Hypothesis
Following a report that captive orangutans spontaneously add water to raise a floating peanut, other great apes, chimpanzees and gorillas, and other orangutan groups, would also solve this liquid-tool problem.
What failed
In the first experiment none of the chimpanzees or gorillas solved the task. In further tests with other populations, about 20% of chimpanzees succeeded but none of the orangutans did, and among children only 8% of 4-year-olds (rising to 58% of 8-year-olds) found the solution.
Broken assumption
Spontaneous use of water as a tool is a robust, general ability across great ape species and groups.
Why
For the chimpanzee population that failed outright, the authors find suggestive evidence that functional fixedness hindered their strategies.
From the published paper: Hanus et al. PLoS ONE, 2011. ↗CC BY