Fruit flies can assess and compare quantities, according to a study published in Cell reports. The authors unveil what areas of the brain are essential for this skill.
In the animal world, animals may not use our numerical system, but they still need to be able to count. They need this information to make decisions, such as estimating the number of other animals before engaging in conflict or deciding whether the amount of food available in a difficult-to-reach position is worth it.
“Numerical sensitivity, i.e., the ability to perceive information related to quantities, exists in many vertebrates and invertebrates. It has been documented in primates, birds, amphibians, fish, and bees, said Mercedes Bengochea from the Paris Brain Institute. You don’t need to enumerate numbers to distinguish between one, two, several and many! However, we didn’t know which neuronal circuits were involved in this skill.”
To solve this mystery, the ideal experiment would be to record the brain activity of animals during numerical tasks and then activate or deactivate certain brain areas to see which ones are involved. This isn’t easy to do in vertebrates, but it can be done in fruit flies. “Drosophila melanogaster is a model of choice for studying cognition. These insects adjust their behavior in the face of a threat according to the number of fellow flies who could help. In the event of imminent danger, the smaller the size of its group, the more likely they are to freeze to stay safe,” said Bengochea.
To find out whether fruit flies can “count” and assign values to specific quantities, Bengochea, and her colleagues put flies in boxes and exposed them to different objects. The researchers measured how long the insects spent inspecting each item.
Results showed that flies stayed longer near the set containing three objects than the set that only had one, independently of the size of the objects. Flies could still distinguish between 8 and 4 objects, 4 and 2 objects, and even 2 and 3 objects, but not 3 and 4 objects. “The flies were unable to distinguish between sets of respectively 3 and 4 objects. It seems that the ratio between these two numbers is not sufficient for them to perceive a difference,” explained Bengochea. “On the other hand, they can very easily compare a group of 4 and a group of 8 objects – a ratio of simple to double”. The authors suggest that the ratio between the quantities must be clear enough to be identified.
This experiment shows that flies can assess quantities, but we still don’t know which neural circuits are involved in this system. To find an answer, the team managed to “switch off” different areas of the insects’ brains, stopping the transmission of information between nerve cells. Using this method, they noticed that the activity of certain neurons located in the optic lobe — called LC11 neurons for lobular columnar neurons 11 — was needed for flies to distinguish different sets of objects.
“In a second experiment, we taught the insects to go against their natural inclination for large numbers, using a simple conditioning method: an appetizing dose of sugar was placed next to the smallest sets of objects,” explained Bengochea. “Momentarily, thanks to the lure of the food, we made them prefer the small numbers. But once the LC11s had been inactivated, the insects no longer showed any preference… for either large or small quantities. This confirms that these neurons are essential for comparing quantities, regardless of the value fruit flies assign to them.”
LC11s are also needed to regulate social behaviour. These neurons activate when insects need to establish a defense strategy according to the number of nearby congeners. “We believe that the ability to assess quantities has been decisive in the evolution of invertebrates,” concluded Bassem Hassan, head of the Brain Development team. “The cognitive solutions insects use to ‘count’ are very simple. Several studies have shown that, in a computational model, a few artificial neurons are enough to perform a numerical task.”
Flies will never learn how to count, but the team believes this work will be useful in unraveling the evolutionary origin regarding how we understand numbers and quantities.
Bengochea M, Sitt JD, Izard V, Preat T, Cohen L, Hassan BA. Numerical discrimination in Drosophila melanogaster. Cell Rep. 2023 Jul 7:112772. doi: 10.1016/j.celrep.2023.112772.