Previously it was known that fish could tell big shoals from small ones, but researchers have now found that they have a limited ability to count how many other fish are nearby. This means that they have similar counting abilities to those observed in apes, monkeys and dolphins and humans with very limited mathematical ability.
Christian Agrillo, an experimental psychologist at the university of Padua in Italy said: “We have provided the first evidence that fish exhibit rudimentary mathematical abilities.”
Last year, he and his colleagues showed that if a female mosquito fish is harassed by a male, she will try to avoid his attentions by seeking solace in the largest nearby shoal; demonstrating that the fish can tell bigger shoals from smaller ones. The team first conducted a series of experiments to see whether a lone mosquito fish would prefer to join a shoal of between two and four others.
This article is fascinating because it describes the notion of numbers, not just among animals, but among non-mathematical humans.
[London Telegraph: Fish can count to four — but no higher]