In 2005 researchers reported the first documented use of tools among lowland gorillas in the wild, with photographs showing two female gorillas using sticks and stumps to test the depth of the water and cross a swamp in the Republic of the Congo.
But there is no documentation of western gorillas using tools in an aggressive way, according to Sunderland Groves, who is now a visiting researcher with the Center for International Forestry Research and is based in Bogor, Indonesia.
"The more we went looking, the less we could find out on this type of gorilla tool use," she said. "I'm sure that it happens in other wild populations, but if it's been witnessed by humans before, we don't know."
The gorillas in Cameroon may have developed the behavior as a response to particular local conditions, the authors write.
The highly endangered gorillas' mountainous habitat reaches into farms and pasture, and researchers believe the animals come into at least visual contact with humans fairly regularly.
"These gorillas are surrounded by so many people, maybe [throwing things] is part of their learned behavior," Sunderland Groves said.
In one of the three documented instances, the researchers observed a local hunter stumbling upon a group of gorillas.
Killing gorillas for meat or sale is taboo in this particular area of Cameroon. But when the hunter tried to scare off the gorillas by banging his machete on the ground, the group stayed put.
When the hunter began picking up stones and throwing them at the gorillas, the gorillas responded by picking up chunks of grass and throwing them back at the hunter.
"This went on for an hour," Sunderland Groves said. "The gorillas seemed more curious than frightened."
Elizabeth Lonsdorf is director of the Center for the Study and Conservation of Apes at the Lincoln Park Zoo in Chicago and a National Geographic Society Emerging Explorer. (The National Geographic Society owns National Geographic News.)
"It's very exciting for me to read about new research findings like these," said Lonsdorf, who was not involved with the study.
"People assume we know everything there is to know about apes, given that they have been studied in the wild for over 40 years now," she added. "However, every year brings about new research that challenges our assumptions about what we know.
"This new observation certainly merits further follow up and contributes to our growing understanding of gorillas' tool-use abilities."
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