The Surprising Complexity of Animal Memories

June 5, 2019

Chimpanzees, birds, and even rats have shown signs of reviewing their own past to prepare for the future.

Chrysippus, a Greek philosopher from the third century B.C., is said to have recounted how a hunting dog arrived at a spot where three roads met. The dog smelled the two roads by which the quarry had not passed, then without hesitation or any further sniffing set off on the third. According to the philosopher, the dog had drawn a logical conclusion, reasoning that if the quarry had not taken two of the roads, it must have taken the third.

Facing a fork in a maze, mice often hesitate for a few seconds before continuing. Recent studies suggest that in order to decide which way to go, a mouse has to project itself into the future. We know that rodents replay previous action sequences in their hippocampus, so the wavering mouse in the maze probably compares the memory of old routes with imagined future ones. In order to do so, it will have to be able to tell the difference between experienced and projected actions, which requires a primal sense of self.

At least this is what the scientists doing these experiments assume. I find this fascinating, because in this thought experiment, we postulate that humans would need a sense of self to make the same decision, which we then take as evidence for a sense of self in another organism. This extrapolation is generally satisfying, but not risk-free, because it hinges on the assumption that there is only one way to solve a problem.

Chrysippus’s dog is a great example of apparent inferential reasoning. Fortunately, we now have tests of inferential reasoning. In the 1990s, the psychologists David and Ann Premack presented their chimpanzee, Sarah, with two boxes, putting an apple in one and a banana in the other. After a few minutes, Sarah would watch one of the experimenters munch on either an apple or a banana. This experimenter then left the room, and Sarah was given a chance to inspect the boxes.

She faced an interesting dilemma, since she had not seen how the experimenter had obtained his fruit. Invariably, however, she would go to the box with the fruit that the experimenter had not eaten. She must have concluded that the experimenter had taken his fruit from the corresponding box and that the second box would still contain its original fruit. Most animals don’t make any such assumptions, the Premacks note; they just see an experimenter consume fruit. Chimpanzees, by contrast, always try to figure out the order of events, looking for logic, filling in the blanks.

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