03/01/2024

Folha: The meaning of number in animals

Reproduction of Marcelo Viana's column in Folha de S. Paulo.

The parasitic wasp Leptopilina heterotoma injects its eggs into caterpillars, which will serve as food for the larva when the egg hatches. It is important that there is no more than one egg in each caterpillar, as the food is not enough for two larvae. Thus, the wasp has developed a sensor in its "syringe" that informs the wasp's brain if the caterpillar is no longer "available," to avoid wasting eggs.

Furthermore, recent research shows that the signal sent by the sensor to the wasp's brain varies depending on whether the caterpillar is hosting zero, one, or two eggs. Therefore, wasps are able to count (up to two), and this ability plays an important role in the survival of their species.

It's not a rare occurrence. Although the case of the horse Kluge Hans , which I recounted here two weeks ago, still raises skepticism about the numerical capabilities of animals, studies conducted in recent decades indicate that a sense of number is widely distributed throughout the animal kingdom, even among species with rudimentary brains.

We know that many insects , fish, birds , and mammals possess an approximate sense of number, which allows them to quickly choose between two groups (of animals, food , etc.) of different sizes. The mosquitofish , for example, uses this sense to always join the largest available school, which offers greater protection against predators. Rats use the same ability to choose the largest of different available food sources.

Other species have a more refined sense, which distinguishes between individual numbers. The tiny Central American tree frog, measuring only 3 cm, is a spectacular example. The male spends hours emitting characteristic calls to potential mates , always ending with a click. But when he hears a rival's call, he raises the stakes: he makes a new call, this time ending with two clicks! And the duel continues, with an increasing number of clicks, until one of them runs out of breath, which usually happens around six or seven clicks.

To read the full text, visit the newspaper's website.