Scientists have known for some time that fish can tell big shoals from small ones, but now they've discovered that they do so by counting:
'We have provided the first evidence that fish exhibit rudimentary mathematical abilities,' says experimental psychologist Christian Agrillo of the University of Padova in Italy, who made the discovery while studying a group mosquitofish (Gambusia holbrooki). [...]
His team first conducted a series of experiments to see whether a lone mosquitofish would prefer to join a shoal of between two and four others. 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. This means the fish were clearly able to count up to four and demonstrates that fish have a rudimentary mathematical ability to visually count how many items are present if the number is small.
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.
Although it doesn't sound much, it is actually on a par with the numerical abilities of monkeys and human infants between six and 12 months old, who are both able to visually count small numbers and less accurately estimate larger ones.