Many animals have a sense of number – 01/02/2024 – Marcelo Viana

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 there is not enough food for two larvae. Thus, the wasp developed a sensor in its “syringe” that informs the wasp’s brain if the caterpillar is no longer “available”, to avoid wasting eggs. What’s more, recent research shows that the signal sent by the sensor to the wasp’s brain varies depending on whether the caterpillar hosts zero, one or two eggs. Therefore, wasps can count (up to two), and this ability plays an important role in the survival of their species. It is not a rare fact. Although the case of the horse Kluge Hans, which I reported here two weeks ago, still causes skepticism about the numerical capabilities of animals, studies carried out in recent decades indicate that number sense is widely distributed in the animal kingdom, even among species with rudimentary brains.

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

Other species have a more refined sense, which distinguishes between individual numbers. The tiny Central American túngara frog, just 3cm long, is a spectacular example. The male spends hours at a time making characteristic calls to potential girlfriends that always end with a click. But when he hears a rival’s call, he ups the ante: he makes another call, this time ending with two clicks! And the duel continues, with an increasing number of clicks, until one of them loses breath, which usually happens around six or seven clicks.

It’s an expensive game, in terms of energy that males need to expend, and dangerous: all this shouting also attracts predators. But they have no choice, because the Túngara girls are listening (and counting!), and they always choose the boys who make the most clicks.

Scientists debate whether number sense in other species is similar to ours, or essentially different. In any case, despite the advantage that speech gives us, there are some aspects in which other animals can exhibit superior numerical capabilities than ours. It will be a topic for another day.

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