Rhesus Monkeys: Using Numbers and Videogames to Demonstrate their Understanding of the Relative Numerical Middle

18.02.2022

If we count from 1 to 9, what number is in the middle? It is easy for us to say that the answer is five, but for monkeys to know this is something different yet a study entitled Relative numerical middle in rhesus monkeys, published by researchers from the University of Padua and the University of Pennsylvania in the journal Biology Letters does just that.

Like humans, monkeys can also identify the middle in a sequence of numbers, and with surprising accuracy.

 Coordinated by Professor Rosa Rugani of the Department of General Psychology at the University of Padua, the published study demonstrates the ability of these animals to identify the central element in a series of elements (numerical middle).

For humans, the representation of numbers takes place mainly through symbols, such as Arabic numerals. While the counting skills of animals take place through non-symbolic representation known as "intuitive number sense."

Researchers have learned much about the animal understanding of primitive mathematics in the last few decades. Including their ability to perform simple arithmetic operations or to estimate proportions, but understanding the numerical middle was a skill that has remained unexplored, until now.

For this particular study, researchers trained the monkeys to identify the central dot in a series of three dots with a video touchscreen connected to a device that distributed a reward for each correct answer.

Increased the difficulty of the experiment, the researcher tested the animals in the face of a series made up of new and larger numbers: 5, 7, or 9 dots. Regardless of the number of dots, the monkeys, stimulated by the reward, consistently chose the middle number.

 However, they could have chosen them based on a spatial strategy when estimating the average distance from each of the extremities, identifying a hypothetical center.

To understand what strategies the animal implemented, the research used asymmetrical sequences in which the dots were closer and denser on one side of the series. The goal was to see if the two distinct dots represented one the spatial middle and the other the numeric middle. The results revealed that these animals could certainly make use of the numerical clues and not just the spatial ones.

The researchers plan to study the same phenomenon in other animal species. Aimed at identifying brain areas involved in numerical bisection, research using animal models could be important to understand the biological basis of number learning difficulties in children.