Fish can count … up to three

Fish can distinguish between larger and smaller quantities, with an additional ability to “count” up to three, according to research on tropical angelfish. Angelfish are regarded as being one of the world’s most intelligent fish, but scientists believe other fish species also possess the math-related skills outlined in a new Animal Cognition paper.

Doing something akin to counting up to three might sound underwhelming, but math itself is a very human-centric concept that may need reconsideration if comparisons are to be made with the abilities of non-human species.

“We all think we know what we mean by 'counting,' but do we really?” asked co-author Robert Gerlai. “Is recounting a series of 1 to 100 counting? Is 2+3=5 counting? Is calculating the square root of a number counting, or perhaps is the mathematics necessary for quantum physics counting?”

Gerlai and Luis Gomez-Laplaza of the University of Oviedo in Spain exploited the previously determined tendency of angelfish to seek protection in unfamiliar environments by joining the largest possible fish group, called a shoal. To rule out possible confounding effects arising from sexual interactions, the researchers only used juvenile angelfish for their experiments.

Test fish placed in special compartmentalized tanks were given a simultaneous choice between shoals containing different numbers of fish. The angelfish were always able to select the larger of two groups so long as the ratio between the shoals was 2:1 or above. Below that ratio, their choices were less predictable, suggesting a limit to their quantity estimation abilities.

After the findings were published, the researchers, according to Gerlai, “have already collected new data suggesting that angelfish can discriminate much more precisely than this. That is, angelfish can tell the difference between 3 and 2, for example.”