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Mosquitofish.

Mosquitofish (above, in a file photo) scored about the same as people in a test involving numerical ratios.

Joel Sartore, National Geographic

Matt Kaplan

for National Geographic News

Published January 7, 2011

Are you as smart as a mosquitofish?

According to a new study, college students showed roughly the same numerical skills as the small fish when presented with a laboratory test.

Mosquitofish, freshwater fish with a taste for mosquito larvae, are highly social. When a mosquitofish finds itself alone, its top priority becomes finding others. A previous study showed that the fish can, in fact, "count"—differentiating between numerical quantities in a lab experiment.

(Related: "Monkeys Can Subtract, Study Finds.")

The new study shows that the fish can not only tell the difference between small numbers such as 4 and 8, but they can also differentiate between quantities as large as 100 and 200.

"You just don't expect interesting results like this when dealing with animals like fish," said study leader Christian Agrillo of the University of Padova in Italy. "We thought this was really incredible."

But numerical skills break down for the fish when the ratios between two numbers change—an effect that was also seen among human volunteers.

Fish That Can "Count" to a Hundred

For the initial experiment, lone fish were trained in the lab to associate a door bearing a certain number of geometric shapes with the path to rejoining a larger group.

The fish were then placed in tanks where they had to chose between one of two identical doors bearing different numbers of symbols. For example, four shapes might be associated with door A and eight shapes with door B.

At the beginning of the test, the fish did not know where to go, and they chose randomly. Over time, however, the fish started to chose the correct door more often than by chance alone.

(Related: "Coot Birds Can Count, Study Says.")

The new study repeated this experiment—but then the researchers started using bigger numbers of shapes on the doors.

"It was kind of funny, most of them appeared to be surprised when we switched from small numbers to hundreds. They swam inside the tank for a while, looking at the new stimuli as if they were trying to understand what was going on," Agrillo said.

"However, after a short while they started to solve the task as well."

Ratios Boggle Fish and Humans

As the researchers tinkered with the amounts on the doors, they found that when the sets of shapes were closer in number, the fish were less successful.

For example, when the ratio between the shapes on the two doors was 1:2 (8 versus 16) or 2:3 (8 versus 12), the fish chose the correct door more often than by chance. But when the ratio became 3:4 (9 versus 12), the fish showed no indication that they could detect a difference.

Keen to see how the numerical skills of fish compare to those of humans, the researchers asked a group of 25 undergraduate students to engage in a test that presented them with the same types of challenges.

The test forced the students to determine the difference between large numbers in two seconds, without stopping to deliberately count the shapes.

While humans were universally more accurate than fish, they showed the same degraded ability to judge number differences as ratios shifted from 2:3 to 3:4.

According to Agrillo and colleagues, the results add to evidence that humans, fish, and other vertebrates share the same abilities for processing numbers as a distant but common ancestor.

(Also see "Fossil Fish With 'Limbs' Is Missing Link, Study Says.")

The counting-fish study was published December 22 in the journal PLoS ONE.

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