Philip Ball, writing for BBC Future:
Newborn fish not only can count, but also can be taught to count better. This discovery by a team of psychologists at the University of Padova in Italy, might seem bizarre, even frivolous at first glance, but in fact it bears on a deep and interesting question: how do we assess the number of people, animals or objects based on what we see?
As Ball goes on to explain, there are two ways to assess numbers—a numerical system, like the kind we’re taught as children, and a ratio system, like the kind we use to guess whether one pile of cookies is larger than another. Fish, like humans, can improve their performance in counting. You just have to train them.
Parrots are another example of an animal that can be trained to perform better at counting tasks. Unlike fish, though, they can also be taught to tell researchers the right answer, literally, as Alex the African Grey parrot did for many years.