Deciphering Quantities: The Remarkable Numeric Skills of Fruit Flies

Artwork depicting numeric differentiation in Drosophila. Credit: Mercedes Bengochea, Maria Ines Oviedo

A study conducted at the Paris Brain Institute revealed the impressive numeric sensitivity of fruit flies, who displayed a preference for larger numbers and could differentiate between quantities based on distinct ratios. The study emphasized the significance of LC11 neurons in this skill, drawing attention to insects’ cognitive abilities and their relevance to the study of human brain function.

In the world of animals, a formal numeric system, like our common ten-digit Indo-Arabic system, isn’t necessary for counting. Animals consistently use numeric information from their environment to inform their decisions. Whether estimating the size of a rival group before conflict, the quantity of food available in a hard-to-reach location, or the count of possible mates in a new territory, these calculations are essential for survival and reproduction. Some species exhibit extraordinary precision in this skill; for example, certain ants navigate the desert by estimating the steps needed to reach a destination.

Numeric Sensitivity and Neural Connections in Animals

“Numeric sensitivity, or the capacity to perceive quantity-related information, is seen in a wide variety of vertebrates and invertebrates. This includes primates, birds, amphibians, fish, and bees,” explains Mercedes Bengochea, a post-doctoral researcher in Bassem Hassan’s team at the Paris Brain Institute. “You don’t need to enumerate numbers to distinguish between one, two, several, and many! However, we were unaware of the neuronal circuits involved in this ability.”

To answer this, researchers need to track an animal’s brain activity during numeric tasks, then activate or deactivate certain neural cells to identify which brain regions are implicated. Although these tasks are challenging with vertebrates, appropriate tools are already available for working with fruit flies. “Drosophila melanogaster is an ideal model for cognition studies. These insects adapt their behavior in response to a threat based on the count of fellow flies who could provide assistance,” adds the researcher. “In situations of impending danger, the smaller the group size, the more likely they are to freeze for safety.”

Probing Numeric Perception in Fruit Flies

To investigate if fruit flies can accurately assess numbers and assign values to perceived quantities, Mercedes Bengochea and her team utilized a proven experimental setup. The flies were placed in enclosures known as “Buridan arenas,” where they were subjected to visual stimuli consisting of two sets of objects. The researchers then noted which stimulus the insects favored by tracking the time they spent examining each set.

The results show that fruit flies lingered longer near the set containing three objects than the one with only a single object – irrespective of object size or the total volume of the set. This preference for larger quantities was maintained when the insects had to choose between groups of 2 or 4 objects and 2 or 3 objects. “The flies, however, couldn’t differentiate between sets of 3 and 4 objects, according to Mercedes Bengochea. It appears that the ratio between these numbers isn’t clear enough for them to perceive a difference. Conversely, they can easily compare a group of 4 with a group of 8 objects – a simple to double ratio. Thus, fruit flies are not just limited to counting up to 3: the ratio between evaluated quantities must be clear enough to be discerned.

Evaluating Ratios: A Shared Skill in Animals

Comparing two quantities is a simple visual task common to many animals, including humans. This helps us to estimate the size of a large group quickly, like a concert crowd too large to count individually.

Unveiling the Involved Neural Circuits

Identifying the neural circuits that drive numeric discrimination in Drosophila remains an open question. To solve this, the researchers successively deactivated different areas of the insects’ brains, halting the transmission of nerve signals at synapses. After several tests, they observed that the activity of a column of neurons located in the optic lobe, LC11 neurons, was required for flies to distinguish different sets of objects.

“In a second experiment, we trained the insects to counteract their natural inclination towards larger numbers. We placed a tempting dose of sugar next to the smallest sets of objects,” adds the researcher. “Temporarily, with the promise of food, we made them favor smaller numbers. But once the LC11s were inactivated, the insects no longer showed a preference for either larger or smaller quantities. This confirms that these neurons are vital for comparing quantities, regardless of the value fruit flies attribute to them.”

LC11 neurons also play a role in the social behavior of fruit flies: they activate when insects need to adjust their defense strategy according to the number of nearby flies. “We theorize that the ability to assess quantities has been a crucial factor in the evolution of invertebrates,” explains Bassem Hassan, head of the ‘Brain Development’ team. “The cognitive mechanisms insects use to ‘count’ are quite straightforward. Several studies have shown that, in a computational model, a handful of artificial neurons can perform a numeric task.”

While flies will never assist us in accounting, we often undervalue their cognitive abilities and the complexity of their social behavior. This is a regrettable oversight, as without them, our understanding of the human brain would be considerably constrained.

Reference: “Numerical discrimination in Drosophila melanogaster” by Mercedes Bengochea, Jacobo D. Sitt, Veronique Izard, Thomas Preat, Laurent Cohen and Bassem A. Hassan, 14 July 2023, Cell Reports.
DOI: 10.1016/j.celrep.2023.112772

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More about Fruit flies’ numerical sensitivity

• Understanding Numerical Discrimination in Fruit Flies
• The Cognitive Capabilities of Insects
• Bassem Hassan’s research at the Paris Brain Institute
• Role of LC11 neurons in Drosophila