Fish can count, according to scientists, who have found that North American mosquito fish have the ability to count up to four.
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.
The results, published on the BBC Worldwide's natural history site, loveearth.com, show that females preferred to join shoals that were larger by just one fish significantly more often - consistently preferring shoals of four fish rather than three fish, and consistently preferring shoals of three fish over those containing just two.
A second series of experiments revealed the fish's ability to process larger numbers. The fish were not able to directly count over four, but they were able to distinguish between larger numbers if they differed by a ratio of 2:1.
For example, the fish could distinguish between a shoal of 16, compared to a shoal of eight others. But they could not tell the difference between a shoal of 12 compared to a shoal of eight, a ratio of 3:2. This demonstrates that fish are able to visually estimate larger numbers - but not very accurately.
Prof Angelo Bisazza, who led the latest research, said that fishes' numerical abilities were actually on a par with the numerical abilities of monkeys and human infants between six and 12 months old, who were both able to visually count small numbers and less accurately estimate larger ones.
Adult humans use a third counting mechanism, in which they verbally count much larger numbers.
Dr Agrillo said: "The most interesting thing is that fish performance is very similar to what is observed in adult humans who possess a very limited vocabulary for numbers."
For example, speakers of the Amazonian language Mundurukú lack words for numbers beyond five. "Their limits in quantity tasks closely resemble what we found in pre-verbal organisms such as fish!" he added.
A variety of animals, including pigeons, parrots, raccoons, ferrets, rats, monkeys and apes are to varying degrees capable of either counting, adding or subtracting numbers. Most need to be trained to do so.
Without training, adult rhesus monkeys are capable of subtracting small numbers, and are capable of representing the number zero.
Wild lions apparently have a rudimentary ability to count. When a pride of lions hears the roar of an approaching lion then two or three females, rather than a lone greeter will always go out to meet the stranger. But if two approaching lions can be heard, the resident females send out four of their own.