Oral Presentation (6) February 20

In this presentation, I will report tool-using behaviors by gorillas and orangutans living in social groups at zoological gardens. Three 8-year-old gorillas at the San Diego Wild Animal Park, CA, USA, threw sticks into the foliage of trees, which the gorillas could not climb due to electric wire, to knock down leaves and seeds@(stick-throwing). They selected sticks that were more suitable (i.e. longer or thicker sticks) for throwing. Two of them pulled branches of trees toward themselves by using sticks to capture them (branch-pulling), and beat a branch with a stick to knock down leaves and seeds (branch-beating). One of these two gorillas used much longer sticks for branch-pulling than for stick-throwing, indicating that he was capable of task-dependent selection of sticks.
The second part of this presentation will focus on the use and making of tools to obtain foodstuffs in artificial-mound holes by five captive, group-living Sumatran orangutans at the San Diego Zoo. Three adult orangutans frequently stripped leaves and twigs from a branch provided (tool-making), and then inserted the tool into a hole to obtain foodstuffs (tool-using). A five-year-old female usually used the tools that adult orangutans had previously used, but rarely made tools herself. A two-year-old male did not use any tools. The adult orangutans made good tools: they did not strip all of the leaves but rather left a few at the top of the branch. It seemed likely that with these good tools they were able to obtain more foodstuffs than by using sticks without any leaves. When the orangutans were unable to insert a tool into a hole, they usually modified the tool and/or changed their tool-using technique, such as changing how they grasped the tool.

NAKAMICHI, Masayuki
Laboratory of Ethological Studies,
Faculty of Human Sciences, Osaka University
1-12 Yamadaoka, Suita, Osaka
568-0871, Japan
naka@hus.osaka-u.ac.jp