From small to large: Bees, like people and birds, arrange numbers according to a mental number series, as an experiment has shown. Accordingly, the insects spatially associate small numbers with left and large numbers with right. This suggests that invertebrates also process sets of numbers mentally in a similar way as we do. Contrary to what was long thought, such series of numbers could not be a cultural phenomenon, but a cross-species biological one.

The mental number line is a form of spatial number representation that associates small numbers with the left side and large numbers with the right side. For a long time, research assumed that only we humans deal with numbers in this way and that we first have to learn the method culturally. However, as mental number sequences were found in newborns and eventually in birds, the theory began to falter.

But how widespread are series of numbers in the animal kingdom? Do insects have it too? This question can be answered by testing honey bees. The animals have already demonstrated their excellent sense of mathematics in earlier experiments. You can estimate sets, understand the concept of zero, solve simple arithmetic problems and assign abstract symbols to sets.

But do the honey bees also have mental number sequences? To find out, a research team led by Martin Giurfa from the Research Center for Animal Cognition in Toulouse, France, presented the bees with a new challenge. First, the scientists trained them to associate the number three with a reward of sugary juice. They represented this number as a set of identical shapes, for example three triangles or three circles.

In the next step, the animals had to apply what they had learned in special Y-shaped wooden boxes. As soon as they flew into the test box, they could choose between two exits for the further way, one with a three-point symbol and one with a different number of symbols. Most of the bees successfully recognized the triple symbol and headed towards it in the hope of sugar juice.

But how do animals perceive three compared to other numbers? To answer that, the scientists rebuilt the wooden box. Now when the animals flew in, they had a choice between two identical symbols. Depending on the round, this was twice a one symbol or twice a five symbol. Theoretically, it would not have made any difference which exit the animals chose.

Nevertheless: In the box with the one symbols, 93 percent of the bees chose the left symbol, while in the box with the five symbols, 79 percent of them flew towards the right one. But why? The researchers suspected that this could be related to a mental ranking of the numerical values: Because the bees were trained to do three, it was apparently most logical for them that a lower number had to be on the left and a higher number on the right.

To test this interpretation, Giurfa and her team repeated the experiment in a modified form, for example by initially training the bees to use the one symbol instead of the three symbol. But the result was always the same: if the identical symbols in the second run were larger than the learned comparative value, most of the bees flew to the right; if they were smaller, they flew to the left.

According to the research team, this proves that bees also arrange numbers using a mental number series. The left-right direction of the series of numbers could be related to the fact that many animal species have asymmetrical cerebral hemispheres, the biologists explain. For example, earlier experiments in humans showed that our left hemisphere primarily processes smaller numbers and the right hemisphere larger ones.

But that’s not all. Giurfa and his colleagues suspect “that the mental series of numbers is a form of numerical representation that has evolved in all nervous systems with number sense, regardless of their neuronal complexity.” This form of number processing could therefore be far more widespread and be more general than previously thought.

Taken together, these results suggest that, contrary to what has long been thought, left-right orientation is probably biological rather than cultural. (Proceedings of the National Academy of Sciences, 2022, doi: 10.1073/pnas.2203584119)

Quelle: Proceedings of the National Academy of Sciences