their nests.
They filmed the wild primates using their teeth to fashion the tools.
Writing in the Royal Society journal Biology Letters, the researchers said the probes' frayed ends helped the chimpanzees to collect more termites.
Lead researcher Crickette Sanz, from the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, said: "They have invented a way to improve their termite-fishing technique."
Previous studies have suggested that wild chimpanzees use brush-tipped tools to fish for termites.
The chimps seem to understand the function of the tool and its importance in gathering termites
But until now it has been unclear whether this was a specially crafted design feature or whether the frayed edges were a by-product of repeated tool use.
Using remote cameras to film the chimps as they sought out their insect snacks, the team was able to find an answer.
Dr Sanz told the BBC: "We found that in the Goualougo Triangle in the Republic of Congo, the chimpanzees were modifying their termite-fishing tools with a special brush tip."
To make their rods, the chimps first picked some stems from the Marantaceae plant and plucked off the leaves.
"They then pulled the herb stems through their teeth, which were partially closed, to make the brush and they also attended to the brush by sometimes pulling apart the fibres to make them better at gathering the termites," Dr Sanz added.
Further research revealed that a stem with a frayed tip collected 10 times more termites than a pointed probe.
The chimps create the rods from plant stems
Dr Sanz said: "The chimps seem to understand the function of the tool and its importance in gathering termites."
So far, the team have only found this behaviour in chimps in the Goualougo Triangle.
The apparent absence of this in populations in eastern and western Africa suggests that it is not an innate skill found in all chimpanzees.
Instead it seems that the Goualougo primates are learning the crafting techniques from other chimps.
The researchers say they now want to find out if chimps in this region are creating any other kinds of tools.
Dr Sanz said: "Large areas of central Africa have been largely unstudied and so there are many populations that could have examples of complex tool use that we just do not know about."
However, she added that further research might be hampered as the species was under threat.
"Just as we are learning about these exciting new complex tool behaviours, the chimps that are showing us these behaviours are under danger from logging, poaching and Ebola," she explained.
"There is a lot we need to do to conserve the chimps in the Congo Basin."
Dr Sanz worked on the paper with Josep Call, also from the Max Planck Institute of Evolutionary Anthropology, and David Morgan from the Wildlife Conservation Society and Chicago's Lincoln Park Zoo.
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