The floating peanut.
- Chimpanzees and orangutans can use water as a tool to get a peanut.
- Only a few apes can solve this task spontaneously, but they understand how it works.
- The task is cognitively demanding as only two-thirds of 8-year-old kids solved the problem.
When I give talks about outstanding cognitive skills in the animal kingdom, I always mention the orangutans at the Zoo of Leipzig in Germany. These intelligent creatures have a surprising skill that was actually found accidentally. My former colleague Daniel Hanus had planned an experiment to study causal reasoning, but the test he designed revealed new insights into tool use: He attached a vertical Plexiglas tube outside of the wall of the testing cage. Inside the tube was a yummy peanut. The orangutans could reach into the tube through the mesh with their very long fingers, but they could not reach the nut down at the bottom of the tube. To reach their reward, the orangutans came up with a solution that the scientists had not even thought of. The apes sucked up water from the water dispenser in their cage and then spat it into the tube. In doing so, they raised the water level. They kept spitting new water into the tube until the peanut floated right to the top. Finally, the apes could pick out the peanut with their fingers (Mendes et al., 2007).
The video I show then always impresses my audience.
What Do They Understand?
But do the orangutans really understand what they are doing? Daniel Hanus and his colleagues investigated that skill in detail. It took the five orangutans from Leipzig an average of three charges of water to reach the peanut. They all came up with the solution to the problem in the first trial in which they were presented with the task. In the following trials, they became faster and more skilled. While it had taken them about 10 minutes on the first attempt, in later trials it later took them only 30 seconds. They simply saved themselves the trouble of reaching the peanut with their fingers when the water level was not yet high enough (Hanus et al., 2011). But they also did not spit water indiscriminately into the tube when there was no nut in it at all.
In a very recent experiment, it was shown that orangutans can even solve that problem with an opaque tube. The question of this study was whether the apes learned to solve the task by some simple form of instrumental learning or whether they really understood it (Ebel et al., 2019). The possible learning scenario was that the orangutans somehow came up with the idea to spit the water into the tube and then figured that repeating those actions would bring the reward closer and closer until they then could finally grab it. Indeed, in some easier conditions, there was already some water in the tube, which could have led them to the idea of using water. In that case, the skill to solve the problem would not be so different from the ability of a rat to learn to press a lever to receive a food reward.
But that is not the case. Some orangutans also solved the task in the first trial when the tube was opaque. In this setup, no visual feedback was possible—they could not see how the reward came closer. Thus, it is not simple trial-and-error learning; orangutans understand how to solve the task, or, as psychologists put it, they mentally represent the problem (Sebastian-Enesco et al., 2022).
Thus, all these observations suggest that the red apes solved the peanut problem with insight. But how do other primates perform in this test? Surprisingly, there were not only differences between species, but also between different populations. While the five orangutans from Leipzig had come up with the solution easily, their conspecifics living in a sanctuary on the island of Borneo failed the test. Maybe the apes tested in Borneo were not motivated enough? Two out of three orangutans from a Spanish zoo were successful. In contrast, chimpanzees at Leipzig Zoo had a lot of trouble with the task, but chimps at a sanctuary in Uganda performed much better.
Thus, the task is obviously not easy to solve. Ask your kids what they would do! Indeed, Daniel Hanus actually tested 8-year-old children and found that only 66 percent of them were able to solve the problem. Of course, the children did not have to spit the water inside the tube—they could use water from a watering can. In the case of 4-year-olds, not even one out of 10 children came up with the correct solution! (Hanus et al., 2011). But some great apes do—and the audience of my talks then always agrees that animals are smarter than you think!
Ebel, S. J., Schmelz, M., Herrmann, E., & Call, J. (2019). Innovative problem solving in great apes: the role of visual feedback in the floating peanut task. Animal Cognition, 22(5), 791–805.
Hanus, D., Mendes, N., Tennie, C., & Call, J. (2011). Comparing the performances of apes (Gorilla gorilla, Pan troglodytes, Pongo pygmaeus) and human children (Homo sapiens) in the Floating Peanut Task. PLoS ONE, 6(6), e19555. doi:10.1371/journal.pone.0019555
Mendes, N., Hanus, D., & Call, J. (2007). Raising the Level: Orangutans Use Water as a Tool Biology Letters, 3(5), 453-455. doi:10.1098/rsbl.2007.0198
Sebastián-Enesco, C., Amezcua-Valmala, N., Colmenares, F., Mendes, N., & Call, J. (2022). Raising the level: orangutans solve the floating peanut task without visual feedback. Primates, 63(1), 33-39. doi:Sebastián-Enesco C, Amezcua-Valmala N, Colmenares F, Mendes N, Call J. Raising the level: orangutans solve the floating peanut task without visual feedback. Primates. 2022 Jan;63(1):33-39.
Written by Juliane Bräuer, Ph.D., is the head of the DogStudies Lab at the Max Planck Institute for the Science of Human History in Jena, Germany, where she studies the cognitive aspects of dog domestication.
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