Video: Watch an orang-utan solve the floating peanut problem

Orang-utans are clever enough to use water as a problem-solving tool, an experiment inspired by Aesop’s fables has demonstrated.

When presented with a peanut floating deep down inside a transparent tube, the animals spat their drinking water into the tube to raise the treat to the top, where they could grab it (see video, right). Researchers say that the study is novel because it shows the insightful use of a liquid tool by a non-human primate.

Natacha Mendes at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, recalls that the idea for the study came out of a discussion with her colleagues about Aesop’s fables.


The team focused on one particular story, in which a clever crow throws stones into a pitcher to elevate the water to a level where the bird can access it for drinking. Mendes and her fellow researchers wondered whether the orang-utans they worked with could have a similarly smart insight.

Speedy learning

To test this out, the scientists presented five orang-utans (Pongo abelii) with clear plastic tubes, each containing a small amount of water along with a peanut. But, to the orang-utans’ great dismay, the peanuts floated too far down the tube for them to reach.

The frustrated apes tried everything they could to get to the peanuts – including biting, hitting, and kicking the tube.

It only took them about nine minutes on average, though, to figure out that a little bit of extra water could do the trick. At this point the orang-utans began taking mouthfuls of water from their drink dispenser and spitting the liquid into the tube, a trick that elevated the peanuts to an accessible point.

“This seems to be insightful behaviour because they haven’t seen this test before,” Mendes explains. “And as soon as they got the idea they continued to do it.”

And do it faster, Mendes adds. She presented each orang-utan with the peanut-containing tube 10 times. By the tenth attempt, it took the animals only 30 seconds before they started spitting water into the tube.

Intelligent spitting

Researchers also conducted control experiments, including one in which they taped the peanut to the top of the tube. The orang-utans did not spit into the tube, but instead simply grabbed the treat with their hands.

Mendes notes that archer fish (Toxotes jaculatrix) can spit water at flies to knock them into the water for eating (see Fast food for fish with perfect aim). But she says the orang-utans’ use of water represents a more sophisticated behaviour.

“There’s no comparison,” she says, pointing out that the apes have a conscious idea of what they are doing and consider other options, such as kicking the tube. “With orang-utans we are talking about a flexible strategy – that’s the big difference.”

Journal reference: Biology Letters (DOI: 10.1098/rsbl.2007.0198)