The purpose of this study was to investigate Bornean orangutans’ ability to use water as a tool. In the floating object task, subjects must spit water into a vertical transparent tube to obtain an out-of-reach food reward (peanut). Two zoo-living orangutans were tested: Denda, a 13-year-old male, and Kumang, a 38-year-old female. Three conditions were presented: ‘wet’, in which the tube was quarter-filled with water; ‘dry – stick tools’, in which the tube had no water and six stick tools that were unnecessary for obtaining the reward were provided; and ‘dry’, in which the tube had no water and no stick tools were provided. Both subjects completed two sessions in each condition. The dry conditions were presented first because it is a cognitively more demanding task to use water to obtain the peanut when there is no water in the tube initially to provide a cue. Denda was successful in all six sessions while Kumang was successful in only three sessions. Both subjects’ latency to spit water into the tube decreased exponentially across sessions. Both individuals displayed a variety of strategies to solve the problem, including using and manufacturing tools utilizing objects in their enclosure. Denda is the first orangutan to succeed on the task when presented with the ‘dry’ condition first. This study provides the first evidence of Bornean orangutans spontaneously solving the floating object task, but the mechanism underlying their success remains unclear.
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