While all other species of great apes, including chimpanzees and orang-utans, have been observed using sticks and rocks as tools, wild gorillas had never been recorded behaving in such a way. But conservationists have photographed two gorillas in the rainforests of the Republic of Congo using a branch as a walking stick, and a slender tree trunk as support while gathering food.

The study, co-ordinated by the Wildlife Conservation Society (WCS) and published by the Public Library of Science in its journal PLoS Biology, has been hailed as an important insight into the behavioural evolution of humans. Thomas Breuer, of the WCS, based at the Bronx Zoo in New York, described the behaviour of the western gorillas, nicknamed Leah and Efi, as a “truly astounding discovery”.

“Tool usage in wild apes provides us with valuable insights into the evolution of our own species and the abilities of other species,” Mr Breuer said. “Seeing it for the first time in gorillas is important on many different levels.”

Scientists observed Leah attempting to wade through a pool of water created by elephants. The female gorilla found herself waist-deep after only a few steps, so climbed out of the pool, retrieved a branch from a dead tree and used it to test the depth of the water.

Keeping her upper body above water, she moved some ten yards out into the pool before returning to shore.

On another occasion Efi used a detached trunk to support herself with one hand while digging for herbs with the other. As she moved from location to location, she used the stick for one last job, a bridge over a muddy patch of ground.

The observations were made in Mbeli Bai — a swampy clearing located in Nouabalé-Ndoki National Park where monitoring started in February 1995. In the past, gorillas have been observed using tools in zoos, but not in the wild.

Scientists said that most other observed instances of tool usage in great apes were related to processing food — such as cracking nuts with rocks or extracting termites with long sticks. The authors of the paper suggest that the gorillas’ use of sticks for support shows them adapting to their swampy habitat.

The ancestors of Man are first thought to have used tools more than two million years ago. The WCS has been studying gorillas in the Republic of Congo since the 1980s. Mbeli Bai is being managed to meet long-term gorilla research and eco tourism objectives.

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