The thinking monkey

In an experiment designed to test whether apes can solve problems that do not rely on operating a solid tool, but on using water as a tool, surprising results were discovered

Yonat Ashchar and Noam Levitan

From many studies in apes it became clear that they excel, more than any other animal except man, in solving problems, especially those that require the use of tools. The only animals that come close, and sometimes even surpass them, in this area are - perhaps surprisingly - birds from the crow family.

In a new study conducted at the Max Planck Institute for Evolutionary Anthropology in Leipzig, the researchers tried to check whether apes can solve problems that do not rely on operating a solid tool, but on using water as a tool.

In a study published in the journal Biology Letters, Natasha Mendes and her colleagues placed in front of 5 female orangutans a transparent tube, filled with water up to a quarter of its height, with a peanut (the prize) floating on the surface of the water. The only opening in the tube was on the top side, and too small to allow the man-hoppers to reach the peanut with their fingers.

It was not possible to tilt the receiver in any way, and there were no tools in the room with which the prize could be "fished". The only other thing found in the room was a drinking device - the female orangutans could only get the peanut if they filled their mouths with water and spat it into the tube several times, thus raising the water level and with it - the peanut.

All 5 females solved the problem the first time they encountered it (!), within 9 minutes on average. At first they tried to use their hands and mouth to get the peanut out, but after reaching a solution they abandoned these attempts and focused on filling the water. They continued to use the same method in repeated tests, and in the tenth test it only took them half a minute on average to reach the peanut.

The fact that all the subjects reached the solution in such a short time also surprised the researchers. Mendes said in an interview with the science weekly "Nature" that in their opinion many people were not able to give a quick answer to the problem.

An important aspect of the experiment is that the first spit of water into the tube did not yield the reward - the cashier had to spit into the tube several times until the level rose to a sufficient level. Nevertheless, once the subject "got the idea" of filling the water, she immediately continued this behavior until the reward was obtained.

Such behavior indicates the ability to plan and the ability to think abstractly, since the female orangutans did not see the tool that was used to solve the problem, and had to develop a mental image of the solution. The next step, say the researchers, will be to find out if the subjects spit the water in without realizing what they were doing, and only when they saw that the water was rising did they realize the advantage of this action, or did they plan the entire way of the solution in advance. To test this, another experiment will be conducted in which apes will be presented with a reward inside a sealed and non-transparent tube, so that they cannot clearly see the result of putting the water into it. Success in such a test can indicate advance planning, and hence abstract thinking at a high level.

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