And this skill can reach an astonishing level of refinement, according to the authors.

In the animal kingdom, the ability to count doesn’t rely on learning a specific numeral system as humans do. Instead, animals utilize numerical information from their surroundings to make crucial decisions for survival and reproduction. Estimating the number of competitors in a group, evaluating food availability in challenging locations, or assessing potential mates in a new territory are vital skills. It’s fascinating to note that certain species of ants can orient themselves in the desert by estimating the number of steps required to reach a target.

Numerical sensitivity, which refers to the ability to perceive quantity-related information, exists in various vertebrates and invertebrates. This phenomenon has been observed in primates, birds, amphibians, fish, and bees.

Mercedes Bengochea, a post-doctoral researcher at the Paris Brain Institute, explains that animals can distinguish between one, two, several, and many without the need for enumerating numbers. However, the specific neuronal circuits responsible for this skill have yet to be fully understood.

To investigate this question, researchers have faced challenges in recording the brain activity of vertebrates. However, fruit flies (Drosophila melanogaster) have proven to be an excellent model for studying cognition. These insects adjust their behavior based on the number of fellow flies that could provide assistance in the face of a threat. When danger is imminent, smaller fly groups are more likely to freeze to ensure their safety.

To determine whether fruit flies can accurately evaluate numbers and assign values to perceived quantities, Mercedes Bengochea and her colleagues conducted relevant experiments. They placed the flies in experimental arenas called “Buridan arenas,” exposing them to visual stimuli in the form of two sets of objects. The researchers measured the time the insects spent inspecting each set to determine their preferences.

The results revealed that fruit flies preferred the set containing three objects over the set with only one, regardless of the object sizes or total volume occupied by the sets. This preference for larger quantities was also consistent when the flies had to choose between groups of 2 or 4 objects and 2 or 3 objects.

However, the flies were unable to distinguish between sets of 3 and 4 objects, suggesting that the ratio between these two numbers is insufficient for them to perceive a difference. However, they could easily differentiate between a group of 4 objects and a group of 8 objects, which represents a simple doubling ratio. This indicates that fruit flies are not limited to counting up to 3; they can perceive quantities as long as the ratio is clear.

Assessing the ratio between two quantities is a common visual task among animals. It is also useful for humans, as it allows us to quickly estimate the size of a group that contains too many elements to be counted individually, such as a crowd at a concert.

The specific neural circuits responsible for numerical discrimination in Drosophila are still unknown and require further investigation. To uncover this information, the researchers employed a method of successively deactivating various regions of the fruit flies’ brains, effectively blocking the transmission of nerve signals at synapses.

Through multiple tests, they made a significant observation: the activity of a particular column of neurons known as LC11 neurons (lobular columnar neurons 11) located in the optic lobe was essential for the flies’ ability to differentiate between different sets of objects.

In a second experiment, the researchers trained the fruit flies to favor small numbers by placing an appetizing dose of sugar near the smallest sets of objects. The flies temporarily showed a preference for small numbers due to the lure of the food. However, once the LC11 neurons were inactivated, the insects no longer exhibited a preference for either large or small quantities. This confirmed that the LC11 neurons play a crucial role in comparing quantities, regardless of the specific value assigned to them by fruit flies.

LC11 neurons are also involved in the social behavior of fruit flies, particularly in adapting their defense strategy based on the number of nearby flies. Bassem Hassan, the head of the ‘Brain Development’ team, believes that the ability to assess quantities has been pivotal in the evolution of invertebrates. Insects employ simple cognitive solutions to perform “counting” tasks, as demonstrated by several studies using computational models with just a few artificial neurons.

While fruit flies may not assist us in accounting tasks, it is crucial not to underestimate their cognitive abilities and the complexity of their social behavior. By recognizing their capabilities, we gain a deeper understanding of the human brain. Without studying these remarkable creatures, our knowledge of the intricacies of the human brain would remain significantly limited.

Source: 10.1016/j.celrep.2023.112772

Image Credit: Mercedes Bengochea, Maria Ines Oviedo