Wild gorillas seen using walking sticks and plank bridges.
Gorillas have been spotted using tools in the wild for the first time, after decades of observation.
Researchers in the Congolese jungle saw one of the great apes using a branch to test the depth of a pond, and another using the trunk of a small shrub as an improvised bridge.
Unlike chimpanzees, which use a range of tricks to get food, gorillas rely more on size and strength: they shell nuts with their teeth, or smash open termite mounds with their fists.
But not needing tools to get food does not mean that gorillas aren't smart enough to use them. Captive western gorillas (Gorilla gorilla) are known to use tools in captivity, for example. And mountain gorillas (G. beringei) use a variety of different techniques which, although not involving tools, are clever ways of stripping leaves from hard-to-reach plants.
The new observations show that western gorillas have the mental wherewithal to use tools to solve other problems too, such as navigating through an ominously deep-looking pond, says Thomas Breuer of the New York-based Wildlife Conservation Society, and a member of the team that spotted the innovative behaviour.
From their vantage point in a clearing at Mbeli Bai in the northern Republic of Congo, Breuer and his colleagues spotted a female gorilla, called Leah, studying a pond before venturing a few steps into it. She then turned back and grabbed a handy metre-long branch from the bank, which she proceeded to use as a 'walking stick', repeatedly prodding the pond's bottom with it. After apparently assessing the depth of the pond with each prod, she eventually moved almost 10 metres from the shore.
"It was clear to us that she was testing the depth of the water," says Breuer. "The way she did it was incredible because it was so similar to the way that we humans solve the problem of deep water." And deep water probably is a danger to gorillas, Breuer adds. "I've never seen a gorilla swim; I would assume that they can't," he says.
The researchers saw another female, Efi, using the trunk of a dead shrub as a bridge to cross swampy ground. Efi ripped the 1.3-metre-long, 5-centimetre-wide section from the ground and leant on it like a crutch while she trawled the marshy ground for food. Then she laid it down like a plank and strode over it.
Two of a kind
So far, only two of the roughly 140 gorillas who visit the clearing have been spotted using these tricks, says Breuer, whose observations are reported in PLoS Biology1. "But it wouldn't surprise me if there are other cases," he says. "Gorillas are incredibly smart."
"This is remarkable," comments Andrew Whiten, who studies animal tool use at the University of St Andrews, UK. "Most scientists had given up on seeing it. Gorillas had almost been written off as tool users in the wild."
But given that we have waited so long to see it, tool use might be the preserve of only the most quick-witted gorillas, Whiten speculates. "It is striking how innovative these behaviours are, and how intelligent they are in that sense. But most gorillas are probably not in the same category as chimpanzees."
Breuer,T., Ndoundou-HockembaM., FishlockV., PLoS Biology, 3. e380 (2005).
Wildlife Conservation Society, Republic of Congo
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Hopkin, M. Gorillas branch out into tool use. Nature (2005). https://doi.org/10.1038/news050926-14