How Primates Understand Newton

Legend says that in the summer of 1666, Isaac Newton lay down under an apple tree in the garden at his parents’ house near Cambridge. Normally, we imagine a beautiful day with a gentle breeze, just strong enough to pull an apple from its branch. Whether the apple hit Newton’s head or bounced to the ground can only be speculated; however, according to the legend, it made Newton wonder why apples always fall perpendicularly to the ground. Why not sideways or upwards? Why always toward the center of the Earth? In this way, the great physicist discovered the general law of gravity.

The scale

But can animals also use this understanding? Maybe they don’t contemplate universal forces or the center of the Earth, but do they know that food has weight—and, thus, can they find it using the effects of gravity? In Leipzig (Germany), chimpanzees were presented with a beam scale that was in equilibrium. At each end of the scale was an opaque container. Experimenters then hid a piece of banana in one of the two containers but didn’t let the chimps see which container they placed it in. But the chimps could see how the scales tilted to one side.

Without hesitation, the chimpanzees chose the correct container, namely the one that had gone down with the banana. They came to the conclusion that the banana pulled the beam scale down with its weight. Control conditions were used to rule out the possibility that the chimpanzees had followed any other non-causal cue. Still, the experimental animals reliably selected the lower vessel only when the banana was the sole cause of the beam scale’s movement. If an experimenter moved the beam by hand, for example, or if it did not tilt at all, the chimps were indecisive and chose randomly (Hanus & Call 2008).

Checking the weight

The disadvantage of this test is that the animals only take a passive role and cannot investigate the whole scenario themselves, as they would under natural conditions. Therefore, another test was developed. It resembles a situation that we humans may encounter every day.

Imagine you have two tetra packs of milk in your refrigerator. You know that one of the two packs has already been opened. To find out which carton you’ve been drinking from, you don’t make the effort of opening up both and peeking inside. After all, you want to keep one of them sealed. Instead, you take both tetra packs in your hands and estimate the weight. Then you conclude that the lighter pack must be the one you’ve been drinking from.

Capuchin monkeys select the right nuts.

Source: Adèle, used with permission.

Capuchin monkeys in the wild do something very similar. They have to decide which nuts are worth the work of cracking open. After all, cracking open a nut requires a great deal of effort and energy.

In the lab, it was shown that capuchin monkeys could use the weight of a nut to learn about its contents. Subjects had to choose between a nutshell containing food and an empty nutshell, using only the difference in weight. The minimal difference in perceived weight ranged between 2.1 and 3 g! Thus, these monkeys seek information to discriminate effectively between full and empty nuts before going through the costly opening process (Visalberghi & Neel 2003).

The chimpanzees in Leipzig were tested in a very similar way. These apes love to drink fruit juice. In this study, five identical-looking bottles were placed in the test room. All the bottles were tightly closed, making it difficult for the chimpanzees to open them. Only one of the bottles was filled with juice. It was about eight times heavier than the other bottles, weighing 570 grams. The question was, which bottle would the chimpanzees try to open first?

Indeed, the apes very quickly began to open the correct bottle, i.e., the heavy one. But does that mean that they really understood the causal relationship between weight and juice in the bottle? There are two possibilities. Either the chimpanzees had learned very quickly an arbitrary connection: heavy bottle means juice. Or, they had grasped the causal connection: One bottle is heavier than the others because there is juice in it.

To find out which possibility was true, causal reasoning or associative learning, the same chimpanzees were tested again. This time, all five bottles weighed the same, but only one contained the delicious juice; the others were filled with water. However, this time the juice bottle differed from the others in one artificial feature. It was marked white, while all the other bottles were black. In 15 trials, the chimpanzees did not learn to open the correct bottle first, i.e., the marked one.

Although the color was a completely reliable indicator of the presence of juice here, there was no direct causal relationship between color and juice—just an artificial one. Of course, chimpanzees are in principle able to learn such artificial features, like colors; it just takes a bit longer. But if they find a causal connection (the juice makes the bottle heavier), they use their logical abilities and learn very quickly to choose the right bottle (Hanus & Call 2011).

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So maybe apes and monkeys don’t know how or why Newton’s general law of gravity works, but they certainly know how to make it work for them!