When rats go shopping: Three is the magic number

As humans we use numbers in almost everything that we do. They are used to quantify, categorise and measure all aspects of everyday life. The same is true of animals: numbers could be useful to them to identify how many animals there are in a competing group, the amount of food available and so on.

Humans are clearly capable of representing numbers in abstract terms. Likewise, several studies have shown that a wide variety of animals are also capable of some of the precursors of this ability. For example, chimps can order numbers even after only seeing them very briefly and Ayuma out performs humans in this sort of task.

Most of the work in abstract numerical discrimination been conducted with primates and big brained birds, like parrots. Surprisingly, we know very little about this ability in rats—even though rats are the go-to animals for experimental psychologists. Thus, in a recent article published in the Psychonomic Society’s journal Learning and Behavior, Tohru Taniuchi and his colleagues at Kanazawa University examined whether rats had the capacity to discriminate on the basis of an abstract number.
“Much research has been conducted on rats’ numerical capacity, but the ‘abstractness’ of numerical concepts in rats has not been elucidated well. So, we planned to demonstrate truly abstract numerical concepts in rats by examining the transfer of numerical discrimination to novel stimuli,” says Professor Taniuchi.

In an ingenious experimental design the researchers presented rats with rows of objects which had little doors behind them. The animals had to learn to choose the third object, counting from the beginning of a walkway. The door behind this object had a treat hidden behind it and if the rat chose that door then it would be rewarded with the treat. There were 10 different doors altogether, but the objects weren’t in front of every door so that the animals could not learn a spatial cue to solve the task.The animals had to use the object number to obtain their reward. The video illustrates what the rats did:

The objects were placed in a number of different spatial configurations so that the third object was not always in the same place. The rats were also tested on different absolute numbers of objects present, and they were even able to perform well when totally novel objects were used. This suggests that the rats had learned the abstract idea of “go to the third object”, irrespective of the objects’ identity placement, and absolute number.

Despite this impressive performance the experimenters still had concerns, as related by Professor Taniuchi:

“Although we found significant transfer of numerical discrimination, we could not exclude the possibility that such good transfer was due to the rats’ ignorance or insensitivity to differences in object stimuli. To solve this problem, we came up with a ‘selective counting task’ in which the rats had to ‘exclude’ an odd object from their counting.”

In the selective counting task the rats were given an odd (non-matching) object in the line-up, sometimes it was before the third matching object and sometimes it was after. This object was not to be counted towards the number that identified the door with the reward. To perform well on this task, the rats had to discriminate different objects from each other by attending to the features and position of the objects.

Interestingly, the rats learned this task surprisingly quickly and responded correctly even when entirely new objects were used at test. In a final experiment the rats were trained to include the non-matching object in the count, and they readily learned to do that as well.

“We believe the results provide strong evidence of abstract numerical concepts in rats. The findings also suggested ‘categorical flexibility’ in rats. That is, humans can count two apples and three oranges separately, but also in combination as five pieces of fruit. In our study, the same rats could learn not only to exclude an odd object from counting but also to count different objects non-specifically, depending on task demands” says Professor Taniuchi.

This is impressive evidence to showing that the rats are able to make abstract numerical discriminations and builds on a large number of experiments revealing remarkable cognitive abilities in these animals. Three seems to be a magic number for rats.

No wonder they are so successful at living in human environments.

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