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Gorillas Found Tossing "Weapons," Study Says

Stefan Lovgren
for National Geographic News
January 30, 2008
 
As if size and strength were not enough to scare off human intruders, gorillas may have another tactic at their disposal: improvised weapons.

Researchers studying western gorillas in Cameroon have documented three cases in which the great apes threw clumps of grass or tree branches at humans.

"At first we didn't think too much of it, but then we realized that this is quite remarkable," said Jacqueline Sunderland Groves, who established the Wildlife Conservation Society research team working in the area.

"I don't think gorillas have been documented using this kind of weaponry before in the wild."

The observations were made during a three-year ecological study of Cross River gorillas (Gorilla gorilla diehli) on Cameroon's Kagwene Mountain.

A paper describing the incidents was recently published in the American Journal of Primatology. In it, the scientists suggest that the animals might have learned their unusual behavior from interactions with humans.

Testing the Waters

Great apes have long been known to use tools. Many studies have shown that wild chimpanzees use a variety of objects, including sticks and rocks, for foraging and other activities.

(Related: "Chimps Use 'Spears' to Hunt Mammals, Study Says" [February 22, 2007].)

Even the throwing of objects at predators or rivals has been seen before—it is considered one of the six primary types of tool use among primates.

At the Limbe Wildlife Center in Cameroon, for instance, captive gorillas have been observed hurling stones and grit toward human visitors, something they may have learned from chimps housed next door.

But captive behavior can differ significantly from behavior in the wild—and observing behavior in wild gorillas is especially difficult, because few groups are habituated to human presence.

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."

Throwing Match

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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