Researchers from the University of Padova in Italy have found that fish have rudimentary counting abilities and can count up to four.
Christian Agrillo, Marco Dadda, Giovanna Serena and Angelo Bisazza studied the ability of Eastern mosquitofish (Gambusia affinis) to discriminate numbers by giving lone female mosquitofishes the choice of joining shoals of between two to eight other individuals in an experimental tank setup.
The authors found that lone fishes more often preferred joining larger shoals that had one more fish.
Test individuals consistently preferred joining shoals of four over shoals of three, shoals of three over shoals of two, and shoals of two over one fish.
However, experiments with larger numbers (5 vs. 4, 6 vs. 5, 7 vs. 6 and 8 vs. 7) showed that the fishes failed to discriminate quantities larger than four.
This demonstrated that the fish had the ability to count up to four and possessed a rudimentary mathematical ability to visually count items if the number is small.
Larger numbersThe authors next tested the abilities of the fish to distinguish between groups of different numerical sizes with numbers larger than four, by performing the same experiment, but with larger groups of fishes (e.g. 8 vs. 12, 8 vs. 16) as choices.
They found that the fishes were able to clearly discriminate larger numbers only when the ratio was at least 2:1, i.e. the fishes could distinguish between a shoal of 16 and a shoal of 8, but not between a shoal of 12 and a shoal of 8 (a ratio of 3:2).
However, the authors did not record an abrupt decrease in the ability to discriminate numbers with ratios below 2:1; instead a steady decrease in preference with ratios decreasing from 5:2 to 5:4 was observed.
This indicates that a difference is also perceived below the 2:1 ratio, but that discrimination of the two quantities becomes more and more difficult when the relative distance between the quantities is reduced.
As clever as monkeys...The current findings indicate that fish have counting and numerical discrimination abilities similar to that of monkeys and human infants.
The authors write: Even though we presently lack sufficient knowledge to determine whether the same cognitive processes are involved in all species, our results show that organism as diverse as primates and teleost fishes, whose divergence occurred more than 450 million years ago, share similar capacities in precise discrimination of small quantities.
The study is due to be published in the journal Animal Cognition.
For more information, see the paper: Agrillo, C, M Dadda, G Serena and A Bisazza (2008) Do fish count? Spontaneous discrimination of quantity in female Mosquitofish. Animal Cognition doi 10.1007/s10071-008-0140-9