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Fish ‘count’ in similar way to humans

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26 May 2011 in

According to a new study, fish can differentiate between quantities as large as 100 and 200. Surprisingly, they show similar numerical skills to college students' when presented with a laboratory test.


Recent research has shown that fish show basic numerical abilities similar to those observed in mammals and birds. This latest study investigated whether the fish can discriminate between large numbers.

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."

Mosquitofish, freshwater fish with a taste for mosquito larvae, are highly social. When a mosquitofish finds itself alone, its top priority becomes finding others.

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

The fish were then placed in tanks where they had to choose 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 choose the correct door more often than by chance alone.

The new study repeated this experiment, but then the researchers started using larger 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."

The researchers then changed the ratios of shapes on the doors. 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 v 16) or 2:3 (8 v 12), the fish chose the correct door more often than by chance. But when the ratio became 3:4 (9 v 12), the fish showed no indication that they could detect a difference.

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

The test asked 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.

Clearly there is a lot more going on inside fish's heads than they are usually given credit for. Yet we do not treat them in the same way as we do other vertebrate animals. It is now widely accepted that fish can feel pain and the more we continue to learn about their complex minds the more urgent the need to change our practices and laws to protect fish from suffering.

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