According to Christian Agrillo, an experimental psychologist at the University of Padua in Italy, the North American mosquito fish can count up to four. This rudimentary mathematical ability makes it possible for the North American mosquito fish to count how many other fish that are nearby – but only up to four. Similar counting abilities have already been observed in dolphins, but until now researchers only new that fish could tell big shoals from small ones, not that they were able to actually count.

In earlier research, Agrillo and his colleagues found that a female specimen of the North American mosquito fish will swim to the largest nearby shoal to protect herself from a harassing male. In order to do so, she must of course be able to somehow tell the difference between a larger shoal and a smaller one.

To establish exactly how advanced the counting ability of the North American mosquito fish was, Agrillo et al continued their research by testing if a lone specimen would prefer to join a shoal consisting of 2, 3 or 4 other fish. The study showed that female fish could tell the difference between two shoals even when the shoal size differed by only one specimen. Females would significantly more often prefer to join a shoal consisting of four others rather than three, and would also favour shoals comprised of three fish rather than of two.

When a new series of experiments were conducted using even larger shoals, researchers found that the female North American mosquito fish were unable to directly count over four. If two shoals differed in size by a ratio of 2:1 she would go for the larger one, but if the difference was smaller she seemed to conclude that they were “both big”. She would for instance not favour a shoal consisting of 12 fish over a shoal of 8, but could clearly tell a shoal of 16 from a shoal of 8. The female North American mosquito fish therefore seems to have the ability to estimate larger numbers, but not very exactly.

The results of the study can be found on BBC’s site Loveearth.com